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Place a charged particle in an electromagnetic field and the particle will accelerate and give off radiation. Typically, the emitted radiation has little effect on the particle’s motion. However, if the acceleration is extremely large, as is the case for high-energy electrons or positrons in strong electromagnetic fields, the emitted radiation will drastically slow down the particle. The effect, known as radiation reaction, has been recognised since the beginning of the twentieth century, and is relevant in several branches of physics, from accelerator physics to astrophysics. But until now it has been difficult to pin down the maths that best describes the phenomenon. In a paper recently published in Physical Review D, the NA63 collaboration reports a high-precision study of the phenomenon that shows that an equation proposed long ago does the job remarkably well.
The NA63 team has previously investigated radiation reaction by firing a beam of high-energy positrons from the Super Proton Synchrotron at a silicon crystal. The phenomenon has also been studied by colliding a high-intensity laser beam with a high-energy electron beam. However, these two types of study were conducted in a regime in which quantum effects were dominant, and the laser-based experiments also used relatively small data samples with large data fluctuations, all of which prevented a high-precision study of the effect.
Enter the latest NA63 study. By directing a beam of high-energy charged particles (electrons or positrons) from the Super Proton Synchrotron at several (silicon or diamond) crystals of different thickness, one crystal at a time and with different angles at which the beam strikes the crystal, the NA63 team succeeded in studying with high precision the radiation reaction for the charged particles in the crystal’s strong electromagnetic field. In all cases, the researchers measured the energy spectrum of the photons emitted by the charged particles, that is, they measured how the number of photons emitted by the charged particles varied with the photon energy.
They found that all of the measured energy spectra are in remarkable agreement with predictions based on the Landau–Lifshitz equation describing the dynamics of charged particles in a strong electromagnetic field if these predictions also include small changes from quantum effects.
“This classical equation was proposed in the 1950s to account for the effect of radiation reaction,” said NA63 spokesperson Ulrik Uggerhøj. “Our new study has investigated for the first time the experimental regime in which the effect is dominant, and it showed that the equation does seem to describe this regime well.”
It is with great sadness that we inform you that Philippe Mermod, a member of the ATLAS and SHiP collaborations at CERN, passed away on 20 August 2020.
Philippe was born in Geneva in 1978 He obtained his Master’s degree in 2002 from the University of Geneva and his PhD in 2006 from Uppsala University. He joined ATLAS in 2007, affiliated first with Stockholm University and then with Oxford University and, in 2011, rejoined the Particle Physics Department (DPNC) at the University of Geneva as a research associate, becoming SNF Assistant Professor in 2014.
Philippe made several contributions to ATLAS. Among them, he pioneered the search for displaced heavy neutral leptons and led the effort on the search for highly-ionising particles in Run 2. He also made important contributions to the trigger system. His preferred topic was the search for magnetic monopoles, which he performed using various techniques in ATLAS, MoEDAL and other small experiments.
Always looking for more science, he also participated in the SHiP experiment. Moreover, he recently made significant contributions to the design and construction of the Time of Flight (ToF) detector for the near detector upgrade of the T2K collaboration in Japan; this is the first modern neutrino detector applying this technology.
We all followed with great interest the journey of this magnificent human being, passionate about physics, where he sought the answer to questions rather than personal promotion. Philippe was an intense scientist, curious to explore new paths, who devoted his attention and efforts to fundamental phenomena. He was also an active citizen, conscious of the need for fairness and sustainability if humanity was to have a future, whether he would see it himself or, alas, not. We will miss his energy, ideas and vision.
We would like to express our sympathies and heartfelt condolences to his wife and his family.
His colleagues and friends
The LHCb experiment at CERN has developed a penchant for finding exotic combinations of quarks, the elementary particles that come together to give us composite particles such as the more familiar proton and neutron. In particular, LHCb has observed several tetraquarks, which, as the name suggests, are made of four quarks (or rather two quarks and two antiquarks). Observing these unusual particles helps scientists advance our knowledge of the strong force, one of the four known fundamental forces in the universe. At a CERN seminar held virtually on 12 August, LHCb announced the first signs of an entirely new kind of tetraquark with a mass of 2.9 GeV/c²: the first such particle with only one charm quark.
First predicted to exist in 1964, scientists have observed six kinds of quarks (and their antiquark counterparts) in the laboratory: up, down, charm, strange, top and bottom. Since quarks cannot exist freely, they group to form composite particles: three quarks or three antiquarks form “baryons” like the proton, while a quark and an antiquark form “mesons”.
The LHCb detector at the Large Hadron Collider (LHC) is devoted to the study of B mesons, which contain either a bottom or an antibottom. Shortly after being produced in proton–proton collisions at the LHC, these heavy mesons transform – or “decay” – into a variety of lighter particles, which may undergo further transformations themselves. LHCb scientists observed signs of the new tetraquark in one such decay, in which the positively charged B meson transforms into a positive D meson, a negative D meson and a positive kaon: B+→D+D−K+. In total, they studied around 1300 candidates for this particular transformation in all the data the LHCb detector has recorded so far.
The well-established quark model predicts that some of the D+D− pairs in this transformation could be the result of intermediate particles – such as the ψ(3770) meson – that only manifest momentarily: B+→ψ(3770)K+→D+D−K+. However, theory does not predict meson-like intermediaries resulting in a D−K+ pair. LHCb were therefore surprised to see a clear band in their data corresponding to an intermediate state transforming into a D−K+ pair at a mass of around 2.9 GeV/c², or around three times the mass of a proton.
The band associated with the new tetraquark transforming into a D− and a K+ at a mass of 2.9 GeVc² (Image: LHCb Collaboration/CERN)The data have been interpreted as the first sign of a new exotic state of four quarks: an anticharm, an up, a down and an antistrange (c̄uds̄). All previous tetraquark-like states observed by LHCb always had a charm–anticharm pair, resulting in net-zero “charm flavour”. The newly observed state is the first time a tetraquark containing a sole charm has been seen, which has been dubbed an “open-charm” tetraquark.
“When we first saw the excess in our data, we thought there was a mistake,” says Dan Johnson, who led the LHCb analysis. “After years of analysing the data, we accepted that there really is something surprising!”
Why is this important? It so happens that the jury is still out as to what a tetraquark really is. Some theoretical models favour the notion that tetraquarks are pairs of distinct mesons bound together temporarily as a “molecule”, while other models prefer to think of them as a single cohesive unit of four particles. Identifying new kinds of tetraquarks and measuring their properties – such as their quantum spin (their intrinsic spatial orientation) and their parity (how they appear under a mirror-like transformation) – will help paint a clearer picture of these exotic inhabitants of the subatomic domain. Johnson adds: “This discovery will also allow us to stress-test our theories in an entirely new domain.”
While LHCb’s observation is an important first step, more data will be needed to verify the nature of the structure observed in the B+ decay. The LHCb collaboration will also anticipate independent verification of their discovery from other dedicated B-physics experiments such as Belle II. Meanwhile, the LHC continues to provide new and exciting results for experimentalists and theorists alike to dig into.
The Large Hadron Collider (LHC) is renowned for the hunt for and discovery of the Higgs boson, but in the 10 years since the machine collided protons at an energy higher than previously achieved at a particle accelerator, researchers have been using it to try to hunt down an equally exciting particle: the hypothetical particle that may make up an invisible form of matter called dark matter, which is five times more prevalent than ordinary matter and without which there would be no universe as we know it. The LHC dark-matter searches have so far come up empty handed, as have non-collider searches, but the incredible work and skill put by the LHC researchers into finding it has led them to narrow down many of the regions where the particle may lie hidden – necessary milestones on the path to a discovery.
“Before the LHC, the space of possibilities for dark matter was much wider than it is today”, says dark-matter theorist Tim Tait of UC Irvine and theory co-convener of the LHC Dark Matter Working Group. “The LHC has really broken new ground in the search for dark matter in the form of weakly interacting massive particles, by covering a wide array of potential signals predicted by either production of dark matter, or production of the particles mediating its interactions with ordinary matter. All of the observed results have been consistent with models that don’t include dark matter, and give us important information as to what kinds of particles can no longer explain it. The results have both pointed experimentalists in new directions for how to search for dark matter, and prompted theorists to rethink existing ideas for what dark matter could be – and in some cases to come up with new ones.”
During the International Conference on High-Energy Physics (ICHEP 2020), the ATLAS collaboration presented the first observation of photon collisions producing pairs of W bosons, elementary particles that carry the weak force, one of the four fundamental forces. The result demonstrates a new way of using the LHC, namely as a high-energy photon collider directly probing electroweak interactions. It confirms one of the main predictions of electroweak theory – that force carriers can interact with themselves – and provides new ways to probe it.
According to the laws of classical electrodynamics, two intersecting light beams would not deflect, absorb or disrupt one another. However, effects of quantum electrodynamics (QED), the theory that explains how light and matter interact, allow interactions among photons.
Indeed, it is not the first time that photons interacting at high energies have been studied at the LHC. For instance, light-by-light “scattering”, where a pair of photons interact by producing another pair of photons, is one of the oldest predictions of QED. The first direct evidence of light-by-light scattering was reported by ATLAS in 2017, exploiting the strong electromagnetic fields surrounding lead ions in high-energy lead–lead collisions. In 2019 and 2020, ATLAS further studied this process by measuring its properties.
The new result reported at this conference is sensitive to another rare phenomenon in which two photons interact to produce two W bosons of opposite electric charge via (among others) the interaction of four force carriers[1]. Quasi-real photons from the proton beams scatter off one another to produce a pair of W bosons. A first study of this phenomenon was previously reported by ATLAS and CMS in 2016, from data recorded during LHC Run 1, but a larger dataset was required to unambiguously observe it.
The observation was obtained with a highly significant statistical evidence of 8.4 standard deviations, corresponding to a negligible chance of being due to a statistical fluctuation. ATLAS physicists used a considerably larger dataset taken during Run 2, the four-year data collection in the LHC that ended in 2018, and developed a customised analysis method.
Owing to the nature of the interaction process, the only particle tracks visible in the central detector are the decay products of the two W bosons, an electron and a muon with opposite electric charge. W-boson pairs can also be directly produced from interactions between quarks and gluons in the colliding protons considerably more often than from photon–photon interactions, but these are accompanied by additional tracks from strong interaction processes. This means that the ATLAS physicists had to carefully disentangle collision tracks to observe this rare phenomenon.
This observation opens up a new facet of experimental exploration at the LHC using photons in the initial state”, said Karl Jakobs, spokesperson of the ATLAS collaboration. “It is unique as it only involves couplings among electroweak force carriers in the strong-interaction-dominated environment of the LHC. With larger future datasets it can be used to probe in a clean way the electroweak gauge structure and possible contributions of new physics.
Indeed, the new result confirms one of the main predictions of electroweak theory, namely that, besides interacting with ordinary particles of matter, the force carriers, also known as gauge bosons – the W bosons, the Z boson and the photon – are also interacting with each other. Photon collisions will provide a new way to test the Standard Model and to probe for new physics, which is necessary for a better understanding of our Universe.
Links, related articles & scientific material:
[1] The four force-carrier interaction is one of the predictions of the electroweak theory that explains how force-carrier particles, also known as gauge bosons, interact not only with matter particles, but also with one another.
The neutrino is the most ethereal of particles. Tens of billions of them emanating from nuclear reactions in the sun’s core pass through every square centimetre of Earth’s surface each second without notice. They have vanishingly small masses, a trillion times smaller than the top quark, and oscillate weirdly between their three flavours – electron, muon and tau – as they travel.
Since the first direct detection of a neutrino from a nuclear power plant in 1956, a vast and varied experimental programme employing reactor, solar, accelerator, atmospheric, cosmic and geological neutrino sources has grown up to explore its still-mysterious nature.
The latest issue of CERN Courier describes the state of the art in experimental neutrino physics, including recent results from the Tokai-to-Kamioka (T2K) facility in Japan that hint at differences in the way neutrinos and antineutrinos oscillate. It also celebrates the key role being played by Europe in contributing to a globally coordinated programme of neutrino research via the CERN Neutrino Platform.
Established in 2013, the CERN Neutrino Platform has enabled significant European participation in the US Long-Baseline Neutrino Facility, which will see neutrinos sent 1300 km from Fermilab in Chicago to the Deep Underground Neutrino Experiment (DUNE) in South Dakota, and in T2K, which sends neutrinos from Japan’s J-PARC accelerator facility to the Super-Kamiokande detector 295 km away. DUNE, T2K and its successor, the Hyper-Kamiokande project, will refine physicists’ understanding of neutrino oscillations, while a series of shorter baseline experiments are exploring the existence of a possible fourth, “sterile” neutrino.
For the US-based programme, the CERN Neutrino Platform has provided a large-scale demonstration of DUNE’s kilotonne-scale liquid-argon time-projection chambers (TPCs), with the construction and operation of two large-scale single- and dual-phase prototypes. The single-phase ProtoDUNE detector, which has recently completed two years of continuous recording of high-quality data, paves the way for the first DUNE module. At over 70 000 tonnes, the full DUNE detector will be the largest ever deployment of liquid-argon technology, which was first proposed by former CERN Director-General Carlo Rubbia in 1977 and serves as both target and tracker for neutrino interactions.
The first large-scale liquid-argon detector, ICARUS, has also been completely refurbished via the CERN Neutrino Platform. ICARUS was one of two detectors (along with OPERA) at Gran Sasso National Laboratory in Italy that studied neutrinos generated by CERN’s Super Proton Synchrotron (SPS) between 2006 and 2012. The refitted detector was shipped to the US in 2017 and is about to take data at Fermilab’s short-baseline neutrino facility.
For neutrino projects in Japan, the CERN Neutrino Platform has participated in the development of the BabyMIND magnetic spectrometer and upgrades to T2K’s “near-detector”, ND280. This detector, which was built inside the magnet from the UA1 experiment at CERN’s SPS, is crucial for understanding the neutrino flux prior to oscillations – one of the main measurement uncertainties at T2K and, in the future, at Hyper-Kamiokande. Independently, the SPS Heavy Ion and Neutrino Experiment (NA61/SHINE) at CERN has also contributed to a better understanding of T2K data, and has an important role to play in the future neutrino physics programmes in the US and Japan.
The 2020 update of the European strategy for particle physics, which was released on 19 June, recommends that Europe, and CERN through its neutrino platform, should continue to support neutrino projects in Japan and the US for the benefit of the worldwide neutrino community. “Experimental neutrino physics is back in town at CERN, and it looks like it is there to stay,” says Albert de Roeck, leader of the CERN EP-Neutrino group.
Geneva. At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.
Physicists at CERN have been studying the Higgs boson since its discovery in 2012 in order to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analysing how it decays into the various fundamental particles and the rate of disintegration.
CMS achieved evidence of this decay with 3 sigma, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s two-sigma result means the chances are one in 40. The combination of both results would increase the significance well above 3 sigma and provides strong evidence for the Higgs boson decay to two muons.
“CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.
The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.
“This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics programme. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.
What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimised to categorise physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.
The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.
LINKS
Scientific materials
Papers:
CMS physics analysis summary: https://cds.cern.ch/record/2725423
ATLAS paper on arXiv: https://arxiv.org/abs/2007.07830
Physics briefings:
CMS: https://cmsexperiment.web.cern.ch/news/cms-sees-evidence-higgs-boson-decaying-muons
ATLAS: https://atlas.cern/updates/physics-briefing/new-search-rare-higgs-decays-muons
Event displays and plots:
CMS: https://cds.cern.ch/record/2720665?ln=en
http://cds.cern.ch/record/2725728
ATLAS: https://cds.cern.ch/record/2725717?ln=en
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-14
Photos
CMS detector:
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https://cds.cern.ch/record/1431473/files/bul-pho-2007-079.jpg?subformat=icon-1440
ATLAS detector: https://mediastream.cern.ch/MediaArchive/Photo/Public/2007/0706038/0706038_02/0706038_02-A4-at-144-dpi.jpg
https://mediastream.cern.ch/MediaArchive/Photo/Public/2007/0705021/0705021_01/0705021_01-A4-at-144-dpi.jpg
CMS muon system:
https://cds.cern.ch/record/2016944/files/IMG_0267.jpg?subformat=icon-1440
https://cds.cern.ch/record/1431505/files/DSC_1432.jpg?subformat=icon-1440
ATLAS muon spectrometer:
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A team of researchers using the ISOLDE nuclear-physics facility at CERN has measured for the first time the so-called electron affinity of the chemical element astatine, the rarest naturally occurring element on Earth. The result, described in a paper just published in Nature Communications, is important for both fundamental and applied research. As well as giving access to hitherto unknown properties of this element and allowing theoretical models to be tested, the finding is of practical interest because astatine is a promising candidate for the creation of chemical compounds for cancer treatment by targeted alpha therapy.
The electron affinity is the energy released when an electron is added to a neutral atom in the gas phase to form a negative ion. It is one of the most fundamental properties of a chemical element. Together with the ionization energy, the energy it takes to remove an electron from the atom, it defines several other traits of an element, such as its electronegativity – the ability of the element to attract shared electrons in chemical bonds between atoms.
Although astatine was discovered in the 1940s, knowledge of its properties has mostly been based on theoretical calculations or on extrapolation from the properties of its relatives in the periodic table; astatine is a member of the halogen family, which includes chlorine and iodine. This is because astatine is scarce on Earth, and the tiny amounts of the element that can be produced in the lab prevent the use of traditional techniques to measure its properties. One notable exception was a previous measurement at ISOLDE of the element’s ionization energy.
In the new ISOLDE study, astatine atoms were first produced along with other atoms by firing a high-energy beam of protons from the Proton Synchrotron Booster at a thorium target. The astatine atoms were then negatively ionized, and ions of the isotope 211At were extracted and sent to a special measurement device in which laser light of tunable energy was shone on the ions to measure the energy required to extract the extra electron of the 211At ion and turn the ion into a neutral atom.
From this measurement, the ISOLDE researchers obtained a value of 2.415 78 eV for the electron affinity of astatine. This value, which agrees with the value that the authors derived using state-of-the-art theoretical calculations, indicates that the electron affinity of astatine is the lowest of all halogens but is nonetheless greater than that of any other elements outside the halogen family that have been measured so far.
If that wasn’t enough the researchers went on to use the derived electron affinity and the previous measurement of the ionization energy to determine several other properties of astatine, such as its electronegativity.
These properties are relevant for studies investigating the possible use of 211At compounds in targeted alpha therapy, a treatment that delivers alpha radiation to cancer cells. Astatine 211At is an ideal source of alpha radiation but most of the 211At compounds under investigation suffer from the rapid release of 211At negative ions, which could damage healthy cells before the compounds reach the cancer cells.
“Our results could be used to improve our knowledge of this release reaction and the stability of the 211At compounds being considered for targeted alpha therapy,” says lead author of the study David Leimbach. “In addition, our findings pave the way to measurements of the electron affinity of elements heavier than astatine, potentially of the superheavy elements, which are produced one atom at a time.”
“With the present result, we conclude a 10-year research effort at ISOLDE to determine the fundamental properties of astatine, the ionization energy and the electron affinity, which together finally enabled us to derive the electronegativity of astatine,” adds Sebastian Rothe, lead author of the earlier ISOLDE study.
Physicists look for new physics phenomena in many ways. One is by observing and measuring processes that are predicted to be extremely rare and looking for differences between data and theoretical predictions. The NA62 detector – the 62nd experiment located in CERN’s North Area – is designed to observe with high precision one such process, in which a positively charged particle known as a kaon transforms into a positively charged pion and a neutrino–antineutrino pair (denoted by K+→π+νν). Yesterday, at the 40th International Conference on High Energy Physics, the NA62 collaboration reported recording 17 candidate events for this particular transformation in data they collected in 2018. By combining the data they collected in 2016 and 2017, NA62 can claim the first evidence for this ultra-rare process, with a statistical significance of three-and-a-half sigma (3.5σ).
Colliding particles – into other particle beams or into fixed targets – at sufficiently high energies can produce heavy, unstable particles, like the kaons sought by NA62. These heavy particles transform (or “decay”) almost instantaneously into lighter particles in various combinations. The Standard Model of particle physics predicts how often a given particle will undergo all possible transformations. In the case of the kaon, only around one in every ten billion are expected to transform into a pion and a neutrino–antineutrino pair, with an uncertainty of about 10%. It is thus one of the rarest processes that can be observed by physicists.
While CERN is famous for the Large Hadron Collider, other accelerators at the laboratory provide particle beams for smaller but highly specialised experiments. The NA62 detector gets its beam from the Super Proton Synchrotron (SPS). Proton beams from the SPS, with an energy of 450 gigaelectronvolts, slam into a fixed target made of beryllium located upstream of NA62. Nearly a billion secondary particles are produced each second as a result and race towards the detector. Of these particles, around 6% are positively charged kaons. The kaons enter the detector, where a dedicated device identifies them before they undergo transformation into lighter particles. The physicists therefore have to first count the kaons produced and identify which of them transformed into a pion and a neutrino–antineutrino pair. Since neutrinos and their antiparticle counterparts leave no trace in the NA62 detector, their presence has to be deduced by calculating the angles between the parent kaon and the daughter pion and by measuring their speed and direction of motion.
In 2018, the NA62 detector collected data for 217 days, at the expense of around a billion billion (1018) protons. By sifting through these data, the collaboration was able to identify 17 new events that fit the K+→π+νν profile, in addition to the first candidate event observed in data from 2016 and the two candidates from 2017. Combining these data allowed NA62 to experimentally determine that the rate at which kaons undergo this rare transformation is around one in ten billion, with an uncertainty of about 35%. The experimental value is compatible with the Standard Model’s prediction at the current level of precision.
This is an important milestone for the experiment. NA62 is now on track to reach the threshold of 5σ statistical significance to claim observation of the process. The detector will receive new batches of kaons when the SPS resumes operations in 2021, following the second long shutdown of CERN’s accelerator complex.
The best-known particle in the lepton family is the electron, a key building block of matter and central to our understanding of electricity. But the electron is not an only child. It has two heavier siblings, the muon and the tau lepton, and together they are known as the three lepton flavours. According to the Standard Model of particle physics, the only difference between the siblings should be their mass: the muon is about 200 times heavier than the electron, and the tau-lepton is about 17 times heavier than the muon. It is a remarkable feature of the Standard Model that each flavour is equally likely to interact with a W boson, which results from the so-called lepton flavour universality. Lepton flavour universality has been probed in different processes and energy regimes to high precision.
In a new study, described in a paper posted today on the arXiv and first presented at the LHCP 2020 conference, the ATLAS collaboration presents a precise measurement of lepton flavour universality using a brand-new technique.
ATLAS physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. “The LHC is a top-quark factory, and produced 100 million top-quark pairs during Run 2,” says Klaus Moenig, ATLAS Physics Coordinator. “This gave us a large unbiased sample of W bosons decaying to muons and tau leptons, which was essential for this high-precision measurement.”
They then measured the relative probability that the lepton resulting from a W-boson decay is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity, as the strength of the interaction with a W boson should be the same for a tau-lepton and a muon. But there has been tension about this ever since the 1990s when experiments at the Large Electron-Positron (LEP) collider measured R(τ/μ) to be 1.070 ± 0.026, deviating from the Standard Model expectation by 2.7 standard deviations.
The new ATLAS measurement gives a value of R(τ/μ) = 0.992 ± 0.013. This is the most precise measurement of the ratio to date, with an uncertainty half the size of that from the combination of LEP results. The ATLAS measurement is in agreement with the Standard Model expectation and suggests that the previous LEP discrepancy may be due to a fluctuation.
“The LHC was designed as a discovery machine for the Higgs boson and heavy new physics,” says ATLAS Spokesperson Karl Jakobs. “But this result further demonstrates that the ATLAS experiment is also capable of measurements at the precision frontier. Our capacity for these types of precision measurements will only improve as we take more data in Run 3 and beyond.”
Although it has survived this latest test, the principle of lepton flavour universality will not be completely out of the woods until the anomalies in B-meson decays recorded by the LHCb experiment have also been definitively probed.
Read more on the ATLAS website.
By: Ana Lopes
7 AUGUST, 2020 · Voir en français
Our fourth story in the LHC Physics at Ten series discusses the LHC’s hunt for the hypothetical particle that may make up dark matter
The Large Hadron Collider (LHC) is renowned for the hunt for and discovery of the Higgs boson, but in the 10 years since the machine collided protons at an energy higher than previously achieved at a particle accelerator, researchers have been using it to try to hunt down an equally exciting particle: the hypothetical particle that may make up an invisible form of matter called dark matter, which is five times more prevalent than ordinary matter and without which there would be no universe as we know it. The LHC dark-matter searches have so far come up empty handed, as have non-collider searches, but the incredible work and skill put by the LHC researchers into finding it has led them to narrow down many of the regions where the particle may lie hidden – necessary milestones on the path to a discovery.
“Before the LHC, the space of possibilities for dark matter was much wider than it is today,” says dark-matter theorist Tim Tait of UC Irvine and theory co-convener of the LHC Dark Matter Working Group.“The LHC has really broken new ground in the search for dark matter in the form of weakly interacting massive particles, by covering a wide array of potential signals predicted by either production of dark matter, or production of the particles mediating its interactions with ordinary matter. All of the observed results have been consistent with models that don’t include dark matter, and give us important information as to what kinds of particles can no longer explain it. The results have both pointed experimentalists in new directions for how to search for dark matter, and prompted theorists to rethink existing ideas for what dark matter could be – and in some cases to come up with new ones.”
Simulation of the dark-matter distribution in the universe. (V. Springel et al. 2005) Make it, break it and shake itTo look for dark matter, experiments essentially “make it, break it or shake it”. The LHC has been trying to make it by colliding beams of protons. Some experiments are using telescopes in space and on the ground to look for indirect signals of dark-matter particles as they collide and break themselves out in space. Others still are chasing these elusive particles directly by searching for the kicks, or “shakes”, they give to atomic nuclei in underground detectors.
The make-it approach is complementary to the break-it and shake-it experiments, and if the LHC detects a potential dark-matter particle, it will require confirmation from the other experiments to prove that it is indeed a dark-matter particle. By contrast, if the direct and indirect experiments detect a signal from a dark-matter particle interaction, experiments at the LHC could be designed to study the details of such an interaction.
Missing-momentum signal and bump hunting An ATLAS detector event with missing transverse momentum. A photon with transverse momentum of 265 GeV (yellow bar) is balanced by 268 GeV of missing transverse momentum (red dashed line on the opposite side of the detector). (Image: ATLAS/CERN)So how has the LHC been looking for signs of dark-matter production in proton collisions? The main signature of the presence of a dark-matter particle in such collisions is the so-called missing transverse momentum. To look for this signature, researchers add up the momenta of the particles that the LHC detectors can see – more precisely the momenta at right angles to the colliding beams of protons – and identify any missing momentum needed to reach the total momentum before the collision. The total momentum should be zero because the protons travel along the direction of the beams before they collide. But if the total momentum after the collision is not zero, the missing momentum needed to make it zero could have been carried away by an undetected dark-matter particle.
Missing momentum is the basis for two main types of search at the LHC. One type is guided by so-called complete new physics models, such as supersymmetry (SUSY) models. In SUSY models, the known particles described by the Standard Model of particle physics have a supersymmetric partner particle with a quantum property called spin that differs from that of its counterpart by half of a unit. In addition, in many SUSY models, the lightest supersymmetric particle is a weakly interacting massive particle (WIMP). WIMPs are one of the most captivating candidates for a dark-matter particle because they could generate the current abundance of dark matter in the cosmos. Searches targeting SUSY WIMPs look for missing momentum from a pair of dark-matter particles plus a spray, or “jet”, of particles and/or particles called leptons.
Another type of search involving the missing-momentum signature is guided by simplified models that include a WIMP-like dark-matter particle and a mediator particle that would interact with the known ordinary particles. The mediator can be either a known particle, such as the Z boson or the Higgs boson, or an unknown particle. These models have gained significant traction in recent years because they are very simple yet general in nature (complete models are specific and thus narrower in scope) and they can be used as benchmarks for comparisons between results from the LHC and from non-collider dark-matter experiments. In addition to missing momentum from a pair of dark-matter particles, this second type of search looks for at least one highly energetic object such as a jet of particles or a photon.
In the context of simplified models, there’s an alternative to missing-momentum searches, which is to look not for the dark-matter particle but for the mediator particle through its transformation, or “decay”, into ordinary particles. This approach looks for a bump over a smooth background of events in the collision data, such as a bump in the mass distribution of events with two jets or two leptons.
Narrowing down the WIMP territoryWhat results have the LHC experiments achieved from these WIMP searches? The short answer is that they haven’t yet found signs of WIMP dark matter. The longer answer is that they have ruled out large chunks of the theoretical WIMP territory and put strong limits on the allowed values of the properties of both the dark-matter particle and the mediator particle, such as their masses and interaction strengths with other particles. Summarising the results from the LHC experiments, ATLAS experiment collaboration member Caterina Doglioni says “We have completed a large number of dedicated searches for invisible particles and visible particles that would occur in processes involving dark matter, and we have interpreted the results of these searches in terms of many different WIMP dark-matter scenarios, from simplified models to SUSY models. This work benefitted from the collaboration between experimentalists and theorists, for example on discussion platforms such as the LHC Dark Matter Working Group (LHC DM WG), which includes theorists and representatives from the ATLAS, CMS and LHCb collaborations. Placing the LHC results in the context of the global WIMP search that includes direct- and indirect-detection experiments has also been a focus of discussion in the dark-matter community, and the discussion continues to date on how to best exploit synergies between different experiments that have the same scientific goal of finding dark matter.”
Giving a specific example of a result obtained with data from the ATLAS experiment, Priscilla Pani, ATLAS experiment co-convener of the LHC Dark Matter WG, highlights how the collaboration has recently searched the full LHC dataset from the machine’s second run (Run 2), collected between 2015 and 2018, to look for instances in which the Higgs boson might decay into dark-matter particles. “We found no instances of this decay but we were able to set the strongest limits to date on the likelihood that it occurs,” says Pani.
Phil Harris, CMS experiment co-convener of the LHC Dark Matter Working Group, highlights searches for a dark-matter mediator decaying into two jets, such as a recent CMS search based on Run 2 data.
“These so-called dijet searches are very powerful because they can probe a large range of mediator masses and interaction strengths,” says Harris.Xabier Cid Vidal, LHCb experiment co-convener of the LHC Dark Matter WG, in turn notes how data from Run 1 and Run 2 on the decay of a particle known as the Bs meson has allowed the LHCb collaboration to place strong limits on SUSY models that include WIMPs. “The decay of the Bs meson into two muons is very sensitive to SUSY particles, such as SUSY WIMPs, because the frequency with which the decay occurs can be very different from that predicted by the Standard Model if SUSY particles, even if their masses are too high to be directly detected at the LHC, interfere with the decay,” says Cid Vidal.
Casting a wider net“10 years ago, experiments (at the LHC and beyond) were searching for dark-matter particles with masses above the proton mass (1 GeV) and below a few TeV. That is, they were targeting classical WIMPs such as those predicted by SUSY. Fast forward 10 years and dark-matter experiments are now searching for WIMP-like particles with masses as low as around 1 MeV and as high as 100 TeV,” says Tait. “And the null results from searches, such as at the LHC, have inspired many other possible explanations for the nature of dark matter, from fuzzy dark matter made of particles with masses as low as 10−22 eV to primordial black holes with masses equivalent to several suns. In light of this, the dark-matter community has begun to cast a wider net to explore a larger landscape of possibilities.”
The possible explanations for the nature of dark matter. (Image: G. Bertone and T. M. P. Tait)On the collider front, the LHC researchers have begun to investigate some of these new possibilities. For example, they have started looking at the hypothesis that dark matter is part of a larger dark sector with several new types of dark particles. These dark-sector particles could include a dark-matter equivalent of the photon, the dark photon, which would interact with the other dark-sector particles as well as the known particles, and long-lived particles, which are also predicted by SUSY models.
“Dark-sector scenarios provide a new set of experimental signatures, and this is a new playground for LHC physicists,” says Doglioni.“We are now expanding upon the experimental methods that we are familiar with, so we can try to catch rare and unusual signals buried in large backgrounds. Moreover, many other current and planned experiments are also targeting dark sectors and particles interacting more feebly than WIMPs. Some of these experiments, such as the newly approved FASER experiment, are sharing knowledge, technology and even accelerator complex with the main LHC experiments, and they will complement the reach of LHC searches for non-WIMP dark matter, as shown by the CERN Physics Beyond Colliders initiative.”
Finally, the LHC researchers are still working on data from Run 2, and the data gathered so far, from Run 1 and Run 2, is only about 5% of the total that the experiments will record. Given this, as well as the immense knowledge gained from the many LHC analyses thus far conducted, there’s perhaps a fighting chance that the LHC will discover a dark-matter particle in the next 10 years. “It’s the fact we haven’t found it yet and the possibility that we may find it in the not-so-distant future that keeps me excited about my job,” says Harris. “The last 10 years have shown us that dark matter might be different from what we had initially thought, but that doesn’t mean it is not there for us to find,” says Cid Vidal.
“We will leave no stone unturned, no matter how big or small and how long it will take us,” says Pani.Further reading:
A new era in the search for dark matter
Searching for Dark Matter with the ATLAS detector
Hero header image: NASA, ESA, H.Teplitz and M.Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (ASU), Z. Levay (STScI)
Don't miss the next articles of our series, which will cover the Standard Model, the early universe and more.
Our fourth story in the LHC Physics at Ten series discusses the LHC’s hunt for the hypothetical particle that may make up dark matter“For me, it’s an incredible thing that it happened in my lifetime!”
Peter Higgs was at a loss for words. The CMS and ATLAS collaborations had just announced the discovery of a new, Higgs-boson-like particle at the Large Hadron Collider.
It had been 48 years since the publication of his paper that first predicted the existence of the particle that bears his name, not long after Robert Brout and François Englert proposed a new mechanism that would give mass to elementary bosons. More than 30 years had elapsed since the LHC was first conceived and around 20 years since the ATLAS and CMS collaborations were formed. After those long years filled with anticipation, it only took the Swedish Academy of Sciences a little over one year to award Englert and Higgs the 2013 Nobel Prize in Physics.
For Peter Higgs, the discovery of the Higgs boson was the end of a remarkable journey. For particle physics, it was the beginning of a new one.
All known elementary particles have electric charges that are integer multiples of a third of the electron charge. But some theories predict the existence of “millicharged” elementary particles that would have a charge much smaller that the electron charge and could account for the elusive dark matter that fills the universe. An international team of researchers has now reported the first search at the Large Hadron Collider (LHC) – and more generally at any hadron collider – for elementary particles with charges smaller than a tenth of the electron charge.
Many previous studies have tried and failed to find millicharged particles, both directly, at collider and non-collider experiments, and indirectly, using astronomical observations. But millicharged particles with masses between about 1 billion electron volts (GeV) and 100 GeV remain largely unexplored owing to the lack of sensitivity of current detectors to such particles.
This is where a proposed detector called milliQan could make a difference. The detector would be sensitive to 1–100 GeV millicharged particles produced in proton–proton collisions at the LHC, through the flash of light created in its interior by the passage of such a particle. The detector has yet to be approved, and if approved then built, but a demonstrator detector that is a mere 1% of the full detector and was installed at the LHC in 2017 and gathered data in 2018 has now delivered promising results.
The data taken by the milliQan demonstrator rule out the existence of millicharged particles with masses between 20 and 4700 MeV for charges varying between 0.006 and 0.3 times the electron charge, depending on the mass. The results are consistent with those previously obtained by other experiments and represent a hadron collider’s first venture into the territory of particles with a charge smaller than 0.1 times the electron charge.
“We are very pleased by these results from the demonstrator. It has certainly achieved the original goal of providing feedback on our design and giving us experience with its operation, but to demonstrate that with only a 1% prototype we were already able to place new constraints on the properties of millicharged particles was a nice bonus. We are now quite confident that the full-scale milliQan detector will perform as expected, and we look forward to securing the funding to make this happen,” says Chris Hill, co-spokesperson of the milliQan collaboration.
Claude Détraz was born on 20 March 1938 in Albi, in the south of France. He graduated from the École Normale Supérieure and began his career at CNRS in 1962 as a researcher studying atomic nuclei.
Détraz then joined the Institut de Physique Nucléaire d’Orsay, founded by Irène and Fréderic Joliot Curie, which has now been merged with its neighbouring laboratories in Orsay to form the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJCLab).
At CERN’s Proton Synchrotron (PS), in collaboration with Robert Klapisch’s team, he contributed to the discovery of the first evidence of deformation in exotic nuclei at a shell closure. Drawing on these results, he became convinced that the beams at GANIL could also become a unique tool in this field.
Détraz was a great scientist and a true visionary, who played a major role in nuclear and particle physics in France and Europe. As the Director of GANIL (the Grand Accélérateur National d’Ions Lourds in Caen) from 1982 to 1990, he launched several research projects on exotic nuclei. The legacy of these projects is still with us today and will continue into the future. He was one of the main founders of NuPECC (the Nuclear Physics Collaboration Committee) and was its first Chair from 1989 to 1992, cementing its position as the main coordinating committee for nuclear physics in Europe.
In 1991, Claude Détraz became a technical adviser in the office of the French Minister for Research, Hubert Curien, who was later the President of the CERN Council at the time of the LHC's approval in 1994. Through his involvement with decision-making bodies at all levels in France, Détraz made a major contribution to ensuring that the LHC project was approved. For example, he played a key role in Hubert Curien’s appointment as the President of the CERN Council, a position from which he was able to exert a major influence in the final phases of the decision.
As the Director of IN2P3 (Institut National de Physique Nucléaire et des Particules at CNRS) from 1992 to 1998, he helped to give the impetus, first with Robert Aymar and then with Catherine Cesarsky of the CEA, to France’s wholehearted participation in the LHC adventure. His involvement was essential in ensuring that France and its institutes played a leading role in the project.
In 1999, Luciano Maiani, CERN Director-General at that time, appointed him Director of Research, jointly with Roger Cashmore, until 2003. This was a period filled with important events for CERN, including the shutdown of LEP, the excavation of new caverns for the LHC and the start of a project to send neutrinos from CERN to the underground laboratory at Gran Sasso, to which Claude contributed substantially.
Throughout his career, Détraz promoted and supported interaction between scientific disciplines. As a nuclear physicist, he established strong links with particle physics. He was also one of the architects of the emergence of astroparticle physics, a discipline connecting the two infinities.
He received multiple honours both in France (commander of the Order of Merit, SFP Joliot-Curie prize, CNRS silver medal) and abroad (Gay Lussac-Humboldt prize from the Humboldt Foundation and an honoris causa doctorate from JINR Dubna).
I knew Claude Détraz throughout his time at GANIL, IN2P3 and CERN, and even afterwards. As well as being a brilliant scientist and occupying several high-level positions, he was a true “Enlightenment man” whom I appreciated for his commitment, efficiency, foresight and humanity. A man of great culture and finesse, he expressed himself in an elegant, convincing and moving way. His passing is a great loss that greatly saddens me. He was a shining light of our generation.
Michel Spiro, President of the International Union of Pure and Applied Physics, Chair of the CERN and Society Foundation Board, former Director of IN2P3 and former President of the CERN Council
The LHCb collaboration has observed a type of four-quark particle never seen before. The discovery, presented at a recent seminar at CERN and described in a paper posted today on the arXiv preprint server, is likely to be the first of a previously undiscovered class of particles.
The finding will help physicists better understand the complex ways in which quarks bind themselves together into composite particles such as the ubiquitous protons and neutrons that are found inside atomic nuclei.
Quarks typically combine together in groups of twos and threes to form particles called hadrons. For decades, however, theorists have predicted the existence of four-quark and five-quark hadrons, which are sometimes described as tetraquarks and pentaquarks, and in recent years experiments including the LHCb have confirmed the existence of several of these exotic hadrons. These particles made of unusual combinations of quarks are an ideal “laboratory” for studying one of the four known fundamental forces of nature, the strong interaction that binds protons, neutrons and the atomic nuclei that make up matter. Detailed knowledge of the strong interaction is also essential for determining whether new, unexpected processes are a sign of new physics or just standard physics.
“Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type, specifically two charm quarks and two charm antiquarks,” says the outgoing spokesperson of the LHCb collaboration, Giovanni Passaleva. “Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type.”
“These exotic heavy particles provide extreme and yet theoretically fairly simple cases with which to test models that can then be used to explain the nature of ordinary matter particles, like protons or neutrons. It is therefore very exciting to see them appear in collisions at the LHC for the first time,” explains the incoming LHCb spokesperson, Chris Parkes.
The LHCb team found the new tetraquark using the particle-hunting technique of looking for an excess of collision events, known as a “bump”, over a smooth background of events. Sifting through the full LHCb datasets from the first and second runs of the Large Hadron Collider, which took place from 2009 to 2013 and from 2015 to 2018 respectively, the researchers detected a bump in the mass distribution of a pair of J/ψ particles, which consist of a charm quark and a charm antiquark. The bump has a statistical significance of more than five standard deviations, the usual threshold for claiming the discovery of a new particle, and it corresponds to a mass at which particles composed of four charm quarks are predicted to exist.
As with previous tetraquark discoveries, it is not completely clear whether the new particle is a “true tetraquark”, that is, a system of four quarks tightly bound together, or a pair of two-quark particles weakly bound in a molecule-like structure. Either way, the new tetraquark will help theorists test models of quantum chromodynamics, the theory of the strong interaction.
Read more on the LHCb website.
There is strong evidence that dark matter exists and permeates the cosmos, yet all searches for the hypothetical particles that may make up this invisible form of matter have drawn a blank so far. In light of these null results, researchers have started to spread a wider net in their searches, exploring as many types of particle as possible, new regions in which the particles may lie hidden and new ways to probe them. The NA64 experiment collaboration has now widened the scope of its searches with a search for axions and axion-like particles – hypothetical particles that could mediate an interaction between dark matter and visible matter or comprise dark matter itself, depending on their exact properties.
The NA64 team targeted an unexplored area for axions and axion-like particles, a gap in the two-dimensional area of possible values of their mass and interaction strength with a pair of photons. This gap doesn’t include the regions where axions and axion-like particles could make up dark matter, but it includes an area where axions could explain the long-puzzling symmetry properties of the strong force, for which axions were originally proposed, as well as an area where axion-like particles could mediate an interaction between dark matter and visible matter.
To explore this gap, the NA64 team used an electron beam of 100 GeV energy from the Super Proton Synchrotron and directed it onto a fixed target. They then searched for axions and axion-like particles that would be produced in interactions between high-energy photons generated by the 100 GeV electrons in the target and virtual photons from the target’s atomic nuclei. The researchers looked for the particles both through their transformation, or “decay”, into a pair or photons in a detector placed right after the target or through the “missing energy” that the particles would carry away if they decayed downstream of the detector.
The NA64 team analysed data that was collected over the course of three years, between 2016 and 2018. Together, these data corresponded to some three hundred billion electrons hitting the target. The NA64 researchers found no sign of axions and axion-like particles in this dataset, but the null result allowed them to set limits on the allowed values of the interaction strength of axions and axion-like particles with two photons for particle masses below 55 MeV.
“We’re very excited to have added NA64 to the list of experiments that are hunting for axions as well as axion-like particles, which are a popular candidate for a mediator of a new force between visible and dark matter”, says NA64 collaboration spokesperson Sergei Gninenko. “Little by little, and together, these experiments are narrowing down the regions of where to look for, and perhaps find, these particles.”
Following almost two years of discussion and deliberation, the CERN Council today announced that it has updated the strategy that will guide the future of particle physics in Europe within the global particle-physics landscape. Presented during the open part of the Council’s meeting, held remotely due to the ongoing COVID-19 pandemic, the recommendations highlight the scientific impact of particle physics, as well as its technological, societal and human capital.
By probing ever-higher energy and thus smaller distance scales, particle physics has made discoveries that have transformed the scientific understanding of the world. Nevertheless, many of the mysteries about the universe, such as the nature of dark matter, and the preponderance of matter over antimatter, are still to be explored. The 2020 update of the European Strategy for Particle Physics proposes a vision for both the near- and the long-term future of the field, which maintains Europe's leading role in addressing the outstanding questions in particle physics and in the innovative technologies being developed within the field.
The highest scientific priorities identified in this update are the study of the Higgs boson - a unique particle that raises scientific profound questions about the fundamental laws of nature - and the exploration of the high-energy frontier. These are two crucial and complementary ways to address the open questions in particle physics.
“The Strategy is above all driven by science and thus presents the scientific priorities for the field,” says Ursula Bassler, President of the CERN Council. “The European Strategy Group (ESG) – a special body set up by the Council – successfully led a strategic reflection to which several hundred European physicists contributed.” The scientific vision outlined in the Strategy should serve as a guideline to CERN and facilitate a coherent science policy across Europe.
The successful completion of the High-Luminosity LHC in the coming decade, for which upgrade work is currently in progress at CERN, should remain the focal point of European particle physics. The strategy emphasises the importance of ramping up research and development (R&D) for advanced accelerator, detector and computing technologies, as a necessary prerequisite for all future projects. Delivering the near and long-term future research programme envisaged in this Strategy update requires both focused and transformational R&D, which also has many potential benefits to society.
The document also highlights the need to pursue an electron-positron collider acting as a “Higgs factory” as the highest-priority facility after the Large Hadron Collider (LHC). The Higgs boson was discovered at CERN in 2012 by scientists working on the LHC, and is expected to be a powerful tool to look for physics beyond the Standard Model. Such a machine would produce copious amounts of Higgs bosons in a very clean environment, would make dramatic progress in mapping the diverse interactions of the Higgs boson with other particles and would form an essential part of a rich research programme, allowing measurements of extremely high precision. Construction of this future collider at CERN could begin within a timescale of less than 10 years after the full exploitation of the High-Luminosity LHC, which is expected to complete operations in 2038.
The exploration of significantly higher energies than the LHC will allow new discoveries to be made and the answers to existing mysteries, such as the nature of dark matter, to potentially be found. In acknowledgement of the fact that the particle physics community is ready to prepare for the next step towards even higher energies and smaller scales, another significant recommendation of the Strategy is that Europe, in collaboration with the worldwide community, should undertake a technical and financial feasibility study for a next-generation hadron collider at the highest achievable energy, with a view to the longer term.
It is further recommended that Europe continue to support neutrino projects in Japan and the US. Cooperation with neighbouring fields is also important, such as astroparticle and nuclear physics, as well as continued collaboration with non-European countries.
“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach. We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s Member States and beyond,” declares CERN Director-General Fabiola Gianotti. “These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”
“The natural next step is to explore the feasibility of the high-priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains ESG chair Halina Abramowicz. “Europe should keep the door open to participating in other headline projects that will serve the field as a whole, such as the proposed International Linear Collider project.”
Beyond the immediate scientific return, major research infrastructures such as CERN have broad societal impact, thanks to their technological, economic and human capital. Advances in accelerators, detectors and computing have a significant impact on areas like medical and biomedical technologies, aerospace applications, cultural heritage, artificial intelligence, energy, big data and robotics. Partnerships with large research infrastructures help drive innovation in industry. In terms of human capital, the training of early-career scientists, engineers, technicians and professionals provides a talent pool for industry and other fields of society.
The Strategy also highlights two other essential aspects: the environment and the importance of Open Science. “The environmental impact of particle physics activities should continue to be carefully studied and minimised. A detailed plan for the minimisation of environmental impact and for the saving and reuse of energy should be part of the approval process for any major project,” says the report. The technologies developed in particle physics to minimise the environmental impact of future facilities may also find more general applications in environmental protection.
The update of the European Strategy for Particle Physics announced today got under way in September 2018, when the CERN Council, comprising representatives from CERN’s Member and Associate Member States, established a European Strategy Group (ESG) to coordinate the process. The ESG worked in close consultation with the scientific community. Nearly two hundred submissions were discussed during an Open Symposium in Granada in May 2019 and distilled into the Physics Briefing Book, a scientific summary of the community’s input, prepared by the Physics Preparatory Group. The ESG converged on the final recommendations during a week-long drafting session held in Germany in January 2020. The group’s findings were presented to the CERN Council in March and were scheduled to be announced on 25 May, in Budapest. This was delayed due to the global Covid-19 situation but they have now been made publicly available.
For more information, consult the documents of the Update of the European Strategy for Particle Physics:
Geneva, 19 June 2020. Today, the CERN Council announced that it had unanimously updated the strategy intended to guide the future of particle physics in Europe within the global landscape (the document is available here). The updated recommendations highlight the scientific impact of particle physics and its technological, societal and human capital.
The 2020 update of the European Strategy for Particle Physics proposes a vision for both the near- and the long-term future of the field, which maintains Europe's leading role in particle physics and in the innovative technologies developed within the field.
The highest-priority physics recommendations are the study of the Higgs boson and the exploration of the high-energy frontier: two crucial and complementary ways to address the open questions in particle physics.
“The Strategy is above all driven by science and thus presents the scientific priorities for the field,” said Ursula Bassler, President of the CERN Council. “The European Strategy Group (ESG) – a special body set up by the Council – successfully led a strategic reflection to which several hundred European physicists contributed.” The scientific vision outlined in the Strategy should serve as a guideline to CERN and facilitate a coherent science policy across Europe.
The successful completion of the High-Luminosity LHC in the coming years, for which upgrade work is currently in progress at CERN, should remain the focal point of European particle physics.
The Strategy emphasises the importance of ramping up research and development (R&D) for advanced accelerator, detector and computing technologies as a necessary prerequisite for all future projects. Delivering the near and long-term future research programme envisaged in this Strategy update requires both focused and transformational R&D, which also has many potential benefits to society.
The document also highlights the need to pursue an “electron-positron Higgs factory” as the highest-priority facility after the Large Hadron Collider (LHC). Construction of this future collider at CERN could start within a timescale of less than 10 years after the full exploitation of the High-Luminosity LHC, which is expected to complete operations in 2038. The electron-positron collider would allow the properties of the Higgs boson to be measured with extremely high precision. The Higgs boson was discovered at CERN in 2012 by scientists working on the LHC, and is expected to be a powerful tool in the search for physics beyond the Standard Model.
Another significant recommendation of the Strategy is that Europe, in collaboration with the worldwide community, should undertake a feasibility study for a next-generation hadron collider at the highest achievable energy, in preparation for the longer-term scientific goal of exploring the high-energy frontier, with an electron-positron collider as a possible first stage.
It is further recommended that Europe continue to support neutrino projects in Japan and the US. Cooperation with neighbouring fields is also important, such as astroparticle and nuclear physics, as well as continued collaboration with non-European countries.
“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach. We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s Member States and beyond,” declared CERN Director-General Fabiola Gianotti. “These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”
“The natural next step is to explore the feasibility of the high-priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains ESG Chair Halina Abramowicz. “Europe should keep the door open to participating in other headline projects that will serve the field as a whole, such as the proposed International Linear Collider project.”
Beyond the immediate scientific return, major research infrastructures such as CERN have vast societal impact, thanks to their technological, economic and human capital. Advances in accelerators, detectors and computing have a significant impact on areas like medical and biomedical technologies, aerospace applications, cultural heritage, artificial intelligence, energy, big data and robotics. Partnerships with large research infrastructures help drive innovation in industry.
In terms of human capital, the training of early-career scientists, engineers, technicians and professionals from diverse backgrounds is an essential part of high-energy physics programmes, which provide a talent pool for industry and other fields.
The Strategy also highlights two other essential aspects: the environment and the importance of Open Science. “The environmental impact of particle physics activities should continue to be carefully studied and minimised. A detailed plan for the minimisation of environmental impact and for the saving and reuse of energy should be part of the approval process for any major project,” says the report. The technologies developed in particle physics to minimise the environmental impact of future facilities may also find more general applications in environmental protection.
The update of the European Strategy for Particle Physics announced today got under way in September 2018 when the CERN Council, comprising representatives from CERN’s Member and Associate Member States, established a European Strategy Group (ESG) to coordinate the process. The ESG worked in close consultation with the scientific community. Nearly two hundred submissions were discussed during an Open Symposium in Granada in May 2019 and distilled into the Physics Briefing Book, a scientific summary of the community’s input, prepared by the Physics Preparatory Group. The ESG converged on the final recommendations during a week-long drafting session held in Germany in January 2020. The group’s findings were presented to the CERN Council in March and were scheduled to be announced on 25 May, in Budapest. This was delayed due to the global Covid-19 situation but they have now been made publicly available.
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference was held entirely online due to the COVID-19 pandemic.
At the LHCP conference this year, the ATLAS and CMS collaborations presented new results relating to a physics process called vector boson scattering. CMS also reported the first observation of the so-called “massive triboson production. Studying these processes to test the Standard Model is important as it could shed light on new physics. The results were presented online at the virtual LHCP conference, originally due to be held in Paris.
During proton collisions at the LHC, many particles, including the carriers of the electroweak force – photons and W and Z bosons – are produced. These bosons are often referred to simply as vector bosons, in the Standard Model, and one of the processes that leads to their pair production is called vector boson scattering.
Vector boson processes are an excellent probe to seek deviation from theoretical predictions. Two rare processes that are of particular interest as they probe the self-interactions of four vector bosons are diboson production via vector boson scattering and triboson production”. The observation and measurement of these processes are important as they test the electroweak symmetry breaking mechanism, whereby the unified electroweak force separates into electromagnetic and weak forces in the Standard Model, and are complementary to the measurements of Higgs boson production and decay.
In a vector boson scattering process, a vector boson is radiated from a quark in each proton and these vector bosons scatter off one another to produce a diboson final state. Triboson production refers instead to the production of three massive vector bosons.
At the LHCP conference, physicists from the ATLAS and CMS collaborations presented new searches for the production of a pair of Z bosons via electroweak production including the vector boson scattering mechanism. ATLAS observed this process at 5.5 sigma and CMS reported strong evidence. CMS also reported the first observation of a W boson produced in association with a photon through the vector boson scattering process, as well as more precise measurements of the same-sign WW production, and an observation of the vector boson scattering production of a W and a Z boson, complementing earlier ATLAS observations.
Another way to probe four-boson interaction is to study the very rare production of three massive bosons or tribosons. This April, the CMS experiment released a 5.7 sigma result of the triboson phenomenon, establishing it as a firm observation, following the first evidence of this process seen by the ATLAS experiment last year.
Most physics processes of fundamental particles involve two or more individual particles that interact with each other via an intermediary particle that is emitted or absorbed in the process.
“The more bosons produced, the rarer the event. This new observation of tribosons was very difficult because it is a much rarer process than the one that led to the Higgs boson discovery, and very interesting because it may reveal signs of new particles and anomalous interactions,” says Roberto Carlin, CMS spokesperson.
In the triboson and vector boson scattering processes, W and Z can interact with themselves to create more W and Z particles, producing two or three bosons. W and Z being highly unstable particles, they quickly decay into leptons (electrons, muons, taus and their corresponding neutrinos) or quarks. But such processes are extremely rare and the diboson and triboson events that physicists look for are mimicked by background processes, making them even more difficult for physicists to analyse.
“To separate signal from background, physicists have to be ingenious and employ advanced machine learning algorithms. This is a challenging task for such rare processes, and requires meticulous and thorough studies,” says Karl Jakobs, ATLAS spokesperson.
The measurements of vector boson scattering and triboson production presented at LHCP 2020 are consistent with the predictions made by the Standard Model, which remains our best understanding of fundamental particles and their interactions. The above observations also provide physicists with tools to probe quartic self-interaction between massive electroweak bosons. The current measurements place constraints on the strength at which these quartic interactions take place and increased precision from the use of new datasets could open up horizons for new physics at higher energy scales in the LHC and lead to possible discoveries of new particles.
By: Achintya Rao
4 JULY, 2020 · Voir en français
Our third story in the LHC Physics at Ten series takes us on a deeper dive into the Higgs boson
(Image: CERN)“For me, it’s an incredible thing that it happened in my lifetime!”
Peter Higgs was at a loss for words. The CMS and ATLAS collaborations had just announced the discovery of a new, Higgs-boson-like particle at the Large Hadron Collider.
4 July 2012: François Englert (left) listens as Peter Higgs speaks, after ATLAS and CMS announce their discovery (Image: Maximilien Brice/CERN)It had been 48 years since the publication of his paper that first predicted the existence of the particle that bears his name, not long after Robert Brout and François Englert proposed a new mechanism that would give mass to elementary bosons. More than 30 years had elapsed since the LHC was first conceived and around 20 years since the ATLAS and CMS collaborations were formed. After those long years filled with anticipation, it only took the Swedish Academy of Sciences a little over one year to award Englert and Higgs the 2013 Nobel Prize in Physics.
For Peter Higgs, the discovery of the Higgs boson was the end of a remarkable journey. For particle physics, it was the beginning of a new one.
(Image: CERN)
Higgs-like? Higgs-ish? Higgs-y?
“When you find something new, you have to understand exactly what it is that you have found,” remarks Giacinto Piacquadio, one of the conveners of the ATLAS collaboration’s Higgs group
This understanding is built up gradually over time. Back in July 2012, physicists were cautious about calling the new particle a Higgs boson, let alone the Higgs boson predicted by the Standard Model of particle physics. And with good reason: while the simplest theoretical formulations required there to be only one kind of Higgs boson, some extensions of the Standard Model proposed that there could be as many as five kinds of bosons that are involved in the mass-giving mechanism. So for the first few months after the discovery, it was referred to as Higgs-like, shorthand for “a particle that seems to behave like the Higgs boson predicted by the Standard Model but we need more data to be sure”.
The identification of two quantum-mechanical properties of the particle – quantum spin and parity – gave credence to the Standard-Model interpretation. Spin is the intrinsic spatial orientation of quantum particles, and parity refers to whether the properties of the particle remain the same when some of its spatial coordinates are flipped, like comparing the particle with a hypothetical mirror image. In the Standard Model, the Higgs boson has no spin (“0”) and “even” parity. At the time of the discovery, the fact that the Higgs boson transformed into photons meant that – unlike all other elementary bosons we know – its spin could not be 1: photons have a quantum spin of 1 themselves, so a particle transforming into two photons would have a spin of 0 (with the two spins of the photon cancelling out) or 2 (if the two spins add up).
Differences between the positive- and negative-parity theoretical scenarios (solid and dashed lines respectively) for a particle with spin 0. The data do not show evidence for the negative-parity scenario (Image: ATLAS/CERN)In science, you can never know something with 100% certainty, but you can rule out things that are not likely. Because spin-2 particles or parity-odd particles with spin 0 would leave subtly different signatures in the ATLAS and CMS detectors than the spin-0-parity-even particle they were looking for, the scientists were eventually able to rule out these more exotic possibilities by examining many more collision events and finding no evidence to support them. “We had to analyse two-and-a-half-times more data to drop the ‘-like’,” Piacquadio adds. By March 2013, scientists were confident calling the particle a Higgs boson.
The Goldilocks zoneThe Higgs boson was the last missing piece in the Standard Model. Crucially, its mass would determine how it could be observed. At 125 gigaelectronvolts (GeV), it turned out to be just right for studying the particle at the Large Hadron Collider.
We can never directly see a Higgs boson. Like most kinds of particle in nature, it is unstable and – immediately after being produced – transforms into lighter particles through a process known as particle decay. The ATLAS and CMS detectors can therefore see only the remnants of transformations, signatures that a Higgs boson might have been produced in the LHC’s collisions. Further, the downstream remnants of a Higgs transformation hold clues for how the particle was produced in the first place.
The Higgs boson’s mass was not predicted precisely by the Standard Model, but theorists knew that the processes that produced it and the kinds of particles it transformed into would depend on how heavy the boson actually was. They had prepared elaborate plots calculating the various probabilities for a Higgs boson of a given mass to transform into particular pairs of particles. According to these so-called “branching fractions”, a light Higgs boson of around 125 GeV would have the largest variety of transformation candidates that ATLAS and CMS could detect: pairs of W bosons, Z bosons, photons, bottom quarks, tau leptons and many others. The greater the variety of observable particles the Higgs can transform into, the greater the ability of scientists to study the interplay between these particles and the Higgs boson.
The rates at which a Higgs boson could undergo certain transformations (vertical axis) depending on its mass (horizontal axis) (Image: CERN)Although the Higgs field was conceived to explain the masses of the W and Z bosons, scientists realised that it could help account for the masses of the fermions, namely the particles of matter. If, due to its mass, they could only observe the interplay between the Higgs boson on one hand and the W and Z bosons on the other, the puzzle of the fermion masses would remain unsolved. Discovering the particle at a convenient mass was an unexpected kindness from nature. If it were slightly more massive, above 180 GeV or so, the options to study it at the time of its discovery would have been more limited.
The variety of available transformation products means that data from the individual channels can be combined together through sophisticated techniques to build up a greater understanding of the particle. “Doing so is not trivial,” says Giovanni Petrucciani, co-convener of the Higgs analysis group in CMS. “You have to treat the uncertainties similarly across all the individual analyses and interpret the results carefully, once you have applied complicated statistical machinery.” Combining data from the transformation of the Higgs boson to pairs of Z bosons and pairs of photons allowed ATLAS and CMS to discover the Higgs boson in 2012.
Photograph featured in the CERN courier article for issue 2019MarApr. Contains an image of ATLAS Higgs event, accompanied with a piece of event selection code of an CMS analysis reimplemented by theorists in open code CheckMATE. (Image: CERN)
Generation gaps
The LHC started operations at a collision energy of 7 teraelectronvolts (TeV) before ramping up to 8 TeV over the course of its first run (2010–2013). The data collected over this period not only led to the discovery of the Higgs boson but showed the relationship (“coupling”) between the Higgs boson and elementary bosons: it was observed transforming into pairs of Ws, Zs and photons. And, while transformations to gluons are impossible to observe, the scientists could probe this coupling through the Higgs production itself: the most abundant way for a Higgs to be created in proton–proton interactions is for two gluons – one from each proton – to fuse together, accounting for nearly 90% of Higgs bosons produced at the LHC.
A candidate for a Higgs boson transforming into two photons (Image: CMS/CERN)The next challenge was to observe the coupling to fermions, to cement the role of the Higgs field as the origin of mass of all elementary massive particles. These couplings had been probed indirectly: the Standard Model tells us that the gluon-fusion production mechanism and the Higgs transformation to photon pairs require the creation and annihilation of “virtual” top–antitop pairs. However, a direct observation of Higgs couplings to fermions was lacking.
Curiously, both kinds of fermions – quarks, which make up compound particles like protons, and leptons, like the familiar electron – come in three generations of particles, each heavier than the previous. And unlike bosons, whose coupling strengths to the Higgs are proportional to their masses, the Higgs-coupling strengths of fermions is proportional to the square of their masses.
The third generation of fermions – the heaviest – are therefore the most likely particles to manifest in processes involving the Higgs boson. “The connection between the Higgs and the top quark in particular is very exciting to look into,” remarks María Cepeda, Petrucciani’s fellow convener on CMS. Despite their relative abundance in such processes, these particles are challenging to identify. Since quarks cannot exist freely, two bottom quarks (a quark and an antiquark) emerging from a Higgs transformation rapidly combine with other quarks pulled out of the vacuum and form jets of particles. The experimentalists have to then tag jets of particles that carry the signature of a bottom quark, in order to isolate the signal. The top quark on the other hand is heavier than the Higgs and so a Higgs can never be observed transforming into two top quarks. Scientists have to therefore measure its coupling with the Higgs by looking for collision events in which a Higgs boson is produced in association with two top quarks. The second run of the LHC (2015–2018) was at an energy of 13 TeV and the large data volume collected allowed ATLAS and CMS to observe the interplay between the Higgs boson and the bottom quark, the top quark and the tau lepton.
A candidate for a Higgs boson transforming into a b-quark and a b-antiquark (Image: ATLAS/CERN)Couplings to the second generation of fermions are much weaker and neither ATLAS nor CMS have so far observed Higgs transformations into charm quarks, strange quarks or muons. The next run of the LHC (2021 onwards) is expected to provide enough data to begin to shed light on some of these interactions. “The LHC’s instantaneous luminosity – the rate at which it collides protons – has increased dramatically over its first two runs,” notes Piacquadio with excitement. “This means that the number of Higgs bosons produced by the LHC continues to rise, as do the odds that we observe them undergoing rarer transformations.”
But for the second generation of fermions, the LHC’s data volume over its whole operational life may not be enough to breach the 5σ statistical threshold to claim a Higgs transformation to all these particles. Although the High-Luminosity LHC, which will be the collider’s incarnation from 2026, is expected to allow ATLAS and CMS to see the Higgs transforming into pairs of muons, transformations to second-generation quarks will probably remain out of reach.
More data, more precisionThe Higgs boson holds the key to our understanding of nature beyond what is shown by the Standard Model.
ATLAS and CMS are, for example, looking for so-called “invisible decays” of the Higgs boson, in which it transforms into particles that the detectors cannot observe. These invisible particles might be manifestations of dark matter. And measurements of couplings that deviate from the theoretical predictions could provide an alternative explanation for the masses of the different generations of fermions, explaining why they exist in distinct generations to start with and possibly hinting at the existence of other Higgs bosons.
Yet, the Brout-Englert-Higgs mechanism remains among the least-understood phenomena in the Standard Model. Indeed, while scientists have dropped the “-like” suffix and have understood the Higgs boson remarkably since its discovery, they still do not know if what was observed is the Higgs boson predicted by the Standard Model. Couplings to the second-generation fermions remain elusive and the couplings that have been observed are known with an uncertainty of 10 to 20%, expected to reduce to the 2–4% range with the High-Luminosity LHC. Observation of as-yet-unseen phenomena and precision measurements of those that have been seen may require data volumes far greater than the LHC can provide over its lifetime.
The global particle-physics community is therefore keen on building a “Higgs factory”, a dedicated accelerator with a focus on producing Higgs bosons in unimaginably large quantities, to allow the continued exploration of this strange particle. A high-energy Higgs factory would also enable scientists to produce two Higgs bosons at a time, to address the question of the so-called “Higgs self-interaction”, the process through which the Higgs boson itself gains mass.
Since its discovery nearly eight years ago, ATLAS and CMS have published hundreds of papers on the Higgs boson and our understanding of the particle has grown incrementally but greatly. Today, we know with great precision what its mass is, what its most abundant transformation channels are and how it is produced in the first place. But a lot remains unknown, about both the Higgs boson and the quantum world in general.
The Higgs may be the most important discovery of the LHC so far, but there is much still to learn from this remarkable machine. Our next story in this series will take a look at searches for dark matter at the Large Hadron Collider.
Our third story in the LHC Physics at Ten series takes us on a deeper dive into the Higgs boson
The Hellenic Society for the Study of High Energy Physics (HeSSHEP) promotes the scientific work of the Greek scientists, informs the general public and the Greek state on matters concerning the subject of High Energy Physics, organizes an workshop and conferences.
