Isaac Newton's historic work on gravity was apparently inspired by watching an apple fall to the ground from a tree. But what about an “anti-apple” made of antimatter, would it fall in the same way if it existed? According to Albert Einstein’s much-tested theory of general relativity, the modern theory of gravity, antimatter and matter should fall to Earth in the same way. But do they, or are there other long-range forces beyond gravity that affect their free fall?
In a paper published today in Nature, the ALPHA collaboration at CERN’s Antimatter Factory shows that, within the precision of their experiment, atoms of antihydrogen – a positron orbiting an antiproton – fall to Earth in the same way as their matter equivalents.
“In physics, you don't really know something until you observe it,” says ALPHA spokesperson Jeffrey Hangst. “This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the Universe.”
Gravity is the attractive force between any two objects with mass. It is by far the weakest of the four fundamental forces of nature. Antihydrogen atoms are electrically neutral and stable particles of antimatter. These properties make them ideal systems in which to study the gravitational behaviour of antimatter.
The ALPHA collaboration creates antihydrogen atoms by taking negatively charged antiprotons, produced and slowed down in the Antimatter Factory’s AD and ELENA machines, and binding them with positively charged positrons accumulated from a sodium-22 source. It then confines the neutral – but slightly magnetic – antimatter atoms in a magnetic trap, which prevents them from coming into contact with matter and annihilating.
Until now, the team has concentrated on spectroscopic studies in the ALPHA-2 device, shining laser light or microwaves onto the antihydrogen atoms to measure their internal structure. But the ALPHA team has also built a vertical apparatus called ALPHA-g, which received its first antiprotons in 2018 and was commissioned in 2021. The ‘g’ denotes the local acceleration of gravity, which, for matter, is about 9.81 metres per second squared. This apparatus makes it possible to measure the vertical positions at which the antihydrogen atoms annihilate with matter once the trap’s magnetic field is switched off, allowing the atoms to escape.
This is exactly what the ALPHA researchers did in their new investigation, following a proof-of-principle experiment with the original ALPHA set-up in 2013. They trapped groups of about 100 antihydrogen atoms, one group at a time, and then slowly released the atoms over a period of 20 seconds by gradually ramping down the current in the top and bottom magnets of the trap. Computer simulations of the ALPHA-g set-up indicate that, for matter, this operation would result in about 20% of the atoms exiting through the top of the trap and 80% through the bottom, a difference caused by the downward force of gravity. By averaging the results of seven release trials, the ALPHA team found that the fractions of anti-atoms exiting through the top and bottom were in line with the results of the simulations.
The full study involved repeating the experiment several times for different values of an additional “bias” magnetic field, which could either enhance or counteract the force of gravity. By analysing the data from this “bias scan”, the team found that, within the precision of the current experiment (about 20% of g), the acceleration of an antihydrogen atom is consistent with the familiar, attractive gravitational force between matter and the Earth.
“It has taken us 30 years to learn how to make this anti-atom, to hold on to it, and to control it well enough that we could actually drop it in a way that it would be sensitive to the force of gravity,” says Hangst. “The next step is to measure the acceleration as precisely as we can,” continues Hangst. “We want to test whether matter and antimatter do indeed fall in the same way. Laser-cooling of antihydrogen atoms, which we first demonstrated in ALPHA-2 and will implement in ALPHA-g when we return to it in 2024, is expected to have a significant impact on the precision.”
CERN’s Antimatter Factory is a unique facility in the world for producing and studying antimatter. Two other experiments at this facility, AEgIS and GBAR, share with ALPHA the goal of measuring with high precision the gravitational acceleration of atomic antimatter. Also at the Antimatter Factory is the BASE experiment. Its main focus is to compare with high precision the properties of the proton with those of its antimatter twin, and it has recently compared the gravitational behaviour of these two particles.
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We are deeply saddened to learn that Maria Fidecaro, an experimental physicist who joined CERN in 1957, passed away on 17 September. Maria was a familiar face to the CERN community until long into her retirement, often seen arms linked with her husband Giuseppe as they made their way through the CERN corridors. She was also well-known to CERN visitors, featuring prominently in the Synchrocyclotron exhibition’s film.
Born in Rome in 1930, Maria obtained her PhD in physics from the University of Rome in 1951, studying cosmic rays using a detector located on the Matterhorn. After continuing there as a postdoc, in 1954 she and her future husband went to the University of Liverpool. Maria had obtained a fellowship from the International Federation of University Women while Giuseppe had obtained a CERN fellowship to carry out research at the Synchrocyclotron, CERN’s first accelerator. After their marriage in July 1955, they carried out pion experiments: Maria with a diffusion chamber and Giuseppe with a lead-glass Cherenkov counter.
In summer 1956, the couple moved to Geneva, joining only a few hundred people at CERN, the Laboratory having been established just two years earlier. Maria obtained a CERN fellowship in 1957 and began working in a team of three developing a novel method to provide polarised proton beams at the Synchrocyclotron, later carrying out polarisation experiments at the PS and SPS. She remained at CERN for the rest of her career, where her early research interests included charge-exchange nucleon-nucleon scattering and proton-proton elastic scattering.Maria and Giuseppe Fidecaro inside the spark chamber apparatus of the Proton Synchrotron in 1964. (Image: CERN)
During the 1990s, Maria worked on detectors and analysis for the CPLEAR experiment, which was designed to enable precision measurements of CP, T and CPT violation in the neutral kaon system. She designed and led the construction of a high-granularity electromagnetic calorimeter, helping CPLEAR to achieve new levels of precision in the study of fundamental symmetries. From 1991 to 1995, she was group leader of the CPL group in the Particle Physics Experiments Division. She also took part in the NA48/2 experiment, searching for CP violation in the decay of charged kaons, and contributed to the early phases of NA62.
Maria celebrated her retirement in 1995, but this did not mean the end of her research. She and Giuseppe continued their work at CERN as honorary members of the personnel. As Maria explained in an interview in 2012 “every day or every week there is something new connected with our old work”.
A funeral service will be held at the Italian Catholic Mission of Geneva on Friday 22 September at 2.30 p.m.
A full obituary article will appear in the CERN Courier.anschaef Wed, 09/20/2023 - 13:30 Publication Date Wed, 09/20/2023 - 13:28
Binding together quarks into protons, neutrons and atomic nuclei is a force so strong, it’s in the name. The strong force, which is carried by gluon particles, is the strongest of all fundamental forces of nature – the others being electromagnetism, the weak force and gravity. Yet, it’s the least precisely measured of these four forces. In a paper just submitted to Nature Physics, the ATLAS collaboration describes how it has used the Z boson, the electrically neutral carrier of the weak force, to determine the strength of the strong force with an unprecedented uncertainty of below 1%.
The strength of the strong force is described by a fundamental parameter in the Standard Model of particle physics called the strong coupling constant. While knowledge of the strong coupling constant has improved with measurements and theoretical developments made over the years, the uncertainty on its value remains orders of magnitude larger than that of the coupling constants for the other fundamental forces. A more precise measurement of the strong coupling constant is required to improve the precision of theoretical calculations of particle processes that involve the strong force. It is also needed to address important unanswered questions about nature. Could all of the fundamental forces be of equal strength at very high energy, indicating a potential common origin? Could new, unknown interactions be modifying the strong force in certain processes or at certain energies?
In its new study of the strong coupling constant, the ATLAS collaboration investigated Z bosons produced in proton–proton collisions at CERN's Large Hadron Collider (LHC) at a collision energy of 8 TeV. Z bosons are typically produced when two quarks in the colliding protons annihilate. In this weak-interaction process, the strong force comes into play through the radiation of gluons off the annihilating quarks. This radiation gives the Z boson a “kick” transverse to the collision axis (transverse momentum). The magnitude of this kick depends on the strong coupling constant. A precise measurement of the distribution of Z-boson transverse momenta and a comparison with equally precise theoretical calculations of this distribution allows the strong coupling constant to be determined.
In the new analysis, the ATLAS team focused on cleanly selected Z-boson decays to two leptons (electrons or muons) and measured the Z-boson transverse momentum via its decay products. A comparison of these measurements with theoretical predictions enabled the researchers to precisely determine the strong coupling constant at the Z-boson mass scale to be 0.1183 ± 0.0009. With a relative uncertainty of only 0.8%, the result is the most precise determination of the strength of the strong force made by a single experiment to date. It agrees with the current world average of experimental determinations and state-of-the-art calculations known as lattice quantum chromodynamics (see figure below).
This record precision was accomplished thanks to both experimental and theoretical advances. On the experimental side, the ATLAS physicists achieved a detailed understanding of the detection efficiency and momentum calibration of the two electrons or muons originating from the Z-boson decay, which resulted in momentum precisions ranging from 0.1% to 1%. On the theoretical side, the ATLAS researchers used, among other ingredients, cutting-edge calculations of the Z-boson production process that consider up to four “loops” in quantum chromodynamics. These loops represent the complexity of the calculation in terms of contributing processes. Adding more loops increases the precision.
“The strength of the strong nuclear force is a key parameter of the Standard Model, yet it is only known with percent-level precision. For comparison, the electromagnetic force, which is 15 times weaker than the strong force at the energy probed by the LHC, is known with a precision better than one part in a billion,” says CERN physicist Stefano Camarda, a member of the analysis team. “That we have now measured the strong force coupling strength at the 0.8% precision level is a spectacular achievement. It showcases the power of the LHC and the ATLAS experiment to push the precision frontier and enhance our understanding of nature.”
abelchio Wed, 09/20/2023 - 12:01 Publication Date Mon, 09/25/2023 - 10:00
Magnets, those everyday objects we stick to our fridges, all share a unique characteristic: they always have both a north and a south pole. Even if you tried breaking a magnet in half, the poles would not separate – you would only get two smaller dipole magnets. But what if a particle could have a single pole with a magnetic charge? For over a century, physicists have been searching for such magnetic monopoles. A new study from the ATLAS collaboration at the Large Hadron Collider (LHC) places new limits on these hypothetical particles, adding new clues for the continuing search.
In 1931, physicist Paul Dirac proved that the existence of magnetic monopoles would be consistent with quantum mechanics and require — as has been observed — the quantisation of the electric charge. In the 1970s, magnetic monopoles were also predicted by new theories attempting to unify all the fundamental forces of nature, inspiring physicist Joseph Polchinski to claim that their existence was “one of the safest bets that one can make about physics not yet seen.” Magnetic monopoles might have been present in the early Universe but diluted to an unnoticeably tiny density during the early exponential expansion phase known as cosmic inflation.
Researchers at the ATLAS experiment are searching for pairs of point-like magnetic monopoles with masses of up to about 4 teraelectronvolts (TeV). These pairs could be produced in 13 TeV collisions between protons via two different mechanisms: “Drell-Yan”, in which a virtual photon produced in the collisions creates the magnetic monopoles, or “photon-fusion”, in which two virtual photons radiated by the protons interact to create the magnetic monopoles.
The collaboration’s detection strategy relies on Dirac’s theory, which says that the magnitude of the smallest magnetic charge (gD) is equivalent to 68.5 times the fundamental unit of electric charge, the charge of the electron (e). Consequently, a magnetic monopole of charge 1gD would ionise matter in a similar way as a high-electric-charge object (HECO). When a particle ionises the detector material, ATLAS records the energy deposited, which is proportional to the square of the particle’s charge. Hence, magnetic monopoles or HECOs would leave large energy deposits along their trajectories in the ATLAS detector. Since the ATLAS detector was designed to record low-charge and neutral particles, the characterisation of these high-energy deposits is vital to the search for monopoles and HECOs.
In their new study, the ATLAS researchers combed through the experiment’s full dataset from Run 2 of the LHC (2015–2018) in search of magnetic monopoles and HECOs. The search made use of the detector’s transition radiation tracker and the finely segmented liquid-argon electromagnetic calorimeter. The result places some of the tightest limits yet on the rate of production of magnetic monopoles.
The search targeted monopoles of magnetic charge 1gD and 2gD and HECOs of electric charge 20e, 40e, 60e, 80e and 100e, with masses between 0.2 TeV and 4 TeV. Compared to the previous ATLAS search, the new result benefited from the larger, complete Run-2 dataset. This was also the first ATLAS analysis to consider the photon-fusion production mechanism.
With no evidence of either magnetic monopoles or HECOs in the dataset, the ATLAS researchers established new limits on the production rate and mass of monopoles with a magnetic charge of 1gD and 2gD. ATLAS remains the experiment with the greatest sensitivity to monopoles in this charge range; the smaller LHC experiment MoEDAL-MAPP has previously studied a larger charge range and has also searched for monopoles with a finite size.
ATLAS physicists will continue their quest to find magnetic monopoles and HECOs, further refining their search techniques and developing new strategies to study both Run-2 and Run-3 data.
Find out more on the ATLAS website.abelchio Fri, 09/15/2023 - 13:38 Byline ATLAS collaboration Publication Date Fri, 09/15/2023 - 13:32
Earlier this month, almost 700 physicists from all over the world met in Houston, Texas, to attend the 30th edition of the Quark Matter conference, the largest conference in the field of heavy-ion physics. At this meeting, the ALICE collaboration presented its first results based on data collected with the upgraded detector in 2022, the first year of Run 3 of the LHC. Before the start of Run 3, ALICE underwent a major upgrade of its experimental apparatus to allow the recording of 50-100 times more Pb-Pb collisions and up to 500 times more proton-proton collisions than in previous runs. In addition, upgrades of the tracking detectors improved the pointing resolution by a factor 3-6. All in all, many new high-precision results will become available in the coming years.
One of the new results presented at the Quark Matter conference was the measurement of the production of two different states of charmonia in proton-proton collisions. Charmonia are particles that consist of a charm and an anti-charm quark, with a total mass of about 3 GeV, more than 3 times that of the proton. Charmonia have a characteristic decay signature, producing an electron-positron pair or a positive and a negative muon.
There are a variety of charmonium states, with different binding energies, from the tightly bound J/ψ (binding energy of approximately 650 MeV) to the weakly bound – and two times larger – ψ(2S) (binding energy of 50 MeV). In heavy-ion collisions, these states melt in the quark–gluon plasma (QGP) and a reduced number of them is observed in the final state, a phenomenon known as charm suppression. Physicists can determine the temperature of the plasma by measuring how the different states are suppressed. Such measurements have played an important role in the field over the years, starting from early measurements at the SPS in the 1990s.
The key to measuring charmonium suppression is knowing the production rates. These rates can be determined by measuring the production of quarkonia in proton-proton collisions, where there is no suppression. This provides the reference for the measurements performed in Pb-Pb collisions.
The upgraded ALICE detector has a broad kinematic coverage that allows it to study J/ψ and ψ(2S) down to zero transverse momentum in two different and complementary regions. In the central region, charmonium is reconstructed from its decay into an e+e- pair in the central barrel detectors, while in the forward region it is detected in its decay channel µ+µ-, in the muon spectrometer.
The proton-proton statistics collected in LHC Runs 1 and 2 allowed ALICE to study the ψ(2S) yields in the forward region, but not in the central region. The data from 2022 represents an increase of the total number of collisions by a factor of 300, making it possible to measure the production rate of the ψ(2S) in the central region for the first time. The results, based on 500 billion minimum-bias proton-proton collisions, show that both the excited and the ground charmonium states can be accessed over the whole ALICE kinematic region and this will constrain the models of quarkonium production and open the way for more detailed measurements in the upcoming heavy-ion run.Ratio of ψ(2S) to J/ψ in LHC Run 3 proton-proton collisions as a function of transverse momentum, showing ALICE’s capability for measurements of the excited and ground charmonium states in the central (red points) and forward (black points) region. (Image: ALICE) ptraczyk Fri, 09/15/2023 - 12:21 Byline ALICE collaboration Publication Date Fri, 09/15/2023 - 11:52
Links between particle physics and gravitational-wave science are strengthening, both in the theory realm and on the ground. A prime example is CERN’s key role in the design of next-generation gravitational-wave observatories, in particular the vacuum tubes for the proposed Einstein Telescope in Europe (p45).
A second in-depth feature by CERN authors explores the potential of this and other gravitational-wave observatories to study high-energy processes in the early universe (p32). Among them are cosmological phase transitions, which are predicted to contribute to a stochastic gravitational-wave background. In late June, networks of radio telescopes around the world spotted tentative evidence for low-frequency waves consistent with such a background (p7).
Take a deep dive into the high-spec world of graphics processing units with the ALICE O 2 computing upgrade (p39), delve into a century of FCC physics (p20), survey the linear-collider marketplace (p23), zoom out on the vast landscape of accelerators in physics and industry (p19), and explore the long-term US vision for particle physics (p50).
This issue also takes a closer look at efforts to understand the wild variation in recent measurements of the W-boson mass (p27), the latest LHC results (p22), careers (p55), reviews (p52) and more.
It goes without saying that all eyes were on the Muon g-2 experiment this summer, as Fermilab announced the latest results on 10 August. I warmly congratulate Fermilab and the experiments on this impressive result with such breathtaking precision.
Muon g-2 had been a hot topic at summer conferences even before the Fermilab announcement. I was fortunate to join physicists, primarily from Asia and Australasia, at the 31st International Symposium on Lepton Photon Interactions at High Energies from 17 to 21 July in Melbourne, Australia. Here, we discussed how interpretation of this experimental result would need a coordinated effort from theorists to achieve the most precise theoretical prediction possible, in order to interpret a possible discrepancy with the Standard Model. The experimental results themselves are a marvellous achievement, and their interpretation will require a common international effort.
Returning to Europe, the experiments at CERN set their sights on the EPS-HEP 2023 conference for their latest findings. The conference has just taken place, from 21 to 25 August, in Hamburg, Germany and showcased another bumper year of results, as the LHC experiments probe the Standard Model at the highest energies ever created.
The plethora of presentations covered topics ranging from precision measurements at different centre-of-mass energies to searches for new phenomena. FASER, as well as the North Area experiment NA62 both presented their latest results on dark matter seraches. Notable highlights from ATLAS included analyses of their complete Run 2 data set to present their latest limits on supersymmetric dark matter and on magnetic monopoles. CMS highlights included how machine-learning techniques are improving both muon and jet flavour studies. LHCb results featured the observation of hypertriton in proton–proton collisions, building on the 2022 results from ALICE and providing important input for astrophysics and the study of neutron stars. The ALICE collaboration is now looking ahead to the Quark Matter conference in Texas, USA from 3 to 9 September.
These summer conferences are not only a chance to find out the latest news in particle physics in the talks themselves, but also allow for more spontaneous discussions, where a chance meeting over coffee can lead to new collaborations and new research directions.
We return refreshed and inspired, looking ahead to the planned LHC heavy-ion run, the first since 2018, five years ago. This run will be important not only for ALICE, but also for the heavy-ion communities of the other LHC experiments, providing fresh data and more new and exciting results.ndinmore Fri, 08/25/2023 - 10:59 Byline Joachim Mnich Publication Date Thu, 08/31/2023 - 10:50
On 23 August, at the EPS-HEP conference 2023, the LHCb collaboration presented its observation of the rare hypernuclei hypertriton and antihypertriton, which surpasses the experiment’s design goals. More than 100 of these rare hypernuclei were found in proton–proton collisions corresponding to 5 fb-1 of LHC Run 2 data recorded between 2016 and 2018.
Both nuclei and antinuclei are produced at the LHC, as well as unstable hypernuclei such as (anti)hypertriton. Hypertriton comprises a proton, a neutron and a Lambda hyperon, which is a baryon containing at least one strange quark. In the case of antihypertriton, the antiparticles of these three particles form the hypernucleus. Their production is rare and fascinating to study. As both hypertriton and antihypertriton contain a hyperon, they are also an object of study in astrophysics: the creation of hyperons with a strange quark is energetically favoured in the inner core of neutron stars, so knowing about formation of hyperons serves as an ingredient for modelling this core.
An equally exciting research object for astrophysics is one of the (anti)hypertriton’s decay products: (anti)helium-3, which occurs in space and could be used as a probe for dark matter. On the one hand, (anti)nuclei are produced in collisions between cosmic rays and the interstellar medium. On the other hand, they could be created theoretically when dark-matter particles annihilate. To determine the expected number of (anti)nuclei reaching Earth and the possible deviations from it, precise knowledge of their creation and annihilation probabilities is fundamental.
The (anti)hypertriton’s decay in the LHCb detector happens after around 240 ps. Thanks to a new reconstruction technique, the experimentalists were able to trace the path of the decay products through the LHCb detector. The (anti)helium-3 nuclei are identified via the energy that they lose through ionisation inside the inner detectors, such as the VELO and other tracking detectors.
Read more here.ndinmore Wed, 08/23/2023 - 14:29 Byline Kristiane Bernhard-Novotny Publication Date Wed, 08/23/2023 - 13:52
The CMS collaboration has recently presented new results in searches for long-lived heavy neutral leptons (HNLs). Also known as “sterile neutrinos”, HNLs are interesting hypothetical particles that could solve three major puzzles in particle physics: they could explain the smallness of neutrino masses via the so-called “see-saw” mechanism, they could explain the matter-antimatter asymmetry of the Universe, and at the same time they could provide a candidate for dark matter. They are however very difficult to detect since they interact very weakly with known particles. The current analysis is an example of researchers having to use increasingly creative methods to detect particles that the detectors were not specifically designed to measure.
Most of the particles studied in the large LHC experiments have one thing in common: they are unstable and decay almost immediately after being produced. The products of these decays are usually electrons, muons, photons and hadrons - well-known particles that the big particle detectors were designed to observe and measure. Studies of the original short-lived particles are performed based on careful analysis of the observed decay products. Many of the flagship LHC results were obtained this way, from the Higgs boson decaying into photon pairs and four leptons to studies of the top quark and discoveries of new exotic hadrons.
The HNLs studied in this analysis require a different approach. They are neutral particles with comparatively long lifetimes that allow them to fly for metres undetected, before decaying somewhere in the detector. The analysis presented here focuses on cases where an HNL would appear after the decay of a W boson in a proton-proton collision, and would then itself decay somewhere in the muon system of the CMS detector.
The muon system constitutes the outermost part of CMS and was designed - as its name suggests - to detect muons. Muons produced in the LHC proton-proton collisions traverse the whole detector, leaving a trace in the inner tracking system and then another one in the muon system. Combining these two traces into the full muon track lets physicists identify muons and measure their properties. In the HNL search, a muon is replaced by a weakly interacting heavy particle that leaves no trace - until it decays. If it decays in the muon system it can produce a shower of particles clearly visible in the muon detectors. But - unlike a muon - it leaves no trace in the inner tracking detector, and no other activity in the muon system. This analysis is based on looking for “out-of-nowhere” clusters of tracks in the muon detectors.
The analysis started by selecting collision events with a reconstructed electron or muon from the decay of the W boson and an isolated cluster of traces in the muon system. Then, the analysis required the removal of cases where standard processes could imitate the HNL signal. After the full analysis, no excess of signal above expectation has been observed. As a result, a range of possible HNL parameters was excluded, setting the most stringent limits to date for HNLs with masses of 2-3 GeV.
Read more in the CMS publication here.ndinmore Fri, 07/28/2023 - 16:24 Byline Piotr Traczyk Publication Date Fri, 07/28/2023 - 16:23
In the 11 years since its discovery at the Large Hadron Collider (LHC), the Higgs boson has become a central avenue for shedding light on the fundamental structure of the Universe. Precise measurements of the properties of this special particle are among the most powerful tools physicists have to test the Standard Model, currently the theory that best describes the world of particles and their interactions. At the Lepton Photon Conference this week, the ATLAS collaboration reported how it has measured the mass of the Higgs boson more precisely than ever before.
The mass of the Higgs boson is not predicted by the Standard Model and must therefore be determined by experimental measurement. Its value governs the strengths of the interactions of the Higgs boson with the other elementary particles as well as with itself. A precise knowledge of this fundamental parameter is key to accurate theoretical calculations which, in turn, allow physicists to confront their measurements of the Higgs boson’s properties with predictions from the Standard Model. Deviations from these predictions would signal the presence of new or unaccounted-for phenomena. The Higgs boson’s mass is also a crucial parameter driving the evolution and the stability of the Universe’s vacuum.
The ATLAS and CMS collaborations have been making ever more precise measurements of the Higgs boson’s mass since the particle’s discovery. The new ATLAS measurement combines two results: a new Higgs boson mass measurement based on an analysis of the particle’s decay into two high-energy photons (the “diphoton channel”) and an earlier mass measurement based on a study of its decay into four leptons (the “four-lepton channel”).
The new measurement in the diphoton channel, which combines analyses of the full ATLAS data sets from Runs 1 and 2 of the LHC, resulted in a mass of 125.22 billion electronvolts (GeV) with an uncertainty of only 0.14 GeV. With a precision of 0.11%, this diphoton-channel result is the most precise measurement to date of the Higgs boson’s mass from a single decay channel.
Compared to the previous ATLAS measurement in this channel, the new result benefits both from the full ATLAS Run 2 data set, which reduced the statistical uncertainty by a factor of two, and from dramatic improvements to the calibration of photon energy measurements, which decreased the systematic uncertainty by almost a factor of four to 0.09 GeV.
“The advanced and rigorous calibration techniques used in this analysis were critical for pushing the precision to such an unprecedented level,” says Stefano Manzoni, convener of the ATLAS electron-photon calibration subgroup. “Their development took several years and required a deep understanding of the ATLAS detector. They will also greatly benefit future analyses.”
When the ATLAS researchers combined this new mass measurement in the diphoton channel with the earlier mass measurement in the four-lepton channel, they obtained a Higgs boson mass of 125.11 GeV with an uncertainty of 0.11 GeV. With a precision of 0.09%, this is the most precise measurement yet of this fundamental parameter.
“This very precise measurement is the result of the relentless investment of the ATLAS collaboration in improving the understanding of our data,” says ATLAS spokesperson Andreas Hoecker. "Powerful reconstruction algorithms paired with precise calibrations are the determining ingredients of precision measurements. The new measurement of the Higgs boson’s mass adds to the increasingly detailed mapping of this critical new sector of particle physics."
Find out more on the ATLAS website.angerard Fri, 07/21/2023 - 15:38 Publication Date Fri, 07/21/2023 - 16:00
In a new paper published in Physical Review Letters, researchers working at CERN’s ISOLDE facility describe how an upgrade to the ISOLTRAP experiment has allowed them to determine the energy necessary to bring the atomic nucleus of indium-99 from its ground state to a long-lived excited state called an isomer. The result follows an earlier ISOLTRAP measurement of indium-99 in the ground state, offering an even closer look at the nucleus of tin-100 – a “doubly magic” nucleus that is a mere proton above indium-99.
Atomic nuclei in which the constituent protons and neutrons each completely fill the orbital shells to capacity are more strongly bound than their nuclear neighbours. Such “doubly magic” nuclei provide stringent tests of theoretical models of the nucleus. This is the case of the tin-100 nucleus, which has 50 protons and 50 neutrons. But this special doubly magic nucleus – it is also the heaviest such nucleus comprising protons and neutrons in equal number – is particularly challenging to produce in the lab and is relatively short-lived. Researchers therefore turn to its more easily produced nuclear neighbours to try and reveal its secrets.
In their latest study, the ISOLTRAP team turned to indium-99, in particular to its isomer, which has a slightly different orbital occupation – and hence higher energy – than the ground state and results in a slightly larger nuclear mass. Using an upgraded version of the experiment’s multireflection time-of-flight mass spectrometer, the researchers were able to measure the difference in the time-of-flight of confined indium-99 nuclei in their ground and isomeric states. This small difference, which is caused by the different mass of the nucleus in these two states, made it possible to determine the energy necessary to excite the isomer.
The team then compared the result with measurements of isomer excitation energies for other indium neighbours, including a new ISOLTRAP measurement of indium-101. This comparison showed that the excitation energies are essentially the same down to the magic neutron number 50. The result is in stark contrast with recent results on the magnetic moments of indium nuclei from ISOLDE’s CRIS experiment, which saw their remarkably constant value undergoing a surprisingly abrupt change at magic neutron number 82.
The researchers also compared the results with several sophisticated types of theoretical calculations, including “ab initio” calculations that attempt to describe nuclei from first principles. They found that all of the calculations struggle to predict the isomer excitation energies and the magnetic moments simultaneously.
The results will guide researchers in their effort to develop a fully ab initio description of the nucleus, which continues to make promising progress.abelchio Thu, 07/20/2023 - 13:32 Publication Date Thu, 07/20/2023 - 13:30
Half a century ago, a series of tiny tracks in a bubble chamber at CERN changed the course of particle physics. The observation of “weak neutral currents”, announced on 19 July 1973 by Paul Musset of the Gargamelle collaboration, suggested that the electromagnetic and weak forces are facets of a more fundamental electroweak interaction that ruled in the early Universe. Exploring this new sector of nature has been a core business of CERN ever since, leading to the discovery of the W and Z bosons in 1983 and culminating with the discovery of the Higgs boson in 2012.
The weak force is one of the four fundamental forces of nature, responsible for crucial processes such as radioactive beta decay. Whereas the electromagnetic force was well understood as the result of neutral photons being exchanged between charged particles, the weak interaction was harder to cast in the language of quantum theory. In the 1960s, theorists posited that the weak interaction was mediated by massive versions of the photon: the charged W boson and the neutral Z boson, both inextricably tied up with the photon of electromagnetism. The W boson enabled weak interactions that involved a rearrangement of electrical charge, while the Z boson was how uncharged particles interacted via the weak force. While the former were already known to occur, the latter had never been seen before.
As physicists mastered the art of firing intense beams of neutrinos into detectors to study fundamental interactions, searches for neutral currents became possible. By the late 1960s André Lagarrigue of LAL Orsay had proposed the world’s biggest bubble chamber, Gargamelle, named after a fictional giantess. The chamber was built by the École Polytechnique Paris in 1968 and assembled at one of the beamlines of CERN’s Proton Synchrotron. Data taking started in 1970, with first results coming in shortly after. Reflecting the focus of experimentalists at the time, the search for neutral currents was placed only eighth in Gargamelle’s top-ten physics goals.
Picking out experimental evidence for neutral currents from among numerous similar-looking events was not easy, especially with the technology of the time. Researchers needed to see both “leptonic” events (whereby a neutrino interacted with an electron in the dense gas Gargamelle was filled with) and “hadronic” events (whereby a neutrino was scattered from a proton or neutron). “I remember spending the evenings with my colleagues scanning the films on special projectors, which allowed us to observe the eight views of the chamber,” recalls Gargamelle member Donatella Cavalli from the University of Milan, who was a PhD student at the time. “When the first leptonic event was found in December 1972, we were convinced that neutral currents existed.”
Further data would reveal candidate hadronic neutral-current events, but it took time for the community to be convinced. Initially, the independent Harvard–Pennsylvania–Wisconsin–Fermilab experiment in the US confirmed Gargamelle’s findings, but when they changed their experimental set-up, the tracks vanished. Only in 1974, after further analysis by both collaborations, was the existence of neutral currents universally accepted – leading to the award of the 1979 Nobel Prize in Physics to electroweak architects Sheldon Glashow, Abdus Salam and Steven Weinberg.
Gargamelle is now an exhibit in CERN’s Van Hove Square, but physicists are still pursuing the path it opened . In providing the first evidence for electroweak theory, Gargamelle’s results guided CERN to convert the Super Proton Synchrotron into a proton–antiproton collider powerful enough to enable the UA1 and UA2 collaborations to discover the W and Z bosons directly – a feat recognised by the award of the 1984 Nobel Prize in Physics to Carlo Rubbia and Simon van der Meer of CERN. During the 1990s, precision measurements of the W and Z bosons at the Large Electron–Positron collider confirmed important “quantum corrections” to electroweak theory (which, together with the theory of the strong force, quantum chromodynamics, makes up the Standard Model of particle physics). This guided physicists towards the discovery of the final piece of the electroweak jigsaw – the Higgs boson – at the Large Hadron Collider (LHC) in 2012, which led theorists François Englert and Peter Higgs to be awarded the 2013 Nobel Prize in Physics.
But the journey does not end there. As the LHC’s ATLAS and CMS experiments continue to probe the Higgs boson and other mysterious sectors of the Standard Model at increasing levels of precision, physicists are investigating the feasibility of a successor collider at CERN – the proposed Future Circular Collider – that would go much further, opening the next chapter in electroweak exploration.
Read more in CERN Courier:
A scientific symposium marking 50 years of neutral currents and 40 years of the W and Z bosons will take place at CERN on 31 October 2023 in the Science Gateway Auditorium.The Gargamelle bubble chamber now sits in the park next to Science Gateway. (Image: CERN)
ckrishna Wed, 07/19/2023 - 10:04 Byline Matthew Chalmers Publication Date Wed, 07/19/2023 - 10:00
The aim of the GBAR experiment at CERN is to measure the acceleration of an antihydrogen atom – the simplest form of atomic antimatter – in Earth's gravitational field, and to compare it with that of the normal hydrogen atom. Such a comparison is a crucial test of Einstein's equivalence principle, which states that the trajectory of a particle is independent of its composition and internal structure when it is only subjected to gravitational forces.
But producing and slowing down an antiatom enough to see it in free fall is no mean feat. GBAR's approach is to first produce an antihydrogen atom and then turn it into a positive ion (the antimatter equivalent of an H- ion). Then the ion can be slowed down using quantum-optical techniques. Finally, the ion is neutralised for free-fall measurement. In a new paper, the GBAR collaboration reports the successful production of its first antiatoms.
To achieve this, the team has developed a complex protocol in which antihydrogen atoms are assembled from antiprotons produced by the Antiproton Decelerator (AD) and positrons produced in GBAR. The AD's 5.3-MeV antiprotons are decelerated and cooled in the ELENA ring and a packet of a few million 100-keV antiprotons is sent to GBAR every two minutes. In GBAR, a device called a pulsed drift tube further decelerates this packet to an adjustable energy of a few keV. In parallel, in another part of GBAR, a linear particle accelerator sends 9-MeV electrons onto a tungsten target, producing positrons, which are accumulated in a series of electromagnetic traps. Just before the antiproton packet arrives, the positrons are sent to a layer of nanoporous silica, from which about one in five positrons emerges as a positronium atom (the bound state of a positron and an electron). When the antiproton packet crosses the resulting cloud of positronium atoms, a charge exchange can take place, with the positronium giving up its positron to the antiproton, forming antihydrogen.
At the end of 2022, during an operation that lasted several days, the GBAR collaboration detected some 20 antihydrogen atoms produced in this way, validating this "in-flight" production method for the first time.
After this essential first step, the collaboration will now improve the production of antihydrogen atoms. This will enable precision measurements to be made on the antihydrogens themselves, in particular a measurement of an energy gap between two specific atomic levels, known as the Lamb shift. This measurement will give a more precise value of the radius of the antiproton. This will be followed by the production of positive antihydrogen ions, and finally by the implementation of the laser systems for cooling and neutralising these ions in order to finally observe the free fall of an antihydrogen atom.
GBAR is not the first experiment to produce antihydrogen: in 1995, an experiment at CERN's LEAR facility produced nine antiatoms, but at an energy too high for any measurement to be made. Following this early success, CERN's Antiproton Accumulator (used for the discovery of the W and Z bosons in 1983) was repurposed as a decelerator, becoming the AD, which is unique worldwide in providing low-energy (5-MeV) antiprotons to antimatter experiments. After the demonstration of holding antiprotons by the ATRAP and ATHENA experiments, ALPHA, a successor of ATHENA, was the first experiment to merge trapped antiprotons and positrons and to trap the resulting antihydrogen atoms. Since then, ATRAP and ASACUSA have also achieved these two milestones, and AEgIS has produced pulses of antiatoms. GBAR now joins this elite club, having produced 6-keV antihydrogen atoms in flight.
GBAR is also not alone in its aim of testing Einstein’s equivalence principle with atomic antimatter. ALPHA and AEgIS are also working towards this goal using other approaches.
This text is a modified version of a story originally published in French here.abelchio Fri, 07/14/2023 - 12:41 Publication Date Fri, 07/14/2023 - 12:40
In the Large Hadron Collider, proton and lead beams travel close to the speed of light. They carry a strong electromagnetic field that acts like a flux of photons as the beam moves through the accelerator. When the two beams at the LHC pass by close to each other without colliding, one of the beams may emit a photon of very high energy that strikes the other beam. This can result in photon—nucleus, photon—proton, and even photon—photon collisions. The ALICE collaboration studies these collisions to investigate protons and the inner structure of nuclei, and has recently released new results on this topic at the LHCP 2023 conference.
Photons are ideal tools to study the interior of nuclei. Usually when a photon collides with a nucleus, two gluons (force carriers of the strong interaction) are exchanged, which results in the production of a quark-antiquark pair. Researchers further distinguish two different classes of these collisions: when a photon interacts with the whole nucleus (a coherent collision), and when a photon interacts with a single nucleon inside the nucleus (an incoherent collision).
Inside nuclei, scientists look for high numbers of gluons, which indicate high levels of gluon density. Theoretical models suggest that the gluon density inside nuclei increases when they approach the speed of light. If the density increases enough, the nucleus will become saturated with gluonic matter, meaning that the number of gluons in the nucleus cannot increase any further. Directly probing gluonic saturated matter is one of the main outstanding challenges in the field of strong interactions, and observing it could lead to further insight into the inner structure of protons and nuclei.
If a charm quark-antiquark pair is produced in a photon—nucleus collision, this is known as J/ψ meson production. Scientists study how coherent J/ψ production varies with photon energy in order to look for gluon saturation effects. As the photon energy increases, it becomes easier and easier to “see” the gluonic matter inside the nuclei. The new ALICE results on J/ψ production using LHC Run 2 data cover a larger momentum range than previous measurements from Run 1, and are in line with expectations of gluon-saturation models.
Incoherent collisions offer the opportunity to study geometrical configurations of the quantum fluctuations in the internal structure of the proton. The ALICE collaboration achieves this by studying the distribution of momentum that is transferred to the J/ψ meson. In a new study, the collaboration has been able to show that this momentum transfer can only be described when areas of saturated gluonic matter, called gluonic hotspots, are introduced into the models.
The ALICE collaboration will continue to investigate these phenomena in LHC Runs 3 and 4, where high-precision measurements with larger data samples will provide more powerful tools to better understand the role of saturation and gluonic hotspots.
The Big Bang is thought to have created equal amounts of matter and antimatter, yet the Universe today is made almost entirely of matter, so something must have happened to create this imbalance.
The weak force of the Standard Model of particle physics is known to induce a behavioural difference between matter and antimatter – known as CP symmetry violation – in decays of particles containing quarks, one of the building blocks of matter. But these differences, or asymmetries, are hard to measure and insufficient to explain the matter–antimatter imbalance in the present-day Universe, prompting physicists to both measure precisely the known differences and to look for new ones.
In 1964, James Cronin and Val Fitch discovered CP symmetry violation through their pioneering experiment at Brookhaven National Laboratory in the US, using decays of particles containing strange quarks. This finding challenged the long-held belief in this symmetry of nature and earned Cronin and Fitch the Nobel Prize in Physics in 1980.
In 2001, the BaBar experiment in the US and the Belle experiment in Japan confirmed the existence of CP violation in decays of beauty mesons, particles with a beauty quark, solidifying our understanding of the nature of this phenomenon. This achievement ignited intense research efforts to further understand the mechanisms behind CP violation. In 2008, Makoto Kobayashi and Toshihide Maskawa received the Nobel Prize in Physics for their theoretical framework that elegantly explained the observed CP violation phenomena.
It its latest studies, using the full dataset recorded by the LHCb detector during the second run of the Large Hadron Collider (LHC), the LHCb collaboration set out to measure with high precision two parameters that determine the amount of CP violation in decays of beauty mesons.
One parameter determines the amount of CP violation in decays of neutral beauty mesons, which are made up of a bottom antiquark and a down quark. This is the same parameter as that measured by the BaBar and Belle experiments in 2001. The other parameter determines the amount of CP violation in decays of strange beauty mesons, which consist of a bottom antiquark and a strange quark.
Specifically, these parameters determine the extent of time-dependent CP violation. This type of CP violation stems from the intriguing quantum interference that occurs when a particle and its antiparticle undergo decay. The particle has the ability to spontaneously transform into its antiparticle and vice versa. As this oscillation takes place, the decays of the particle and antiparticle interfere with each other, leading to a distinctive pattern of CP violation that changes over time. In other words, the amount of CP violation observed depends on the time the particle lives before decaying. This fascinating phenomenon provides physicists with key insights into the fundamental nature of particles and their symmetries.
For both parameters, the new LHCb results, which are more precise than any equivalent result from a single experiment, are in line with the values predicted by the Standard Model.
“These measurements are interpreted within our fundamental theory of particle physics, the Standard Model, improving the precision with which we can determine the difference between the behaviour of matter and antimatter,” explains LHCb spokesperson Chris Parkes. “Through more precise measurements, large improvements have been made in our knowledge. These are key parameters that aid our search for unknown effects from beyond our current theory.”
Future data, from the third run of the LHC and the collider’s planned upgrade, the High-Luminosity LHC, will further tighten the precision on these matter–antimatter asymmetry parameters and perhaps point to new physics phenomena that could help shed light on what is one of the Universe’s best-kept secrets.
Find out more on LHCb's website: precise measurement of the CP-violating phase φs and precise measurement of the unitarity triangle angle β
angerard Tue, 06/13/2023 - 08:17 Publication Date Tue, 06/13/2023 - 12:35