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LHCb investigates the properties of one of physics’ most puzzling particles

Mon, 15/07/2024 - 13:23
LHCb investigates the properties of one of physics’ most puzzling particles The LHCb experiment. (Image: CERN)

χc1(3872) is an intriguing particle. It was first discovered over 20 years ago in B+ meson decays by the BELLE collaboration, KEK, Japan. Since then, the LHCb collaboration reported it in 2010 and has measured some of its properties. But here’s the catch – physicists still don’t know what it is actually made up of.

In the quark model of particle physics, there are baryons (made up of three quarks), mesons (made up of a quark–antiquark pair) and exotic particles (made up of an unconventional number of quarks). To find out what χc1(3872) consists of, physicists must measure its properties, such as its mass or quantum number. Theories suggest that χc1(3872) could be a conventional charmonium state, made up of charm and anticharm quarks, or an exotic particle composed of four quarks. An exotic particle of this type could be a tightly bound tetraquark, a molecular state, a cc-gluon hybrid state, a vector glueball or a mixture of different possibilities.

Previously, the LHCb collaboration has found its quantum number to be 1++ and, in 2020, made precise measurements of the width (lifetime) and mass of the particle. The collaboration also measured what is known as its low-energy scattering parameters. The results showed that its mass is just a tad smaller than the sum of the masses of the D0 and D*0 mesons.

Following these results, the theoretical community was divided. Some argued that χc1(3872) was a molecular state consisting of spatially separated D0 and D*0 mesons. This molecular state would be much larger than the typical size of particles and more comparable to a heavy nucleus. However, this argument encounters a problem, namely that physicists expect molecular objects to be suppressed in hadron–hadron collisions, and the χc1(3872) is produced abundantly. Other theorists interpreted the results as clear evidence that χc1(3872) has a “compact” component. This would mean it is a particle with much smaller size, containing either a tightly bound charmonium or a tetraquark.

One way to help determine what χc1(3872) contains is to calculate the ratio between probabilities of the decays into different lighter particles (branching fractions). By comparing the rate at which it decays either to an excited charmonium state or to a charmonium state and a photon, physicists can gather clues as to what type of particle it is. There is a clear theoretical signature: if the ratio is non-vanishing, it is evidence for some compact component in χc1(3872), disfavouring the pure molecular model.

Now, using the complete set of LHC Run 1 and Run 2 data, the LHCb collaboration has found these ratios to be non-vanishing, with a significance exceeding six standard deviations.  The large measured value of the ratios is inconsistent with the expectations based on the pure D0D*0 molecular hypothesis for the χc1(3872) particle. Instead, it supports a wide range of predictions based on other hypotheses of the χc1(3872) structure, including conventional (compact) charmonium, a compact tetraquark containing a charm quark, charm antiquark, light quark and light antiquark, or a mixture of molecules with a substantial compact core component. In short, the result provides a strong argument in favour of the χc1(3872) structure containing a compact component.

The χc1(3872) particle continues to fascinate the particle physics community. Find out more in the paper or on the LHCb website.

ndinmore Mon, 07/15/2024 - 12:23 Byline LHCb collaboration Publication Date Tue, 07/16/2024 - 09:40

The July/August issue of the CERN Courier is out

Mon, 08/07/2024 - 14:43
The July/August issue of the CERN Courier is out (Image: CERN Courier)

Insulators rather than conductors. Pillars rather than cavities. Microns rather than centimetres. Over the past 30 years, a handful of research groups have been quietly reimagining the electron linac in miniature. Their work is developing rapidly, with focusing, bunching and net acceleration over hundreds of optical cycles all now demonstrated “on chip”. This is acceleration, but not as we know it (p35).

This research is no scholarly abstraction. Tens of thousands of conventional electron linacs are used in medicine and industry. In this edition’s interview, International Cancer Expert Corps’ Manjit Dosanjh and CERN’s Steinar Stapnes tell the Courier about the need to increase access to medical linacs in low- and middle-income countries. Their newly funded project is based on open science and international cooperation – and it will require the full toolkit of the experimental high-energy physicist (p46).

Elsewhere on these pages: Andrzej Buras identifies the six rare decays that are best placed to probe beyond the energy frontier this decade (p30); CALET studies cosmic-ray anomalies on the International Space Station (p24); DUNE takes shape in Ray Davis Jr’s gold mine (p41); ATLAS and CMS hunt the Higgs boson’s self-interaction (p7); Wolfgang Lerche reviews On the Origin of Time (p49); Silvia Pascoli on the next 10 years in astroparticle physics (p45); electroweak delights from Moriond (p18); and how big is a neutrino? (p8).

Read the digital edition of this new issue on the CERN Courier website.

anschaef Mon, 07/08/2024 - 13:43 Publication Date Mon, 07/08/2024 - 13:39

LHCb investigates the rare Σ+→pμ+μ- decay

Thu, 27/06/2024 - 17:45
LHCb investigates the rare Σ+→pμ+μ- decay

The LHCb collaboration reported the observation of the hyperon Σ+→pμ+μ- rare decay at the XV International Conference on Beauty, Charm, Hyperons in Hadronic Interactions (BEACH 2024) in Charleston, South Carolina, USA. A hyperon is a particle containing three quarks, like the proton and neutron, including one or more strange quarks.

Rare decays of known particles are a promising tool for searching for physics beyond the Standard Model (SM) of particle physics. In the SM, the Σ+→pμ+μ- process is only possible through “loop diagrams”: rather than the decay happening directly, intermediate states need to be exchanged in a “loop”, as illustrated in diagrams (a) and (b) below.

In quantum field theory, the probability of such a process occurring is the sum of the probabilities of all possible particles exchanged in this loop, both known and unknown. This is what makes such a process sensitive to new phenomena. If a discrepancy between the experimental measurement and theoretical calculations was observed, it could be caused by a contribution from some unknown particles. These particles could either be exchanged in the loop or mediate this decay directly, interacting with the quarks and then decaying into a pair of muons. In the latter case, shown in diagram (c) below, the new particle would leave a footprint in the properties of the two muons.

Feynman diagrams illustrating the Σ+→pμ+μ- decay in the Standard Model (diagrams a and b) and with a new X0 intermediate particle (diagram c). (Image: LHCb)

Studies of the Σ+→pμ+μ- decay were especially exciting thanks to a hint of a structure that had been observed in the properties of the muon pair in 2005 by the HyperCP (E871) collaboration. With only three events, the structure was far from conclusive, and the LHCb study was expected to shine some light on the situation.

Ultimately, LHCb data does not show any significant peaking structures in the dimuon mass region highlighted by HyperCP, hence disconfirming the hint. The new analysis does, however, observe the decay with high significance, and precise measurement of the decay probability along with other parameters will follow, allowing further searches for discrepancies with SM predictions.

Read more in the LHCb presentations at BEACH as well as in the conference note.

ptraczyk Thu, 06/27/2024 - 16:45 Byline Piotr Traczyk Publication Date Fri, 06/28/2024 - 10:00

Hans Joachim Specht (1936 – 2024)

Mon, 24/06/2024 - 16:28
Hans Joachim Specht (1936 – 2024) (Image: Specht)

Hans Joachim Specht, one of the founders of ultra-relativistic heavy-ion physics and a pioneering figure in hadron cancer therapy, passed away on 20 May 2024, at the age of 87. A graduate of the University of Munich and ETH Zurich, and full Professor at the University of Heidelberg for more than 30 years, his career was distinguished by important contributions across a spectrum of scientific domains.

Hans started his academic career in atomic and nuclear physics in Munich, under the guidance of Heinz Maier-Leibnitz. A highlight was the discovery and precise measurement of shape isomerism in heavy nuclei. His observation of distinct rotational bands in 240Pu showed, for the first time, that nuclei can be in a strongly deformed cigar-shaped state shortly before fission, confirming the concept of a “double-humped” fission barrier. In Munich, and later in Heidelberg, he developed several innovative large-scale detectors for fission fragments and reaction products of heavy-ion collisions, becoming one of the leading experimentalists in the new field of “heavy-ion physics”, with experiments at the MPI for Nuclear Physics in Heidelberg and at the newly founded GSI in Darmstadt.

In the early 1980s, Hans reoriented his research towards the higher energies available at CERN. His contributions and advocacy, alongside a handful of other enthusiastic proponents, were instrumental in establishing CERN’s ultra-relativistic heavy-ion programme at the SPS accelerator, which was approved in 1984. He became the spokesperson of a first-generation heavy-ion experiment (Helios/NA34-2), initiator and spokesperson of a second-generation experiment (CERES/NA45), and a crucial supporter and mentor for the third-generation ALICE experiment at the Large Hardon Collider.

Initially with the bespoke dilepton experiment CERES, and later as a leading force within NA60, Hans succeeded in detecting, for the first time, thermally produced lepton pairs in heavy-ion collisions. Arguably one of the most challenging signals, the “Planck-like” spectrum of thermal radiation at higher masses, and the precise characterisation of the in-medium modification of the rho-meson at lower masses, proved to be crucial in establishing the existence and properties of the quark–gluon plasma – the state of strongly interacting matter thought to have existed in the primordial Universe, just a few microseconds after the Big Bang. The enduring quality and relevance of these measurements remain unsurpassed almost two decades later.

Hans also provided forward-looking impulses for nuclear and particle physics research as Scientific Director of GSI (1992–1999), where he set the technical and science-policy course for the development and application of a groundbreaking innovation in radiation medicine – ion-beam cancer therapy. A pilot project at GSI for the irradiation of tumours with 12C ions, launched under his leadership, successfully treated 450 patients and led to the establishment of the Heidelberg Ion-Beam Therapy Center (HIT), the first European ion-beam therapy facility. Reflecting on his achievements, he was most proud of his contributions to ion-beam therapy, improving human health through scientific advancement.

Hans also had a profound interest in the intersection of physics, music and neuroscience, collaborating with Hans-Günter Dosch on understanding perception of music and its physiological bases. This transdisciplinary approach produced highly cited publications on the differences in the auditory cortex between musicians and non-musicians, expanding the boundaries of how we understand the brain and its response to music.

Hans was an outstanding teacher, a prolific mentor, a successful science manager, but foremost, he was an inquisitive scientist – someone who profoundly loved physics, with a thirst for knowledge, a hunger for understanding and a relentless drive to follow wherever his interests and research would lead him.

His friends and colleagues 

____

A full obituary will appear later in the year in the CERN Courier.

anschaef Mon, 06/24/2024 - 15:28 Publication Date Mon, 06/24/2024 - 15:25

Going the extra mile to squeeze supersymmetry out of CMS data

Thu, 20/06/2024 - 15:28
Going the extra mile to squeeze supersymmetry out of CMS data

Supersymmetry (SUSY) is an exciting and beautiful theory that answers some of the open questions in particle physics. It predicts that all known particles have a “superpartner” with somewhat different properties. For example, the heaviest quark of the Standard Model, the top quark, would have a superpartner called the top squark, or simply the “stop”. In 2021 the CMS collaboration analysed the entire set of collision data collected from 2016 to 2018 and found features suggesting that it might contain stop particles. In that case, “might” meant that there was less than 5% chance that data containing only known particles could look like what was observed. Instead of waiting many years to collect more data with the hope of reproducing this behaviour, the CMS collaboration decided to reanalyse the same data with upgraded analysis techniques.

The new analysis looks for the simultaneous production of pairs of stops. Each stop decays into a top quark accompanied by several lighter quarks or gluons, which then form bound states known as hadrons, ultimately creating clusters of particles reconstructed in the detector as “jets”. The signal footprint is therefore two top quarks and multiple jets. What makes the analysis challenging is that a very similar footprint is produced by one of the most common Standard Model processes in the LHC: the pair production of top quarks. Top quark production with many accompanying jets is a process that is difficult to accurately simulate, so to have a reliable determination of this background, it must be estimated from observed data.

A commonly used method of estimating backgrounds from data is called the “ABCD method”. It requires two uncorrelated observables that can discriminate between signal and background. The data set can then be divided into four regions (A, B, C and D) depending on the value of each observable being “signal-like” or “background-like”. The subdivision then provides a region dominated by the signal, a region dominated by backgrounds and two intermediate regions. The key feature of the ABCD method is that, following the mathematics of probabilities for independent events, one can estimate the background in the signal-dominated region using only the information from the other regions. The problem with using this method for the stop search is that all simple variables are correlated in this search, making the method invalid. To overcome this issue, CMS physicists have implemented an innovative approach based on advanced machine-learning techniques to determine two variables with a minimal level of correlation. These two variables are then used to divide the data into the four aforementioned regions. The figure below shows the correlation between the two variables for the signal and the background and demonstrates that the signal mostly lies in region “A”. 

Distributions of signal (red) and background (grey) in the four (A, B, C and D) regions, defined based on two uncorrelated variables (SNN1 and SNN2) determined using machine learning. (Credit: CMS collaboration)

Using this novel method, the CMS collaboration was able to accurately predict the dominant background in this analysis from observed data, without relying on simulations with large uncertainties associated with the modelling of the jet multiplicity distribution. This resulted in a large gain in analysis sensitivity. If the signal hinted at by the 2021 analysis was real, it would now have been observed without any doubt. The fact that a signal was not seen in this analysis implies that, in specific SUSY scenarios, a stop decaying ultimately to top quarks and jets must have a mass greater than 700 GeV. With a much more sensitive analysis method in place, the physicists are now eagerly looking forward to analysing the data of the ongoing LHC Run 3 to go even further and to find where Nature hides its answers.

Read more in the CMS Physics Analysis Summary.

ptraczyk Thu, 06/20/2024 - 14:28 Byline CMS collaboration Publication Date Thu, 06/27/2024 - 09:00

ATLAS dives deeper into di-Higgs

Tue, 18/06/2024 - 10:37
ATLAS dives deeper into di-Higgs An event display of a di-Higgs candidate event taken in 2017. (Image: ATLAS collaboration/CERN)

Remember how difficult it was to find one Higgs boson? Try finding two at the same place at the same time. Known as di-Higgs production, this fascinating process can tell scientists about the Higgs boson self-interaction. By studying it, physicists can measure the strength of the Higgs boson’s “self-coupling”, which is a fundamental aspect of the Standard Model that connects the Higgs mechanism and the stability of our Universe.

Searching for di-Higgs production is an especially challenging task. It’s a very rare process, about 1000 times rarer than the production of a single Higgs boson. During the entire Run 2 of the Large Hadron Collider (LHC), only a few thousand di-Higgs events are expected to have been produced in ATLAS, compared with the 40 million collisions that happened every second. So how can physicists find these rare needles in the data haystack? One way to make it easier to look for di-Higgs production is to search for it in multiple places. By looking at the different ways di-Higgs can decay (decay modes) and putting them together, physicists are able to maximise their chances of finding and studying di-Higgs production.

Researchers at the ATLAS collaboration have now released the most sensitive search for di-Higgs production and self-coupling yet, achieved by combining five di-Higgs studies of LHC Run 2 data. This new result is their most comprehensive search so far, covering over half of all possible di-Higgs events in ATLAS.

The five individual studies in this combination each focused on different decay modes, each of which has its pros and cons. For example, the most probable di-Higgs decay mode is into four bottom quarks. However, Standard Model QCD processes are also likely to create four bottom quarks, making it difficult to differentiate a di-Higgs event from this background process. The di-Higgs decay to two bottom quarks and two tau leptons has moderate background contamination but is five times less common and has neutrinos that escape undetected, complicating physicists’ ability to reconstruct the decay. The decay to multiple leptons, while not too rare, has complex signatures. Other di-Higgs decays are even more rare, such as the decay to two bottom quarks and two photons. This final state accounts for only 0.3% of total di-Higgs decays but has a cleaner signature and much smaller background contamination.

By combining the results from searches for each of these decays, the researchers were able to find that the probability that two Higgs bosons are produced excludes values more than 2.9 times the Standard-Model prediction. This result is at 95% confidence level, with an expected sensitivity of 2.4 (assuming that this process is not present in nature). They were also able to provide constraints on the strength of the Higgs boson self-coupling, achieving the best-yet sensitivity on this important observable. They found that the magnitude of the Higgs self-coupling constant and the interaction strength of two Higgs bosons and two vector bosons are consistent with Standard Model predictions.

This combined result sets a milestone in the study of di-Higgs production. Now, ATLAS researchers have set their sights on data from the ongoing LHC Run 3 and upcoming High-Luminosity LHC operation. With this data, physicists may be able to observe the elusive Higgs-boson-pair production at last.

Read more:

ATLAS Briefing
Paper

 

ndinmore Tue, 06/18/2024 - 09:37 Byline ATLAS collaboration Publication Date Tue, 06/18/2024 - 10:59

Bringing black hole jets down to Earth

Thu, 13/06/2024 - 11:16
Bringing black hole jets down to Earth

Dive into the heart of an active galaxy and you’ll find a supermassive black hole gobbling up material from its surroundings. In about one out of ten such galaxies, the black hole will also shoot out jets of matter at close to the speed of light. Such relativistic black hole jets are thought to contain, among other components, a plasma of pairs of electrons and their antimatter equivalents, positrons.

This relativistic electron–positron plasma is believed to shape the dynamics and energy budget of the black hole and its environment. But how exactly this happens remains little understood, because it’s difficult both to measure the plasma with astronomical observations and to simulate it with computer programmes.

In a paper just published in Nature Communications, Charles Arrowsmith and colleagues from the Fireball collaboration report how they have used the HiRadMat facility at CERN to produce a relativistic beam of electron–positron plasma that allows this medium to be studied in detail in laboratory experiments.

Relativistic beams of electron–positron pairs can be created in several ways at different types of laboratories, including high-power laser facilities. However, none of the existing ways can produce the number of electron–positron pairs that is required to sustain a plasma – a state of matter in which the constituent particles are very loosely connected. Without sustaining the plasma, researchers cannot investigate how these analogues of black hole jets change as they move through a laboratory equivalent of the interstellar medium. This investigation is key to explaining observations from ground- and space-based telescopes.

Arrowsmith and colleagues found a way to meet these requirements at CERN’s HiRadMat facility. Their approach involved extracting within a mere nanosecond a whopping three hundred billion protons from the Laboratory’s Super Proton Synchrotron and firing them onto a target of graphite and tantalum, in which a cascade of particle interactions generates huge numbers of electron–positron pairs.

By measuring the resulting relativistic electron–positron beam with a set of instruments, and comparing the result with sophisticated computer simulations, Arrowsmith and co-workers showed that the number of electron–positron pairs in the beam – more than ten trillion – is ten to hundred times greater than previously achieved, exceeding for the first time the number needed to sustain the plasma state.

“Electron–positron plasmas are thought to play a fundamental part in astrophysical jets, but computer simulations of these plasmas and jets have never been tested in the laboratory,” says Arrowsmith. ”Laboratory experiments are necessary to validate the simulations, because what seems like reasonable simplifications of the calculations involved in the simulations can sometimes lead to drastically different conclusions.”

The result is the first from a series of experiments that the Fireball collaboration is carrying out at HiRadMat.

“The basic idea of these experiments is to reproduce in the laboratory the microphysics of astrophysical phenomena such as jets from black holes and neutron stars,” says co-author of the paper and lead researcher Gianluca Gregori. “What we know about these phenomena comes almost exclusively from astronomical observations and computer simulations, but telescopes cannot really probe the microphysics and simulations involve approximations. Laboratory experiments such as these are a bridge between these two approaches.”

Next in Arrowsmith and colleagues’ plasma pursuits at HiRadMat is to have these powerful jets propagate through a metre-long plasma and observe how the interaction between them generates magnetic fields that speed up the particles in the jets – one the greatest puzzles in high-energy astrophysics.

“The Fireball experiments are one of the latest additions to HiRadMat’s portfolio,” says operation manager of the facility Alice Goillot. “We’re looking forward to continue reproducing these rare phenomena using the unique properties of CERN’s accelerator complex.”

View of the HiRadMat facility (Image: CERN)

This project has received funding from the European Union’s Horizon Europe Research and Innovation programme under Grant Agreement No 101057511 (EURO-LABS).

abelchio Thu, 06/13/2024 - 10:16 Byline Ana Lopes Publication Date Thu, 06/13/2024 - 09:55

How can AI help physicists search for new particles?

Thu, 13/06/2024 - 10:33
How can AI help physicists search for new particles?

One of the main goals of the LHC experiments is to look for signs of new particles, which could explain many of the unsolved mysteries in physics. Often, searches for new physics are designed to look for one specific type of new particle at a time, using theoretical predictions as a guide. But what about searching for unpredicted – and unexpected – new particles? Trawling through the billions of collisions that occur in the LHC experiments without knowing exactly what to look for would be a mammoth task for physicists. So, instead of sifting through the data and looking for anomalies, the ATLAS and CMS collaborations are letting artificial intelligence (AI) do the job.

At the Rencontres de Moriond conference on 26 March, physicists from the CMS collaboration presented the latest results obtained by using various machine learning techniques to search for pairs of “jets”. These jets are collimated sprays of particles originating from strongly interacting quarks and gluons. They are particularly difficult to analyse, but they could be hiding new physics.

Researchers at ATLAS and CMS use several strategies to train AI algorithms in their searches for jets. By studying the shape of their complex energy signatures, scientists can determine what particle created the jet. Using real collision data, physicists at both experiments are training their AI to recognise the characteristics of jets originating from known particles. The AI is then able to differentiate between these jets and atypical jet signatures, which potentially indicate new interactions. These would show up as an accumulation of atypical jets in the data set.

Another method involves instructing the AI algorithm to consider the entire collision event and look for anomalous features in the different particles detected. These anomalous features may indicate the presence of new particles. This technique was demonstrated in a paper released by ATLAS in July 2023, which featured one of the first uses of unsupervised machine learning in an LHC result. At CMS, a different approach involves physicists creating simulated examples of potential new signals and then tasking the AI with identifying collisions in the real data that are different to regular jets but resemble the simulation.

In the latest results presented by CMS, each AI training method exhibited varying sensitivities to different types of new particles, and no single algorithm proved to be the best. The CMS team was able to limit the rate of production of several different types of particles that produce anomalous jets. They were also able to show that the AI-led algorithms significantly enhanced the sensitivity to a wide range of particle signatures in comparison to traditional techniques.

Event display of one of the CMS events determined by the AI algorithm to be highly anomalous and therefore potentially coming from a new particle. (Image: CMS collaboration)

These results show how machine learning is revolutionising the search for new physics. “We already have ideas about how to further improve the algorithms and apply them to different parts of the data to search for several kinds of particles,” says Oz Amram, from the CMS analysis team.

Read more:

CMS briefing
ATLAS briefing

ndinmore Thu, 06/13/2024 - 09:33 Byline CMS collaboration Publication Date Thu, 06/13/2024 - 11:26

Shaking the box for new physics

Fri, 07/06/2024 - 16:40
Shaking the box for new physics CMS candidate collision event for a B0 meson decaying into a K*0 meson and two muons (red lines). The K*0 meson decays into a K+ meson (magenta line) and a π- meson (green line). (Image: CERN)

When you receive a present on your birthday, you might be the kind of person who tears off the wrapping paper immediately to see what’s inside the box. Or maybe you like to examine the box, guessing the contents from its shape, size, weight or the sound it makes when you shake it.

When physicists at the Large Hadron Collider (LHC) analyse their datasets in search of new physics phenomena such as new particles, they usually take one of two different approaches. They either perform a direct search for a specific new kind of particle, equivalent to tearing off the wrapping paper immediately, or use an indirect strategy based on quantum mechanics and its subtle wonders, similar to shaking the box and guessing what’s inside.

At the annual LHCP conference that took place in Boston last week, the CMS collaboration reported how it used the second approach to look for new physics in a rare decay of a particle called B0 meson.

The physics process that drives the decay of a particle into lighter ones can be influenced by new, unknown particles, which might be too heavy to be produced at the LHC. This influence could change the decay process in ways that can be measured and compared to predictions of the Standard Model of particle physics. In the same way as shaking the box containing your birthday present could give you a clue about what’s inside, any deviation from the Standard Model predictions could give physicists a hint of new physics.

The decay of the B0 meson, which is made up of a bottom quark and a down quark, into a K*0 meson (containing a strange quark and a down quark) and two muons is particularly suited to this approach. This is because it occurs via a rare penguin transition that is highly sensitive to possible contributions from new heavy particles.

In its new study, the CMS team used all the data collected by its detector between 2016 and 2018, during the second run of the LHC, to “shake” this B0 decay “box”. This box offers many ways to look for new physics. One is to weigh the box, i.e. measure the rate at which the decay occurs. Another is to take two twin boxes – for example, one corresponding to the decay into two muons and the other to the decay into two electrons – and check if they weigh the same.

In their new study, the CMS researchers looked at the shape of the box, i.e. they examined how the particles produced in the decay share the energy of the parent B0 meson and measured at what angles they fly away from each other. They then determined a set of parameters using these energies and angles, and compared the results with two sets of predictions from the Standard Model.

For most parameters, the results are in line with these two sets of Standard Model predictions. However, for two parameters, known as P5’ and P2, and for specific energies of the two muons, the results are in tension with the two available predictions. Overall, the results are in agreement with the previous results from the ATLAS, LHCb and Belle experiments, while improving upon their level of precision.

Unfortunately, there is a charming, “naughty” kind of penguin that’s crashing the birthday party: a charm quark that participates in the rare penguin transition. This complicates the Standard Model predictions and makes it difficult to draw a conclusion. To advance, researchers need better predictions, more data and improved analysis techniques.

Find out more on the CMS website.

abelchio Fri, 06/07/2024 - 15:40 Byline CMS collaboration Publication Date Wed, 06/12/2024 - 14:00

The May/June issue of the CERN Courier is out

Mon, 13/05/2024 - 13:49
The May/June issue of the CERN Courier is out (Image: CERN)

Uncovering the fundamental laws and constituents of the universe is not just a source of fascination for particle physicists. Engineers, chemists and materials scientists draw similar motivation in meeting the challenging and strange requirements of cutting-edge accelerator and detector technologies. The interdisciplinary materials, metrology and non-destructive testing section at CERN supports projects such as the HL-LHC magnet development with state of-the-art equipment and analysis, and its services are increasingly in demand from projects outside – including the ITER fusion experiment (p37).

Also explored in depth in this issue are the next steps for the AWAKE plasma-wakefield experiment (p25), the new-physics implications of neutrino masses (p29) and a lesser-known approach to quantum gravity called asymptotic safety (p43). Advanced triggers for the HL-LHC experiments (p8), the selection of the SHiP experiment for CERN’s North Area (p7) and DESI’s first cosmology results (p11) are among other highlights, along with the latest LHC results (p15), conference reports (p19), news in brief (p13), opinion (p49), reviews (p52), careers (p55) and more.

Read the digital edition of this new issue on CDS. 

anschaef Mon, 05/13/2024 - 12:49 Publication Date Mon, 05/13/2024 - 12:46

The May/June issue of the CERN Courier is out

Mon, 13/05/2024 - 13:49
The May/June issue of the CERN Courier is out (Image: CERN)

Uncovering the fundamental laws and constituents of the universe is not just a source of fascination for particle physicists. Engineers, chemists and materials scientists draw similar motivation in meeting the challenging and strange requirements of cutting-edge accelerator and detector technologies. The interdisciplinary materials, metrology and non-destructive testing section at CERN supports projects such as the HL-LHC magnet development with state of-the-art equipment and analysis, and its services are increasingly in demand from projects outside – including the ITER fusion experiment (p37).

Also explored in depth in this issue are the next steps for the AWAKE plasma-wakefield experiment (p25), the new-physics implications of neutrino masses (p29) and a lesser-known approach to quantum gravity called asymptotic safety (p43). Advanced triggers for the HL-LHC experiments (p8), the selection of the SHiP experiment for CERN’s North Area (p7) and DESI’s first cosmology results (p11) are among other highlights, along with the latest LHC results (p15), conference reports (p19), news in brief (p13), opinion (p49), reviews (p52), careers (p55) and more.

Read the digital edition of this new issue on CDS. 

anschaef Mon, 05/13/2024 - 12:49 Publication Date Mon, 05/13/2024 - 12:46

Probing matter–antimatter asymmetry with AI

Tue, 30/04/2024 - 09:54
Probing matter–antimatter asymmetry with AI The open CMS detector during the second long shutdown of CERN’s accelerator complex. (Image: CERN)

When we look at ourselves in a mirror, we see a virtual twin, identical in every detail except with left and right inverted. In particle physics, a transformation in which charge–parity (CP) symmetry is respected swaps a particle with the mirror image of its antimatter particle, which has opposite properties such as electric charge.

The physical laws that govern nature don’t respect CP symmetry, however. If they did, the Universe would contain equal amounts of matter and antimatter, as it is believed to have done just after the Big Bang. To explain the large imbalance between matter and antimatter seen in the present-day Universe, CP symmetry has to be violated to a great extent. The Standard Model of particle physics can account for some CP violation, but it is not sufficient to explain the present-day matter–antimatter imbalance, prompting researchers to explore CP violation in all its known and unknown manifestations.

One way CP violation can manifest itself is in the “mixing” of electrically neutral mesons such as the strange beauty meson, which is composed of a strange quark and a bottom antiquark. These mesons can travel macroscopic distances in the Large Hadron Collider (LHC) detectors before decaying into lighter particles, and during this journey they can turn into their corresponding antimesons and back.

This phenomenon, called meson mixing, could be different for a meson turning into an antimeson versus an antimeson turning into a meson, generating CP violation. To see if that’s the case, researchers need to count how many mesons or antimesons survive a certain duration before decaying, and then repeat the measurement for a given range of durations. To do so, they have to separate mesons from antimesons, a task called flavour tagging. This task is crucial to pinning down CP violation in meson mixing and in the interference between meson mixing and decay.

At a seminar held recently at CERN, the CMS collaboration at the LHC reported the first evidence of CP violation in the decay of the strange beauty meson into a pair of muons and a pair of electrically charged kaons.

By deploying a new flavour-tagging algorithm on a sample of about 500 000 decays of the strange beauty meson into a pair of muons and a pair of charged kaons, collected during Run 2 of the LHC, the CMS collaboration measured with improved precision the parameter that determines CP violation in the interference between this meson’s mixing and decay. If this parameter is zero, CP symmetry is respected. The new flavour-tagging algorithm is based on a cutting-edge artificial intelligence (AI) technique called a graph neural network, which performs accurate flavour tagging by gathering information from the particles surrounding the strange beauty meson and those being produced alongside it.

The collaboration then combined the result with its previous measurement of the parameter based on data from Run 1 of the LHC. The combined result is different from zero and is consistent with the Standard Model prediction and with previous measurements from CMS and the ATLAS and LHCb experiments.

Notably, the combined result is comparable in precision to the world’s most precise measurement of the parameter, obtained by LHCb, a detector specifically designed to perform measurements of this kind. Moreover, the result has a statistical significance that crosses the conventional “3 sigma” threshold, providing the first evidence of CP violation in the decay of the strange beauty meson into a pair of muons and a pair of charged kaons.

The result marks a milestone in CMS’s studies of CP violation. Thanks to AI, CMS has pushed the boundary of what its detector can achieve in the exploration of this fundamental matter–antimatter asymmetry.

Find out more on the CMS website.

abelchio Tue, 04/30/2024 - 08:54 Byline CMS collaboration Publication Date Tue, 04/30/2024 - 08:53

Probing matter–antimatter asymmetry with AI

Tue, 30/04/2024 - 09:54
Probing matter–antimatter asymmetry with AI The open CMS detector during the second long shutdown of CERN’s accelerator complex. (Image: CERN)

When we look at ourselves in a mirror, we see a virtual twin, identical in every detail except with left and right inverted. In particle physics, a transformation in which charge–parity (CP) symmetry is respected swaps a particle with the mirror image of its antimatter particle, which has opposite properties such as electric charge.

The physical laws that govern nature don’t respect CP symmetry, however. If they did, the Universe would contain equal amounts of matter and antimatter, as it is believed to have done just after the Big Bang. To explain the large imbalance between matter and antimatter seen in the present-day Universe, CP symmetry has to be violated to a great extent. The Standard Model of particle physics can account for some CP violation, but it is not sufficient to explain the present-day matter–antimatter imbalance, prompting researchers to explore CP violation in all its known and unknown manifestations.

One way CP violation can manifest itself is in the “mixing” of electrically neutral mesons such as the strange beauty meson, which is composed of a strange quark and a bottom antiquark. These mesons can travel macroscopic distances in the Large Hadron Collider (LHC) detectors before decaying into lighter particles, and during this journey they can turn into their corresponding antimesons and back.

This phenomenon, called meson mixing, could be different for a meson turning into an antimeson versus an antimeson turning into a meson, generating CP violation. To see if that’s the case, researchers need to count how many mesons or antimesons survive a certain duration before decaying, and then repeat the measurement for a given range of durations. To do so, they have to separate mesons from antimesons, a task called flavour tagging. This task is crucial to pinning down CP violation in meson mixing and in the interference between meson mixing and decay.

At a seminar held recently at CERN, the CMS collaboration at the LHC reported the first evidence of CP violation in the decay of the strange beauty meson into a pair of muons and a pair of electrically charged kaons.

By deploying a new flavour-tagging algorithm on a sample of about 500 000 decays of the strange beauty meson into a pair of muons and a pair of charged kaons, collected during Run 2 of the LHC, the CMS collaboration measured with improved precision the parameter that determines CP violation in the interference between this meson’s mixing and decay. If this parameter is zero, CP symmetry is respected. The new flavour-tagging algorithm is based on a cutting-edge artificial intelligence (AI) technique called a graph neural network, which performs accurate flavour tagging by gathering information from the particles surrounding the strange beauty meson and those being produced alongside it.

The collaboration then combined the result with its previous measurement of the parameter based on data from Run 1 of the LHC. The combined result is different from zero and is consistent with the Standard Model prediction and with previous measurements from CMS and the ATLAS and LHCb experiments.

Notably, the combined result is comparable in precision to the world’s most precise measurement of the parameter, obtained by LHCb, a detector specifically designed to perform measurements of this kind. Moreover, the result has a statistical significance that crosses the conventional “3 sigma” threshold, providing the first evidence of CP violation in the decay of the strange beauty meson into a pair of muons and a pair of charged kaons.

The result marks a milestone in CMS’s studies of CP violation. Thanks to AI, CMS has pushed the boundary of what its detector can achieve in the exploration of this fundamental matter–antimatter asymmetry.

Find out more on the CMS website.

abelchio Tue, 04/30/2024 - 08:54 Byline CMS collaboration Publication Date Tue, 04/30/2024 - 08:53

MoEDAL zeroes in on magnetic monopoles

Fri, 26/04/2024 - 11:45
MoEDAL zeroes in on magnetic monopoles The MoEDAL detector (Image: CERN)

The late physicist Joseph Polchinski once said the existence of magnetic monopoles is “one of the safest bets that one can make about physics not yet seen”. In its quest for these particles, which have a magnetic charge and are predicted by several theories that extend the Standard Model, the MoEDAL collaboration at the Large Hadron Collider (LHC) has not yet proven Polchinski right, but its latest findings mark a significant stride forward. The results, reported in two papers posted on the arXiv preprint server, considerably narrow the search window for these hypothetical particles.

At the LHC, pairs of magnetic monopoles could be produced in interactions between protons or heavy ions. In collisions between protons, they could be formed from a single virtual photon (the Drell–Yan mechanism) or the fusion of two virtual photons (the photon-fusion mechanism). Pairs of magnetic monopoles could also be produced from the vacuum in the enormous magnetic fields created in near-miss heavy-ion collisions, through a process called the Schwinger mechanism.

Since it started taking data in 2012, MoEDAL has achieved several firsts, including conducting the first searches at the LHC for magnetic monopoles produced via the photon-fusion mechanism and through the Schwinger mechanism. In the first of its latest studies, the MoEDAL collaboration sought monopoles and high-electric-charge objects (HECOs) produced via the Drell–Yan and photon-fusion mechanisms. The search was based on proton–proton collision data collected during Run 2 of the LHC, using the full MoEDAL detector for the first time.

The full detector comprises two main systems sensitive to magnetic monopoles, HECOs and other highly ionising hypothetical particles. The first can permanently register the tracks of magnetic monopoles and HECOs, with no background signals from Standard Model particles. These tracks are measured using optical scanning microscopes at INFN Bologna. The second system consists of roughly a tonne of trapping volumes designed to capture magnetic monopoles. These trapping volumes – which make MoEDAL the only collider experiment in the world that can definitively and directly identify the magnetic charge of magnetic monopoles – are scanned at ETH Zurich using a special type of magnetometer called a SQUID to look for any trapped monopoles they may contain.

In their latest scanning of the trapping volumes, the MoEDAL team found no magnetic monopoles or HECOs, but it set bounds on the mass and production rate of these particles for different values of particle spin, an intrinsic form of angular momentum. For magnetic monopoles, the mass bounds were set for magnetic charges from 1 to 10 times the fundamental unit of magnetic charge, the Dirac charge (gD), and the existence of monopoles with masses as high as about 3.9 trillion electronvolts (TeV) was excluded. For HECOs, the mass limits were established for electric charges from 5e to 350e, where e is the electron charge, and the existence of HECOs with masses ranging up to 3.4 TeV was ruled out.

“MoEDAL’s search reach for both monopoles and HECOs allows the collaboration to survey a huge swathe of the theoretical ‘discovery space’ for these hypothetical particles,” says MoEDAL spokesperson James Pinfold.

In its second latest study, the MoEDAL team concentrated on the search for monopoles produced via the Schwinger mechanism in heavy-ion collision data taken during Run 1 of the LHC. In a unique endeavour, it scanned a decommissioned section of the CMS experiment beam pipe, instead of the MoEDAL detector’s trapping volumes, in search of trapped monopoles. Once again, the team found no monopoles, but it set the strongest-to-date mass limits on Schwinger monopoles with a charge between 2gD and 45gD, ruling out the existence of monopoles with masses of up to 80 GeV.

“The vital importance of the Schwinger mechanism is that the production of composite monopoles is not suppressed compared to that of elementary ones, as is the case with the Drell–Yan and photon-fusion processes,” explains Pinfold. “Thus, if monopoles are composite particles, this and our previous Schwinger-monopole search may have been the first-ever chances to observe them.”

The MoEDAL detector will soon be joined by the MoEDAL Apparatus for Penetrating Particles, MAPP for short, which will allow the experiment to cast an even broader net in the search for new particles.

abelchio Fri, 04/26/2024 - 10:45 Byline Ana Lopes Publication Date Fri, 04/26/2024 - 10:24

MoEDAL zeroes in on magnetic monopoles

Fri, 26/04/2024 - 11:45
MoEDAL zeroes in on magnetic monopoles The MoEDAL detector (Image: CERN)

The late physicist Joseph Polchinski once said the existence of magnetic monopoles is “one of the safest bets that one can make about physics not yet seen”. In its quest for these particles, which have a magnetic charge and are predicted by several theories that extend the Standard Model, the MoEDAL collaboration at the Large Hadron Collider (LHC) has not yet proven Polchinski right, but its latest findings mark a significant stride forward. The results, reported in two papers posted on the arXiv preprint server, considerably narrow the search window for these hypothetical particles.

At the LHC, pairs of magnetic monopoles could be produced in interactions between protons or heavy ions. In collisions between protons, they could be formed from a single virtual photon (the Drell–Yan mechanism) or the fusion of two virtual photons (the photon-fusion mechanism). Pairs of magnetic monopoles could also be produced from the vacuum in the enormous magnetic fields created in near-miss heavy-ion collisions, through a process called the Schwinger mechanism.

Since it started taking data in 2012, MoEDAL has achieved several firsts, including conducting the first searches at the LHC for magnetic monopoles produced via the photon-fusion mechanism and through the Schwinger mechanism. In the first of its latest studies, the MoEDAL collaboration sought monopoles and high-electric-charge objects (HECOs) produced via the Drell–Yan and photon-fusion mechanisms. The search was based on proton–proton collision data collected during Run 2 of the LHC, using the full MoEDAL detector for the first time.

The full detector comprises two main systems sensitive to magnetic monopoles, HECOs and other highly ionising hypothetical particles. The first can permanently register the tracks of magnetic monopoles and HECOs, with no background signals from Standard Model particles. These tracks are measured using optical scanning microscopes at INFN Bologna. The second system consists of roughly a tonne of trapping volumes designed to capture magnetic monopoles. These trapping volumes – which make MoEDAL the only collider experiment in the world that can definitively and directly identify the magnetic charge of magnetic monopoles – are scanned at ETH Zurich using a special type of magnetometer called a SQUID to look for any trapped monopoles they may contain.

In their latest scanning of the trapping volumes, the MoEDAL team found no magnetic monopoles or HECOs, but it set bounds on the mass and production rate of these particles for different values of particle spin, an intrinsic form of angular momentum. For magnetic monopoles, the mass bounds were set for magnetic charges from 1 to 10 times the fundamental unit of magnetic charge, the Dirac charge (gD), and the existence of monopoles with masses as high as about 3.9 trillion electronvolts (TeV) was excluded. For HECOs, the mass limits were established for electric charges from 5e to 350e, where e is the electron charge, and the existence of HECOs with masses ranging up to 3.4 TeV was ruled out.

“MoEDAL’s search reach for both monopoles and HECOs allows the collaboration to survey a huge swathe of the theoretical ‘discovery space’ for these hypothetical particles,” says MoEDAL spokesperson James Pinfold.

In its second latest study, the MoEDAL team concentrated on the search for monopoles produced via the Schwinger mechanism in heavy-ion collision data taken during Run 1 of the LHC. In a unique endeavour, it scanned a decommissioned section of the CMS experiment beam pipe, instead of the MoEDAL detector’s trapping volumes, in search of trapped monopoles. Once again, the team found no monopoles, but it set the strongest-to-date mass limits on Schwinger monopoles with a charge between 2gD and 45gD, ruling out the existence of monopoles with masses of up to 80 GeV.

“The vital importance of the Schwinger mechanism is that the production of composite monopoles is not suppressed compared to that of elementary ones, as is the case with the Drell–Yan and photon-fusion processes,” explains Pinfold. “Thus, if monopoles are composite particles, this and our previous Schwinger-monopole search may have been the first-ever chances to observe them.”

The MoEDAL detector will soon be joined by the MoEDAL Apparatus for Penetrating Particles, MAPP for short, which will allow the experiment to cast an even broader net in the search for new particles.

abelchio Fri, 04/26/2024 - 10:45 Byline Ana Lopes Publication Date Fri, 04/26/2024 - 10:24

2024 European School of High-Energy Physics | 25 September - 8 October 2023

Fri, 26/04/2024 - 10:40
2024 European School of High-Energy Physics | 25 September - 8 October 2023  The 2024 European School of High-Energy Physics (ESHEP2024) will take place in Peebles, UK, 25 September - 8 October 2024.

The School is targeted particularly at students in experimental HEP who are in the final years of work towards their PhDs, although candidates at an earlier or later stage in their studies may be considered.

The deadline for applications is 3 May 2024. Sponsorship may be available for a few students from developing countries.____ Further details are available on Indico: https://indico.cern.ch/event/1378334/ lburdych Fri, 04/26/2024 - 09:40 Publication Date Fri, 04/26/2024 - 17:03

Searching for new asymmetry between matter and antimatter

Thu, 11/04/2024 - 12:41
Searching for new asymmetry between matter and antimatter The LHCb detector seen in 2018 during its opening (Image: CERN)

Once a particle of matter, always a particle of matter. Or not. Thanks to a quirk of quantum physics, four known particles made up of two different quarks – such as the electrically neutral D meson composed of a charm quark and an up antiquark – can spontaneously oscillate into their antimatter partners and vice versa.

At a seminar held recently at CERN, the LHCb collaboration at the Large Hadron Collider (LHC) presented the results of its latest search for matter–antimatter asymmetry in the oscillation of the neutral D meson, which, if found, could help shed light on the mysterious matter–antimatter imbalance in the Universe.

The weak force of the Standard Model of particle physics induces an asymmetry between matter and antimatter, known as CP violation, in particles containing quarks. However, these sources of CP violation are difficult to study and are insufficient to explain the matter–antimatter imbalance in the Universe, leading physicists to both search for new sources and to study the known ones better than ever before.

In their latest endeavour, the LHCb researchers have rolled up their sleeves to measure with unprecedented precision a set of parameters that determine the matter–antimatter oscillation of the neutral D meson and enable the search for the hitherto unobserved but predicted CP violation in the oscillation.

The collaboration had previously measured the same set of parameters, which are linked to the decay of the neutral D meson into a positively charged kaon and a negatively charged pion, using its full data set from Run 1 of the LHC and a partial data set from Run 2. This time around, the team analysed the full Run-2 data set and, by combining the result with that of its previous analysis, excluding the partial Run-2 data set, it obtained the most precise measurements of the parameters to date – the overall measurement uncertainty is 1.6 times smaller than the smallest uncertainty achieved before by LHCb.

The results are consistent with previous studies, confirming the matter–antimatter oscillation of the neutral D meson and showing no evidence of CP violation in the oscillation. The findings call for future analyses of this and other decays of the neutral D meson using data from the third run of the LHC and its planned upgrade, the High-Luminosity LHC.

Other neutral D meson decays of interest include the decay into a pair of two kaons or two pions, in which LHCb researchers observed CP violation in particles containing charm quarks for the first time, and the decay into a neutral kaon and a pair of pions, with which LHCb clocked the speed of the particle’s matter–antimatter oscillation. No avenue should be left unexplored in the search for clues to the matter–antimatter imbalance in the Universe and other cosmic mysteries.

Find out more on the LHCb website.

abelchio Thu, 04/11/2024 - 11:41 Byline Ana Lopes Publication Date Thu, 04/11/2024 - 17:00

Searching for new asymmetry between matter and antimatter

Thu, 11/04/2024 - 12:41
Searching for new asymmetry between matter and antimatter The LHCb detector seen in 2018 during its opening (Image: CERN)

Once a particle of matter, always a particle of matter. Or not. Thanks to a quirk of quantum physics, four known particles made up of two different quarks – such as the electrically neutral D meson composed of a charm quark and an up antiquark – can spontaneously oscillate into their antimatter partners and vice versa.

At a seminar held recently at CERN, the LHCb collaboration at the Large Hadron Collider (LHC) presented the results of its latest search for matter–antimatter asymmetry in the oscillation of the neutral D meson, which, if found, could help shed light on the mysterious matter–antimatter imbalance in the Universe.

The weak force of the Standard Model of particle physics induces an asymmetry between matter and antimatter, known as CP violation, in particles containing quarks. However, these sources of CP violation are difficult to study and are insufficient to explain the matter–antimatter imbalance in the Universe, leading physicists to both search for new sources and to study the known ones better than ever before.

In their latest endeavour, the LHCb researchers have rolled up their sleeves to measure with unprecedented precision a set of parameters that determine the matter–antimatter oscillation of the neutral D meson and enable the search for the hitherto unobserved but predicted CP violation in the oscillation.

The collaboration had previously measured the same set of parameters, which are linked to the decay of the neutral D meson into a positively charged kaon and a negatively charged pion, using its full data set from Run 1 of the LHC and a partial data set from Run 2. This time around, the team analysed the full Run-2 data set and, by combining the result with that of its previous analysis, excluding the partial Run-2 data set, it obtained the most precise measurements of the parameters to date – the overall measurement uncertainty is 1.6 times smaller than the smallest uncertainty achieved before by LHCb.

The results are consistent with previous studies, confirming the matter–antimatter oscillation of the neutral D meson and showing no evidence of CP violation in the oscillation. The findings call for future analyses of this and other decays of the neutral D meson using data from the third run of the LHC and its planned upgrade, the High-Luminosity LHC.

Other neutral D meson decays of interest include the decay into a pair of two kaons or two pions, in which LHCb researchers observed CP violation in particles containing charm quarks for the first time, and the decay into a neutral kaon and a pair of pions, with which LHCb clocked the speed of the particle’s matter–antimatter oscillation. No avenue should be left unexplored in the search for clues to the matter–antimatter imbalance in the Universe and other cosmic mysteries.

Find out more on the LHCb website.

abelchio Thu, 04/11/2024 - 11:41 Byline Ana Lopes Publication Date Thu, 04/11/2024 - 17:00

ATLAS provides first measurement of the W-boson width at the LHC

Wed, 10/04/2024 - 13:03
ATLAS provides first measurement of the W-boson width at the LHC View of an ATLAS collision event in which a candidate W boson decays into a muon and a neutrino. The reconstructed tracks of the charged particles in the inner part of the ATLAS detector are shown as orange lines. The energy deposits in the detector’s calorimeters are shown as yellow boxes. The identified muon is shown as a red line. The missing transverse momentum associated with the neutrino is shown as a green dashed line. (Image: ATLAS/CERN)

The discovery of the Higgs boson in 2012 slotted in the final missing piece of the Standard Model puzzle. Yet, it left lingering questions. What lies beyond this framework? Where are the new phenomena that would solve the Universe's remaining mysteries, such as the nature of dark matter and the origin of matter–antimatter asymmetry?

One parameter that may hold clues about new physics phenomena is the “width” of the W boson, the electrically charged carrier of the weak force. A particle’s width is directly related to its lifetime and describes how it decays to other particles. If the W boson decays in unexpected ways, such as into yet-to-be-discovered new particles, these would influence the measured width. As its value is precisely predicted by the Standard Model based on the strength of the charged weak force and the mass of the W boson (along with smaller quantum effects), any significant deviation from the prediction would indicate the presence of unaccounted phenomena.

In a new study, the ATLAS collaboration measured the W-boson width at the Large Hadron Collider (LHC) for the first time. The W-boson width had previously been measured at CERN’s Large Electron–Positron (LEP) collider and Fermilab’s Tevatron collider, yielding an average value of 2085 ± 42 million electronvolts (MeV), consistent with the Standard-Model prediction of 2088 ± 1 MeV. Using proton–proton collision data at an energy of 7 TeV collected during Run 1 of the LHC, ATLAS measured the W-boson width as 2202 ± 47 MeV. This is the most precise measurement to date made by a single experiment, and — while a bit larger — it is consistent with the Standard-Model prediction to within 2.5 standard deviations (see figure below).

This remarkable result was achieved by performing a detailed particle-momentum analysis of decays of the W boson into an electron or a muon and their corresponding neutrino, which goes undetected but leaves a signature of missing energy in the collision event (see image above). This required physicists to precisely calibrate the ATLAS detector’s response to these particles in terms of efficiency, energy and momentum, taking contributions from background processes into account.

However, achieving such high precision also requires the confluence of several high-precision results. For instance, an accurate understanding of W-boson production in proton–proton collisions was essential, and researchers relied on a combination of theoretical predictions validated by various measurements of W and Z boson properties. Also crucial to this measurement is the knowledge of the inner structure of the proton, which is described in parton distribution functions. ATLAS physicists incorporated and tested parton distribution functions derived by global research groups from fits to data from a wide range of particle physics experiments.

The ATLAS collaboration measured the W-boson width simultaneously with the W-boson mass using a statistical method that allowed part of the parameters quantifying uncertainties to be directly constrained from the measured data, thus improving the measurement’s precision. The updated measurement of the W-boson mass is 80367 ± 16 MeV, which improves on and supersedes the previous ATLAS measurement using the same dataset. The measured values of both the mass and the width are consistent with the Standard-Model predictions.

Future measurements of the W-boson width and mass using larger ATLAS datasets are expected to reduce the statistical and experimental uncertainties. Concurrently, advancements in theoretical predictions and a more refined understanding of parton distribution functions will help to reduce the theoretical uncertainties. As their measurements become ever more precise, physicists will be able to conduct yet more stringent tests of the Standard Model and probe for new particles and forces.

Comparison of the measured W-boson width with the Standard-Model prediction and with measurements from the LEP and Tevatron colliders. The vertical grey band illustrates the Standard-Model prediction, while the black dots and the associated horizontal bands represent the published experimental results. (Image: ATLAS/CERN) abelchio Wed, 04/10/2024 - 12:03 Byline ATLAS collaboration Publication Date Wed, 04/10/2024 - 11:57

ATLAS provides first measurement of the W-boson width at the LHC

Wed, 10/04/2024 - 13:03
ATLAS provides first measurement of the W-boson width at the LHC View of an ATLAS collision event in which a candidate W boson decays into a muon and a neutrino. The reconstructed tracks of the charged particles in the inner part of the ATLAS detector are shown as orange lines. The energy deposits in the detector’s calorimeters are shown as yellow boxes. The identified muon is shown as a red line. The missing transverse momentum associated with the neutrino is shown as a green dashed line. (Image: ATLAS/CERN)

The discovery of the Higgs boson in 2012 slotted in the final missing piece of the Standard Model puzzle. Yet, it left lingering questions. What lies beyond this framework? Where are the new phenomena that would solve the Universe's remaining mysteries, such as the nature of dark matter and the origin of matter–antimatter asymmetry?

One parameter that may hold clues about new physics phenomena is the “width” of the W boson, the electrically charged carrier of the weak force. A particle’s width is directly related to its lifetime and describes how it decays to other particles. If the W boson decays in unexpected ways, such as into yet-to-be-discovered new particles, these would influence the measured width. As its value is precisely predicted by the Standard Model based on the strength of the charged weak force and the mass of the W boson (along with smaller quantum effects), any significant deviation from the prediction would indicate the presence of unaccounted phenomena.

In a new study, the ATLAS collaboration measured the W-boson width at the Large Hadron Collider (LHC) for the first time. The W-boson width had previously been measured at CERN’s Large Electron–Positron (LEP) collider and Fermilab’s Tevatron collider, yielding an average value of 2085 ± 42 million electronvolts (MeV), consistent with the Standard-Model prediction of 2088 ± 1 MeV. Using proton–proton collision data at an energy of 7 TeV collected during Run 1 of the LHC, ATLAS measured the W-boson width as 2202 ± 47 MeV. This is the most precise measurement to date made by a single experiment, and — while a bit larger — it is consistent with the Standard-Model prediction to within 2.5 standard deviations (see figure below).

This remarkable result was achieved by performing a detailed particle-momentum analysis of decays of the W boson into an electron or a muon and their corresponding neutrino, which goes undetected but leaves a signature of missing energy in the collision event (see image above). This required physicists to precisely calibrate the ATLAS detector’s response to these particles in terms of efficiency, energy and momentum, taking contributions from background processes into account.

However, achieving such high precision also requires the confluence of several high-precision results. For instance, an accurate understanding of W-boson production in proton–proton collisions was essential, and researchers relied on a combination of theoretical predictions validated by various measurements of W and Z boson properties. Also crucial to this measurement is the knowledge of the inner structure of the proton, which is described in parton distribution functions. ATLAS physicists incorporated and tested parton distribution functions derived by global research groups from fits to data from a wide range of particle physics experiments.

The ATLAS collaboration measured the W-boson width simultaneously with the W-boson mass using a statistical method that allowed part of the parameters quantifying uncertainties to be directly constrained from the measured data, thus improving the measurement’s precision. The updated measurement of the W-boson mass is 80367 ± 16 MeV, which improves on and supersedes the previous ATLAS measurement using the same dataset. The measured values of both the mass and the width are consistent with the Standard-Model predictions.

Future measurements of the W-boson width and mass using larger ATLAS datasets are expected to reduce the statistical and experimental uncertainties. Concurrently, advancements in theoretical predictions and a more refined understanding of parton distribution functions will help to reduce the theoretical uncertainties. As their measurements become ever more precise, physicists will be able to conduct yet more stringent tests of the Standard Model and probe for new particles and forces.

Comparison of the measured W-boson width with the Standard-Model prediction and with measurements from the LEP and Tevatron colliders. The vertical grey band illustrates the Standard-Model prediction, while the black dots and the associated horizontal bands represent the published experimental results. (Image: ATLAS/CERN) abelchio Wed, 04/10/2024 - 12:03 Byline ATLAS collaboration Publication Date Wed, 04/10/2024 - 11:57