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The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 marked a significant milestone in particle physics. Since then, the ATLAS and CMS collaborations have been diligently investigating the properties of this unique particle and searching to establish the different ways in which it is produced and decays into other particles.
At the Large Hadron Collider Physics (LHCP) conference this week, ATLAS and CMS report how they teamed up to find the first evidence of the rare process in which the Higgs boson decays into a Z boson, the electrically neutral carrier of the weak force, and a photon, the carrier of the electromagnetic force. This Higgs boson decay could provide indirect evidence of the existence of particles beyond those predicted by the Standard Model of particle physics.
The decay of the Higgs boson into a Z boson and a photon is similar to that of a decay into two photons. In these processes, the Higgs boson does not decay directly into these pairs of particles. Instead, the decays proceed via an intermediate "loop" of “virtual” particles that pop in and out of existence and cannot be directly detected. These virtual particles could include new, as yet undiscovered particles that interact with the Higgs boson.
The Standard Model predicts that, if the Higgs boson has a mass of around 125 billion electronvolts, approximately 0.15% of Higgs bosons will decay into a Z boson and a photon. But some theories that extend the Standard Model predict a different decay rate. Measuring the decay rate therefore provides valuable insights into both physics beyond the Standard Model and the nature of the Higgs boson.
Previously, using data from proton–proton collisions at the LHC, ATLAS and CMS independently conducted extensive searches for the decay of the Higgs boson into a Z boson and a photon. Both searches used similar strategies, identifying the Z boson through its decays into pairs of electrons or muons – heavier versions of electrons. These Z boson decays occur in about 6.6% of the cases.
In these searches, collision events associated with this Higgs boson decay (the signal) would be identified as a narrow peak, over a smooth background of events, in the distribution of the combined mass of the decay products. To enhance the sensitivity to the decay, ATLAS and CMS exploited the most frequent modes in which the Higgs boson is produced and categorised events based on the characteristics of these production processes. They also used advanced machine-learning techniques to further distinguish between signal and background events.
In a new study, ATLAS and CMS have now joined forces to maximise the outcome of their search. By combining the data sets collected by both experiments during the second run of the LHC, which took place between 2015 and 2018, the collaborations have significantly increased the statistical precision and reach of their searches.
This collaborative effort resulted in the first evidence of the Higgs boson decay into a Z boson and a photon. The result has a statistical significance of 3.4 standard deviations, which is below the conventional requirement of 5 standard deviations to claim an observation. The measured signal rate is 1.9 standard deviations above the Standard Model prediction.
“Each particle has a special relationship with the Higgs boson, making the search for rare Higgs decays a high priority,” says ATLAS physics coordinator Pamela Ferrari. "Through a meticulous combination of the individual results of ATLAS and CMS, we have made a step forward towards unravelling yet another riddle of the Higgs boson."
“The existence of new particles could have very significant effects on rare Higgs decay modes,” says CMS physics coordinator Florencia Canelli. “This study is a powerful test of the Standard Model. With the ongoing third run of the LHC and the future High-Luminosity LHC, we will be able to improve the precision of this test and probe ever rarer Higgs decays.”
angerard Fri, 05/26/2023 - 09:43 Publication Date Fri, 05/26/2023 - 11:00Atomic clocks are the world’s most precise timekeepers. Based on periodic transitions between two electronic states of an atom, they can track the passage of time with a precision as high as one part in a quintillion, meaning that they won’t lose or gain a second over 30 billion years – more than twice the age of the Universe.
In a paper published today in Nature, an international team at CERN’s nuclear physics facility, ISOLDE, reports a key step towards building a clock that would be based on a periodic transition between two states of an atomic nucleus – the nucleus of an isotope of the element thorium, thorium-229.
Such a nuclear clock could be more precise than today’s most precise atomic clocks, thanks to the different size and constituents of a nucleus compared to those of an atom. In addition, it could serve as a sensitive tool with which to search for new phenomena beyond the Standard Model, currently the best description there is of the subatomic world. For instance, it could allow researchers to look for variations over time of fundamental constants of nature and to search for ultralight dark matter.
Artist’s impression of a nuclear clock. (Image: APS/Ann. Phys. 531, 1800381 (2019))Ever since 2003, when Ekkehard Peik and Christian Tamm proposed a nuclear clock based on the transition between the ground state of the thorium-229 nucleus and the first, higher-energy state (called an isomer), researchers have been racing to observe and characterise this nuclear transition.
In the two decades, researchers have measured with ever increasing precision the isomer’s energy, the precise value of which is required to develop lasers to drive the transition to the isomer. However, despite much effort, they have not succeeded in observing the light emitted in the transition from the isomer to the ground state. This phenomenon, known in nuclear physicists’ parlance as the radiative decay of the isomer, which has a relatively long lifetime, is a key ingredient in developing a nuclear clock, because it would allow, among other things, the isomer’s energy to be determined with higher precision.
A team working at ISOLDE has now achieved this feat by producing thorium-229 nuclei in the isomeric state in a novel way and investigating the nuclei using a technique called vacuum-ultraviolet spectroscopy. The wavelength of the observed light corresponds to an isomer’s energy of 8.338 electronvolts (eV) with an uncertainty of 0.024 eV – a value that is seven times more precise than the previous most precise measurements.
Significant to the team’s success was the production of isomeric thorium-229 nuclei via the so-called beta decay of actinium-229 isotopes, which were made at ISOLDE and incorporated in calcium fluoride or magnesium fluoride crystals.
“ISOLDE is currently one of only two facilities in the world that can produce actinium-229 isotopes,” says the main author of the paper, Sandro Kraemer. “By incorporating these isotopes in calcium fluoride or magnesium fluoride crystals, we produced many more isomeric thorium-229 nuclei and increased our chances of observing their radiative decay.”
The novel approach of producing thorium-229 nuclei also made it possible to determine the lifetime of the isomer in the magnesium fluoride crystal. Knowledge of this lifetime is needed to predict the precision of a thorium-229 nuclear clock based on this solid-state system. The long lifetime that was measured, namely 16.1 minutes with an uncertainty of 2.5 minutes, confirms theoretical estimates and indicates that a clock precision competitive with that of today’s most precise atomic clocks is attainable.
“Solid-state systems such as magnesium fluoride crystals are one of two possible settings in which to build a future thorium-229 nuclear clock” says the team’s spokesperson, Piet Van Duppen. “Our study marks a crucial step in this direction, and it will facilitate the development of the lasers needed to drive the periodic transition that would make such a clock tick.”
ISOLDE takes a solid tick forward towards a nuclear clock. (Video: CERN)
ssanchis Tue, 05/23/2023 - 14:37 Publication Date Wed, 05/24/2023 - 17:05
Where did all the antimatter go? After the Big Bang, matter and antimatter should have been created in equal amounts. Why we live in a Universe of matter, with very little antimatter, remains a mystery. The excess of matter could be explained by the violation of charge-parity (CP) symmetry, which essentially means that certain processes that involve particles behave differently to those that involve their antiparticles.
However, the CP-violating processes that have been observed so far are insufficient to explain the matter–antimatter asymmetry in the Universe. New sources of CP violation must be out there – and might be hiding in interactions involving the Higgs boson. In the Standard Model of particle physics, Higgs-boson interactions with other particles conserve CP symmetry. If researchers find signs of CP violation in these interactions, they could be a clue to one of the Universe's oldest mysteries.
In a new analysis of its full dataset from Run 2 of the LHC, the ATLAS collaboration tested the Higgs-boson interactions with the carriers of the weak force, the W and Z bosons, looking for signs of CP violation. The collaboration studied Higgs-boson decays into two Z bosons, each of which transforms into a pair of leptons (an electron and a positron or a muon and an antimuon), thus resulting in four charged leptons. The researchers also studied interactions in which two W or Z bosons combine to produce a Higgs boson. In this case, one quark and one antiquark are produced together with the Higgs boson, creating ‘jets’ of particles in the ATLAS detector.
These interactions are ideal testing grounds for CP violation. When CP symmetry is conserved, the pattern of behaviour of the detected jets and leptons should be the same when particles are exchanged with their antiparticles and their directions of flight are reversed. However, if CP symmetry is violated, particles and antiparticles behave differently.
ATLAS scientists summarise all the information about the particles detected in these processes in a single number: the optimal observable. A special feature of this observable is that its value measured for antiparticles should be equal but opposite in sign to that of the particles. If a process conserves CP symmetry, the mean value of the optimal observable in the data should be zero. If it doesn’t, the mean value would shift away from zero.
In its new analysis, ATLAS used the observed values of the optimal observable to directly place limits on the possible amount of CP violation. The researchers also measured how often each value of the optimal observable occurred in the data, after correcting for any experimental effects. This measurement enabled ATLAS to compare the data with theoretical predictions in a model-independent way and to test the validity of the underlying theoretical assumptions. This is the first time that a measurement of a Higgs-boson decay into four leptons has allowed physicists to detect potential signs of CP violation in a model-independent way, without strongly relying on aspects of the Standard Model prediction other than CP symmetry.
All the results look compatible with the Standard Model expectation, representing another important confirmation of the current theory of nature. However, this is just the first step. Small CP-violating signals remain compatible with the data, and ATLAS is already collecting new collision data at unprecedented energies that will allow the precision of these measurements to be increased – further homing in on the nature of the Higgs boson.
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Find out more on the ATLAS website.
abelchio Fri, 04/21/2023 - 09:47 Byline ATLAS collaboration Publication Date Fri, 04/21/2023 - 09:39When atomic nuclei such as gold or lead nuclei collide at high energy in particle colliders, they can produce quark–gluon plasma (QGP) – a hot and dense state of matter predicted to have existed shortly after the Big Bang. One of the key features of QGP formation in such heavy-ion collisions is a long-range spatial correspondence, or correlation, between the particles that are created in the collisions. This collective phenomenon, which manifests as a ridge-like shape in data plots and is known as the ridge, was first observed in 2005 in heavy-ion collisions at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory in the US, and has since been observed at CERN’s Large Hadron Collider (LHC) in smaller collision systems such as collisions between protons.
At the Rencontres de Moriond conference today, the ALICE collaboration reported the observation of a ridge correlation in the simplest collision system yet. The result brings physicists a step closer to finding the origin of QGP-like collective phenomena in small collision systems.
The first observation of a ridge correlation in collisions other than heavy-ion collisions was made in 2010 by the CMS collaboration in “high-multiplicity” proton collisions that produce a relatively large number of particles. Soon after, CMS, ALICE and ATLAS observed the phenomenon also in collisions between protons and lead nuclei. These observations came as a surprise – such collision systems were expected to be too small and simple to develop QGP-like collective behaviour. Further studies have shown that the observed ridge correlations are indeed collective in nature, but the exact mechanisms that underpin this collective behaviour in these smaller and simpler systems remain to be identified.
Number of particle pairs (vertical axis) along two angular directions. A ridge-like shape is seen on the nearside on both sides of the peak. (Image: ALICE collaboration)In its latest study, the ALICE collaboration set out to investigate whether a ridge correlation also occurs in “low-multiplicity” proton collisions that create a relatively small number of particles. The ALICE researchers analysed a large sample of proton collisions recorded by the collaboration during the second run of the LHC to investigate how the ridge effect depends on the number of particles produced in the collisions. They then plotted in a graph the number of particle pairs produced in a set of low-multiplicity collisions along two angular directions relative to the collision axis, and found a clear ridge-like shape.
Next, the ALICE team examined how the number of particle pairs associated with the ridge varied with multiplicity, and compared the results with previous results from electron–positron collisions recorded by the ALEPH experiment at the Large Electron–Positron Collider, the LHC’s predecessor. This comparison showed that, for the same multiplicity, the ridge correlation in proton collisions is stronger than that deduced for electron–positron collisions, in which no ridge correlation has so far been seen.
These new ALICE results, as well as future studies based on data from the third run of the LHC, should help physicists identify the mechanisms that govern collective behaviour in small collision systems.
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Find out more on the ALICE website.
abelchio Fri, 03/31/2023 - 11:15 Byline Ana Lopes Publication Date Fri, 03/31/2023 - 11:07We have all wished for one extra hour in the day. Now, this may become a reality, thanks to new measurements on the second from the BETA experiment at CERN’s Antimatter Factory. The experiment used the caesium fountain clock – one of the most precise clocks in the world, used to define SI units and help set coordinated universal time (UTC) – to precisely measure the second using caesium. In conjunction with this clock, BETA also used anticaesium atoms produced by the Antimatter Factory to compare the two definitions of the second given by matter and antimatter. By taking the average, scientists say that timekeeping can now be more accurate than ever before.
“It is about time we updated the definition of a second using antimatter,” says Dr Qui, spokesperson of the BETA experiment. “This way, several uncertainties cancel out, making the measurement much more precise.”
A brief history of timekeepingMore accurate methods to measure time have always been ticking along. Before the first clocks, humans relied on the geographical position of Earth’s rotation around its axis to determine time. The first clocks contained pendulums, which were also unreliable due to their damping. In 1932, the quartz clock was invented, which was much more accurate. However, the resonant frequency of quartz, which sends the electric signals to drive the clock, can change due to environmental factors. This makes them lose precision.
In 1967, the first atomic clock was invented. These are much more precise, because atoms from the same element will always have the same properties. The caesium atomic clock works by an oscillator sending a wave with a frequency of exactly 9 192 631 770 Hz (using the old second definition), which is the frequency needed to excite the caesium atoms. If the oscillator is incorrect, the non-excited atoms cause an electric signal to jolt the oscillator, creating a feedback loop to keep the clock running. An atomic clock will lose only one second in 138 million years.
BETA’s new measurement on anticaesium found its excitation frequency to be smaller than that of caesium: 8 499 682 790 oscillations of the oscillator were needed to excite its atoms. By taking the average of matter and antimatter, BETA scientists calculated the second to 8 846 157 280 oscillations: around 96% of the current definition. This means the day would last 24 hours, 56 minutes and 24 seconds. This would be rounded up to 25 hours during the week and rounded down during the weekend.
“We hope that this new measurement will make all our lives easier,” Qui continues. “Using antimatter, time will fly by more slowly.”
Good timing...This measurement comes at a time when many countries are investigating the feasibility of a four-day working week. “Having an extra hour in the day means that a 40-hour work week can now be easily implemented, as well as a three-day weekend,” says Anita Chronon, from the HR department. “CERN staff will be trialling this new way of measuring time on Monday.”
ndinmore Fri, 03/31/2023 - 10:27 Publication Date Sat, 04/01/2023 - 09:00The discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 was a triumph of theoretical and experimental physics, yet its implications are only just beginning to be understood. Precise measurements by the ATLAS and CMS collaborations show that this fundamental particle, which is responsible for generating the masses of elementary particles, behaves as predicted by the half-century-old Standard Model of particle physics. But where does the Higgs boson come from? And why is it so light that the LHC is able to produce it in droves? Such conundrums were discussed during a week-long workshop, Exotic Approaches to Naturalness, hosted by the CERN Theoretical Physics department from 30 January to 3 February.
The Higgs boson is the simplest known particle: a “fragment of vacuum” with no charge or spin. As with all elementary particles, it is an excitation, or quantum, of a more fundamental entity called a field – the uniquely featureless Brout–Englert–Higgs field, which fills all space uniformly. This field is understood to have come into existence during an epochal “electroweak” phase transition a fraction of a nanosecond after the Big Bang; whereas, previously, elementary particles such as the electron had moved at the speed of light, they were forever after forced to interact with this quantum molasses, which imbued them with the property of mass. But if this picture is true, the Higgs boson itself should gain mass from the interactions of known particles with its parent field. Totting up these so-called quantum corrections would suggest a value for the Higgs-boson mass that is many orders of magnitude larger than is observed. Apart from putting it beyond the reach of any conceivable experiment, such a heavyweight Higgs would not allow the universe as we know it to have formed.
Aware of this paradox (called the electroweak hierarchy problem) long before the Higgs boson was discovered, and guided by the possible existence of particles and forces beyond those described by the Standard Model, physicists have come up with various explanations. One is that the Higgs boson is made of more basic entities held together by very strong forces, which circumvents the impact of quantum corrections. Another is that space-time possesses additional “supersymmetric” dimensions that would imply the existence of an entirely new mirror-world of particles that cancel out the troublesome quantum corrections from standard ones. So far, however, no evidence for such “natural” solutions to the electroweak hierarchy problem has been found.
Enter Exotic Approaches to Naturalness, which drew on such concepts as generalised symmetries, ultraviolet/infrared mixing, weak-gravity conjectures and “magic zeroes” to try to explain the Higgs boson’s mass, and other unnatural numbers in physics. If the language is abstruse, it’s because participants of the February workshop were encouraged to challenge conventions and to plant seeds of ideas at the edge of knowledge – including those that reject the concept of naturalness entirely. The latter would be a radical break from past successes. After all, the mass of the Higgs boson is not the only seemingly unnatural number in nature: where physicists were once perplexed about why the electrical energy of the electron does not grow infinitely large at short distances, for instance, the mystery vanished with the discovery that the electron has an antimatter partner, the positron, that cancels out the unphysical divergence. The unnatural mass of the Higgs boson might even be linked to the exceedingly small but non-zero value of the cosmological constant, which is responsible for the accelerating expansion of the universe.
“This workshop provided us with a fantastic forum to bring a fresh perspective on naturalness problems, both in a variety of physical systems and for particle physics specifically,” says workshop co-organiser Tim Cohen of CERN. “Our community has been pondering the Higgs’s naturalness problem for decades, and yet many of us suspect that we have not found the right idea yet. If we can eventually understand how nature has addressed the electroweak hierarchy problem, there is a very high likelihood of learning something that will change our perspective on fundamental physics, and the reductionist philosophy that has served us since the beginning of our discipline.”
While theorists let their imaginations run free, the conclusion of the CERN workshop was clear: the path ahead will be guided by data. Larger samples of Higgs bosons to be collected by ATLAS and CMS in the coming years – and by experiments at a dedicated “Higgs factory” proposed to follow the LHC – will enable physicists to study the unique interaction of the Higgs boson with itself. This will provide information about the precise shape and form of the Brout–Englert–Higgs field and the nature of the electroweak phase transition, and possibly tell us whether the Higgs boson is natural or weirdly fine-tuned for our existence.
katebrad Thu, 03/30/2023 - 13:05 Byline Matthew Chalmers Publication Date Thu, 03/30/2023 - 12:54Today, at the Moriond conference, the ATLAS and CMS collaborations have both presented the observation of a very rare process: the simultaneous production of four top quarks. They were observed using data from collisions during Run 2 of the Large Hadron Collider (LHC). Both experiments’ results pass the required five-sigma statistical significance to count as an observation – ATLAS’s observation with 6.1 sigma, higher than the expected significance of 4.3 sigma, and CMS’s observation with 5.5 sigma, higher than the expected 4.9 sigma – making them the first observations of this process.
The top quark is the heaviest particle in the Standard Model, meaning it is the particle with the strongest ties to the Higgs boson. This makes top quarks ideal for looking for signs of physics beyond the Standard Model.
There are a variety of ways to produce a top quark. Most commonly, they are observed in quark and antiquark pairs, and occasionally on their own. According to Standard Model theory, four top quarks – consisting of two top quark–antiquark pairs – can be produced simultaneously. The rate of production is, however, predicted to be 70 thousand times lower than that of top quark–antiquark pairs, which makes four-top-quark production elusive. Evidence for this phenomenon has previously been found by ATLAS in 2020 and 2021, and by CMS in 2022. However, until today, there had never been an observation.
As well as being rare, four-top-quark production is notoriously difficult to detect. When physicists search for a particular event, they look for its “signature”: the properties of the final particles of a decay. These provide clues to the short-lived events they are looking for. Every top quark decays into a W boson and a bottom quark. The W boson can then decay into either a charged lepton and a neutrino or a quark–antiquark pair. This means that the signature of four-top-quark events can be highly varied, containing from zero to four charged leptons and up to 12 jets produced by the quarks. This makes looking for the signature of four-top-quark production challenging.
To help search for these events, both ATLAS and CMS used novel machine-learning techniques to build the algorithms that select four-top-quark candidate events. The analyses use the spectacular four-top-quark signature with multiple electrons, muons and (bottom-quark-tagged) jets to separate the events with four top quarks from the background due to other Standard Model processes with larger production rates. Both ATLAS and CMS searched for event signatures containing two or more leptons.
The first direct observation of four-top-quark production is an exciting new step in learning more about this fascinating particle. Both experiments look forward to continuing to study this phenomenon during LHC Run 3.
Read more:
ndinmore Fri, 03/24/2023 - 10:23 Byline Naomi Dinmore Publication Date Fri, 03/24/2023 - 09:36The International Union of Pure and Applied Physics (IUPAP) will hold its 8th International Conference on Women in Physics (ICWIP 2023), on 10–14 July. Registrations are now open for this virtual event, with India as the host country. The conference is jointly organised by the Gender in Physics working group of the Indian Physics Association and the Tata Institute of Fundamental Research (TIFR). It will be hosted by the Homi Bhabha Centre for Science Education (HBCSE) – a national centre of TIFR, which promotes quality and equity in science and mathematics education from primary school to introductory college levels.
The programme will consist of plenary sessions, interactive workshops, poster presentations, and networking sessions. In addition to the country papers depicting the status of women in physics, registered participants can make contributions related to physics, physics education and gender issues.
For more information and to register, go to: https://icwip2023.hbcse.tifr.res.in/.
anschaef Wed, 03/22/2023 - 12:24 Publication Date Wed, 03/22/2023 - 12:19Although neutrinos are produced abundantly in collisions at the Large Hadron Collider (LHC), until now no neutrinos produced in such a way had been detected. Within just nine months of the start of LHC Run 3 and the beginning of its measurement campaign, the FASER collaboration changed this picture by announcing its first observation of collider neutrinos at this year’s electroweak session of the Rencontres de Moriond. In particular, FASER observed muon neutrinos and candidate events of electron neutrinos. “Our statistical significance is roughly 16 sigma, far exceeding 5 sigma, the threshold for a discovery in particle physics,” explains FASER’s co-spokesperson Jamie Boyd.
In addition to its observation of neutrinos at a particle collider, FASER presented results on searches for dark photons. With a null result, the collaboration was able to set limits on previously unexplored parameter space and began to exclude regions motivated by dark matter. FASER aims to collect up to ten times more data over the coming years, allowing more searches and neutrino measurements.
FASER is one of two new experiments situated at either side of the ATLAS cavern to detect neutrinos produced in proton collisions in ATLAS. The complementary experiment, SND@LHC, also reported its first results at Moriond, showing eight muon neutrino candidate events. “We are still working on the assessment of the systematic uncertainties to the background. As a very preliminary result, our observation can be claimed at the level of 5 sigma,” adds SND@LHC spokesperson Giovanni De Lellis. The SND@LHC detector was installed in the LHC tunnel just in time for the start of LHC Run 3.
Until now, neutrino experiments have only studied neutrinos coming from space, Earth, nuclear reactors or fixed-target experiments. While astrophysical neutrinos are highly energetic, such as those that can be detected by the IceCube experiment at the South Pole, solar and reactor neutrinos generally have lower energies. Neutrinos at fixed-target experiments, such as those from the CERN North and former West Areas, are in the energy region of up to a few hundred gigaelectronvolts (GeV). FASER and SND@LHC will narrow the gap between fixed-target neutrinos and astrophysical neutrinos, covering a much higher energy range – between a few hundred GeV and several TeV.
One of the unexplored physics topics to which they will contribute is the study of high-energy neutrinos from astrophysical sources. Indeed, the production mechanism of the neutrinos at the LHC, as well as their centre-of-mass energy, is the same as for the very-high-energy neutrinos produced in cosmic-ray collisions with the atmosphere. Those “atmospheric” neutrinos constitute a background for the observation of astrophysical neutrinos: the measurements by FASER and SND@LHC can be used to precisely estimate that background, thus paving the way for the observation of astrophysical neutrinos.
Another application of these searches is measuring the production rate of all three types of neutrinos. The experiments will test the universality of their interaction mechanism by measuring the ratio of different neutrino species produced by the same type of parent particle. This will be an important test of the Standard Model in the neutrino sector.
ckrishna Wed, 03/22/2023 - 10:42 Byline Kristiane Bernhard-Novotny Chetna Krishna Publication Date Wed, 03/22/2023 - 10:25Geneva, 23 March 2023. The W boson, a fundamental particle that carries the charged weak force, is the subject of a new precision measurement of its mass by the ATLAS experiment at CERN.
The preliminary result, reported in a new conference note presented today at the Rencontres de Moriond conference, is based on a reanalysis of a sample of 14 million W boson candidates produced in proton–proton collisions at the Large Hadron Collider (LHC), CERN’s flagship particle accelerator.
The new ATLAS measurement concurs with, and is more precise than, all previous W mass measurements except one – the latest measurement from the CDF experiment at the Tevatron, a former accelerator at Fermilab.
Together with its electrically neutral counterpart, the Z boson, the electrically charged W boson mediates the weak force, a fundamental force that is responsible for a form of radioactivity and initiates the nuclear fusion reaction that powers the Sun.
The particle’s discovery at CERN 40 years ago helped to confirm the theory of the electroweak interaction that unifies the electromagnetic and weak forces. This theory is now a cornerstone of the Standard Model of particle physics. CERN researchers who enabled the discovery were awarded the 1984 Nobel Prize in physics.
Since then, experiments at particle colliders at CERN and elsewhere have measured the W boson mass ever more precisely. In the Standard Model, the W boson mass is closely related to the strength of the electroweak interactions and the masses of the heaviest fundamental particles, including the Z boson, the top quark and the Higgs boson. In this theory, the particle is constrained to weigh 80354 million electronvolts (MeV), within an uncertainty of 7 MeV.
Any deviation of the measured mass from the Standard Model prediction would be an indicator of new physics phenomena, such as new particles or interactions. To be sensitive to such deviations, mass measurements need to be extremely precise.
In 2017, ATLAS released its first measurement of the W boson mass, which was determined using a sample of W bosons recorded by ATLAS in 2011, when the LHC was running at a collision energy of 7 TeV. The W boson mass came out at 80370 MeV, with an uncertainty of 19 MeV.
At the time, this result represented the most precise W boson mass value ever obtained by a single experiment, and was in good agreement with the Standard Model prediction and all previous experimental results, including those from experiments at the Large Electron–Positron Collider (LEP), the LHC’s predecessor at CERN.
Last year, the CDF collaboration at Fermilab announced an even more precise measurement, based on an analysis of its full dataset collected at the Tevatron. The result, 80434 MeV with an uncertainty of 9 MeV, differed significantly from the Standard Model prediction and from the other experimental results, calling for more measurements to try to identify the cause of the difference.
In its new study, ATLAS reanalysed its 2011 sample of W bosons, improving the precision of its previous measurement. The new W boson mass, 80360 MeV with an uncertainty of 16 MeV, is 10 MeV lower than the previous ATLAS result and 16% more precise. The result is in agreement with the Standard Model.
Comparison of the measured value of the W boson mass with other published results. The vertical bands show the Standard Model prediction, and the horizontal bands and lines show the statistical and total uncertainties of the results (Image: CERN)To attain this result, ATLAS used an advanced data-fitting technique to determine the mass, as well as more recent, improved versions of what are known as the parton distribution functions of the proton. These functions describe the sharing of the proton’s momentum amongst its constituent quarks and gluons. In addition, ATLAS verified the theoretical description of the W boson production process using dedicated LHC proton–proton runs.
“Due to an undetected neutrino in the particle’s decay, the W mass measurement is among the most challenging precision measurements performed at hadron colliders. It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modelling uncertainties,” says ATLAS spokesperson Andreas Hoecker. “This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions.”
Further measurements of the W boson mass are expected from ATLAS and CMS and from LHCb, which has also recently weighed the boson.
Further information
Video news release : https://videos.cern.ch/record/2297554
News clip : https://videos.cern.ch/record/2297560
ATLAS images gallery : https://home.cern/resources/image/experiments/atlas-images-gallery
gfabre Tue, 03/21/2023 - 14:38 Publication Date Thu, 03/23/2023 - 12:00
The Hellenic Society for the Study of High Energy Physics (HeSSHEP) promotes the scientific work of the Greek scientists, informs the general public and the Greek state on matters concerning the subject of High Energy Physics, organizes an workshop and conferences.
