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Exactly four decades ago today, on 25 January 1983, physicists at CERN announced to the world that they had observed a new elementary particle – the W boson. Together with its electrically neutral counterpart, the Z boson, which was discovered later in the same year, the electrically charged W boson mediates the weak force, one of nature’s four fundamental forces.
Through this force, the W boson enables the nuclear fusion reaction that powers the Sun, without which life as we know it would not be possible. The W boson is also responsible for a form of radioactivity, called radioactive beta decay, that is widely used in medicine.
The W boson’s discovery was the result of an idea proposed in 1976 by Carlo Rubbia, Peter McIntyre and David Cline. The trio of physicists suggested converting CERN’s largest accelerator at the time, the Super Proton Synchrotron (SPS), from an accelerator of protons into a machine to collide protons and antiprotons (the protons’ antimatter equivalents) at a high enough energy to produce W and Z bosons. Together with Simon van der Meer’s ingenious “stochastic cooling” technique, which made it possible to reduce the size and increase the density of a proton and, later, an antiproton beam, this bold idea allowed the UA1 and UA2 experiments that were built around the converted SPS to begin hunting for the W and Z bosons in 1981.
Two years later, in a seminar on 20 January 1983 held in CERN’s Main Auditorium, Rubbia, spokesperson of the UA1 collaboration, revealed six candidate collision events for the W boson. The following afternoon, Luigi Di Lella of the UA2 collaboration presented four candidate W events and, on 25 January 1983, CERN delivered the news of the discovery of the new particle to the world.
And if that wasn’t enough to celebrate and crown the success of the converted SPS, the W boson discovery was followed a few months later by that of the Z boson, indirect evidence for which had been obtained a decade earlier at CERN’s Gargamelle bubble chamber.
The observations of the W and Z bosons further confirmed the theory of the electroweak interaction that unifies the electromagnetic and weak forces and demands the existence of the Higgs boson, which was found at the Large Hadron Collider (LHC) in 2012. Developed in the 1960s by Sheldon Glashow, Abdus Salam and Steven Weinberg and cemented in the 1970s by Gerard ‘t Hooft and Martinus Veltman, this theory is now a cornerstone of the Standard Model of particle physics.
The W and Z discoveries were recognised with the 1984 Nobel Prize in Physics for Rubbia and Van der Meer, and helped secure the decision to build CERN’s next big accelerator, the Large Electron–Positron Collider (LEP), which went on to study the W and Z bosons in detail.
Forty years on, and after many investigations at LEP and other colliders, including the LHC, the W and Z bosons continue to show their stripes and provide physicists with new ways of exploring the properties and behaviour of matter at the smallest scales.
To give a couple of examples, in 2021 the ATLAS collaboration reported the observation of the rare simultaneous production of three W bosons, and CMS obtained a high-precision measurement of the transformation of Z bosons into invisible particles. And in 2022, based on data collected by the former Tevatron accelerator, the CDF collaboration announced the most precise ever measurement of the W boson mass. However, the CDF W boson mass value is in tension with previous results, including the first at the LHC by ATLAS and LHCb, calling for new measurements with increased precision.
Research into these and other facets of the W and Z bosons will continue at the LHC and its planned upgrade, the High-Luminosity LHC.
Carlo Rubbia, spokesperson of the UA1 collaboration, revealing six candidate W boson events in a seminar on 20 January 1983. (Video: CERN)
Read the CERN Courier article remembering the discovery of the particle.
abelchio Tue, 01/24/2023 - 15:52 Byline Ana Lopes Publication Date Wed, 01/25/2023 - 10:00When the Large Hadron Collider (LHC) is operating, it produces more than one billion proton–proton interactions every second. But exactly how many take place in the LHC experiments? Critical to every analysis of LHC data is a high-precision measurement of what is known as luminosity, that is, the total number of proton–proton interactions in a given dataset. It allows physicists to evaluate the probability of interesting proton–proton collision events occurring, as well as to predict the rates of similar-looking background processes. Isolating such events from the background processes is crucial for both searches for new phenomena and precision measurements of known Standard Model processes.
The ATLAS collaboration has recently released its most precise luminosity measurement to date. They studied data taken over the course of four years (2015–2018), covering the entire Run 2 of the LHC, to assess the total amount of luminosity delivered to the ATLAS experiment in that dataset.
What exactly did this measurement entail? When proton beams circulate in the LHC, they are arranged in “bunches” each containing more than 100 billion protons. As two bunches circulating in opposite directions cross, some of the protons interact. Determining how many interactions there are in each bunch crossing provides a measure of the luminosity. Its value depends on the number of protons per bunch, how tightly squeezed the protons are and the angle at which the bunches cross. The luminosity also depends on the number of colliding proton bunches in each beam.
ATLAS has several detectors that are sensitive to the number of particles produced in proton–proton interactions, and the average number of measured particles is often proportional to the average number of proton–proton interactions per bunch crossing. Researchers can therefore use this average to monitor the “instantaneous” luminosity in real time during data-taking periods, and to measure the cumulative (“integrated”) luminosity over longer periods of time.
While ATLAS’s luminosity-sensitive detectors provided relative measurements of the luminosity during data taking, measurement of the absolute luminosity required a special LHC beam configuration that allows the detector signals to be calibrated. Once a year, the LHC proton beams are displaced from their normal position in order to record the particle counts in the luminosity detectors. This method is called a van der Meer beam separation scan, named after physics Nobel Prize winner Simon van der Meer, who developed the idea in the 1960s for application at CERN’s Intersecting Storage Rings. It allows researchers to estimate the size of the beam and measure how densely the protons are packed in the bunches. With that information in hand, they can calibrate the detector signals.
Working in close collaboration with ATLAS researchers, LHC experts carried out van der Meer scans under low-luminosity conditions, with an average of about 0.5 proton–proton interactions per bunch crossing and very long gaps between the bunches. For comparison, the LHC typically operates with 20–50 interactions per bunch crossing, and with bunches closer together in a “train” structure. The researchers therefore need to extrapolate the results of the van der Meer scans to the normal data-taking regime using the measurements from the luminosity-sensitive detectors.
Using this approach, and after careful evaluation of the systematic effects that can influence a luminosity measurement, ATLAS physicists determined the integrated luminosity of the full Run 2 dataset that had been recorded by ATLAS and certified as good for physics analysis, to be 140.1 ± 1.2 fb–1. For comparison, 1 inverse femtobarn (fb–1) corresponds to about 100 trillion proton–proton collisions. With its uncertainty of 0.83%, the result represents the most precise luminosity measurement at a hadron collider to date. It improves upon previous ATLAS measurements by a factor of 2 and is comparable with results achieved at the ISR experiments (0.9%).
abelchio Tue, 01/24/2023 - 10:12 Byline ATLAS collaboration Publication Date Tue, 01/24/2023 - 09:59Today the international LHCb collaboration at the Large Hadron Collider (LHC) presented new measurements of rare particle transformations, or decays, that provide one of the highest-precision tests yet of a key property of the Standard Model of particle physics, known as lepton flavour universality. Previous studies of these decays had hinted at intriguing tensions with the theoretical predictions, potentially due to the effects of new particles or forces. The results of the improved and wider-reaching analysis based on the full LHC dataset collected by the experiment during Run 1 and Run 2, which were presented at a seminar at CERN held this morning, are in line with the Standard Model expectation.
A central mystery of particle physics is why the 12 elementary quarks and leptons are arranged in pairs across three generations that are identical in all but mass, with ordinary matter comprising particles from the first, lightest generation. Lepton flavour universality states that the fundamental forces are blind to the generation to which a lepton belongs. In recent years, however, an accumulation of results from LHCb and experiments in Japan and the US have suggested that this might not be the case, generating cautious excitement among physicists that a more fundamental theory – perhaps one that sheds light on the Standard Model’s mysterious flavour structure – might reveal itself at the LHC.
Interest in the “flavour anomalies” peaked in March 2021, when LHCb presented new results comparing the rates at which certain B mesons, composite particles that contain beauty quarks, decay into muons and electrons. According to the theory, decays involving muons and electrons should occur at the same rate, once differences in the leptons’ masses are accounted for. But the LHCb results hinted that B mesons decay into muons at a lower rate than predicted, as indicated by the results’ statistical significance of 3.1 standard deviations from the Standard Model prediction.
The new LHCb analysis, which has been ongoing for the past five years, is more comprehensive. It considers two different B-meson decay modes simultaneously for the first time and provides better control of the background processes that can mimic the decays of B-mesons to electrons. In addition, the two decay modes are measured in two different mass regions, thus yielding four independent comparisons of the decays. The results, which supersede previous comparisons, are in excellent agreement with the principle of lepton flavour universality.
“Measurements of the ratios of rare B-meson decays to electrons and muons have generated much interest in recent years because they are theoretically ‘clean’ and show consistency with a pattern of anomalies seen in other flavour processes,” explains LHCb spokesperson Chris Parkes of the University of Manchester and CERN. “The results shown today are the product of a comprehensive study of the two main modes using our full data sample and applying new, more robust techniques. These results are compatible with the expectation of our theory.”
New datasets will allow LHCb – one of the four large experiments at the LHC at CERN – to investigate lepton flavour universality further, in addition to conducting a wider research programme that includes studies of new hadrons, including the search for exotic tetraquarks and pentaquarks and investigation of the differences between matter and antimatter. An upgraded version of the experiment now in operation for LHC Run 3 will collect larger datasets that will allow even higher-precision tests of rare particle decays.
“Earlier LHCb indications of anomalies concerning lepton flavour universality triggered excitement,” says theoretical physicist Michelangelo Mangano of CERN. “That such anomalies could potentially have been real shows just how much remains unknown, since theoretical interpretations exposed a myriad of unanticipated possible phenomena. The latest LHCb findings take nothing away from our mission to push the boundary of our knowledge further, and the search for anomalies, guided by experimental hints, goes on!”
Read more on the LHCb website and in the CERN Courier.
ndinmore Tue, 12/20/2022 - 09:56 Publication Date Tue, 12/20/2022 - 09:50After over three years of upgrade and maintenance work, the Large Hadron Collider began its third period of operation (Run 3) in July 2022. Since then, the world’s most powerful particle accelerator has been colliding protons at a record-breaking energy of 13.6 TeV. The ATLAS collaboration has just released its first measurements of these record collisions, studying data collected in the first half of August 2022.
The researchers measured the rates of two well-known processes: the production of top-quark pairs and the production of a Z boson, which proceed through strong and electroweak interactions, respectively. The ratio of their cross sections is sensitive to the inner structure of the proton, and their measurement sets constraints on the relative probabilities that reactions are initiated by quarks and gluons.
These early measurements also validate the functionality of the ATLAS detector and its reconstruction software, which underwent many improvements in preparation for Run 3.
Physicists focused on Z-boson decays to electron and muon pairs, and on top-quark decays to a W boson and a jet – collimated sprays of particles – originating from a bottom quark. The W boson subsequently decays into one electron or muon and an invisible neutrino. As the analysis uses very early Run 3 data, physicists relied on preliminary calibrations of the leptons, jets and luminosity. These were derived promptly after the first data became available.
ATLAS measured a top-quark pair to Z boson production ratio that is consistent with the Standard Model prediction within the current experimental uncertainty of 4.7%.
The calibration and corresponding uncertainties will be improved as more data is processed. Future updates of the calibration will allow researchers to measure the cross sections with greater precision.
To validate their results, physicists performed a series of cross-checks. These included measuring the ratio of the cross section each time the LHC was injected with a new fill of protons for a data-taking run.
More analyses using the Run 3 data will follow, exploiting the unprecedented energies and the increased LHC data set.
Read more on the ATLAS website.
kbernhar Thu, 12/15/2022 - 17:06 Byline ATLAS collaboration Publication Date Fri, 12/16/2022 - 17:05The Tomalla prize is attributed every few years by the Tomalla Foundation for outstanding work in gravity research (see http://www.tomalla-foundation.ch/ for previous prize winners and other activities of the Tomalla Foundation).
The Italian physicist Alessandra Buonanno has been awarded the Tomalla Prize 2022 for her outstanding work
on gravitational wave physics, especially for the effective one-body approach to describe the gravitational waves emitted by binary black holes or neutron stars, but also for other important contributions relevant for the detection of gravitational waves.
Alessandra Buonanno studied physics in Pisa where she finished her PhD in 1996. After a brief period spent at the theory division of CERN, she held postdoctoral positions at the Institut des Hautes Etudes Scientifiques (IHES) in France and at the California Institute of Technology in the USA. She was a permanent researcher at the Institut d'Astrophysique de Paris (IAP) and Laboratoire Astroparticule et Cosmologie (APC) in Paris with CNRS before joining the University of Maryland as physics professor.
Since 2014 she is a director at the Max Planck Institute for Gravitational Physics in Potsdam, and holds a professorship position at the University of Maryland. Since 2017 she is also honorary professor at Berlin’s Humboldt University and at Potsdam University. She is member and principal investigator of the LIGO Scientific Collaboration, which discovered the first gravitational wave from two coalescing black holes in 2015.
In her theoretical work, Prof. Buonanno made seminal contributions to the modelling of gravitational
waves from the coalescence of compact-binary systems, most importantly two orbiting black holes, by combining analytical and numerical-relativity methods. A precise modelling of this signal is crucial to distinguish gravitational waves from the many contributions to the ‘noise’ in the detectors. In particular, Prof. Buonanno and her group contributed importantly to the generation of gravitational wave ‘templates’ which observers use to compare their signals with and interpret their astrophysical origin.
The Tomalla Prize ceremony 2022 will take place on December 16 at 17:30
in the Grand Auditoire de l’Ecole de Physique (24, Quai Ernest Ansermet, Genève)
The laureate will give a colloquium about her work.
The ceremony is open to the public.
anschaef Tue, 12/13/2022 - 10:26 Publication Date Tue, 12/13/2022 - 10:20The antimatter counterpart of a light atomic nucleus can travel a long distance in the Milky Way without being absorbed, shows the international ALICE collaboration in an article published today in Nature Physics. The finding, obtained by feeding data on antihelium nuclei produced at the Large Hadron Collider (LHC) into models, will help space- and balloon-based searches for antimatter that may have originated from dark matter.
Light antimatter nuclei such as antideuteron and antihelium have been produced on Earth, at particle accelerators, but they have yet to be observed with certainty coming from outer space. In space, such antinuclei, as well as antiprotons, could be created in collisions between cosmic rays and the interstellar medium, but they could also be produced when hypothetical particles that may make up the dark matter that pervades the Universe annihilate each other.
Space-based experiments such as AMS, which was assembled at CERN and is installed on the International Space Station, are therefore looking for light antimatter nuclei in an effort to search for dark matter, as will the upcoming GAPS balloon mission.
To find out whether dark matter is the source behind any potential detections of light antinuclei from outer space, physicists need to determine the number, or more precisely the “flux”, of light antinuclei that is expected to reach the near-Earth location of these experiments. This flux depends on features such as the exact type of antimatter source in our Galaxy and the rate at which it produces antinuclei, but also on the rate at which the antinuclei should later disappear through annihilation or absorption when they encounter normal matter on their journey to Earth.
The latter is where the new study from the ALICE collaboration comes in. By investigating how antihelium-3 nuclei1 produced in collisions of heavy ions and of protons at the LHC interact with the ALICE detector, the ALICE researchers were able to measure, for the first time, the rate at which antihelium-3 nuclei disappear when they encounter normal matter. In this analysis, the ALICE detector’s material serves as the normal matter with which the antinuclei interact.
Next, the ALICE researchers incorporated the obtained disappearance rate into a publicly available computer programme called GALPROP, which simulates the propagation of cosmic particles, including antinuclei, in the Galaxy. They considered two models of the flux of antihelium-3 nuclei expected near Earth after the nuclei’s journey from sources in the Milky Way. One model assumes that the sources are cosmic-ray collisions with the interstellar medium, and the other describes them as hypothetical dark-matter particles called weakly interacting massive particles (WIMPs).
For each model, the ALICE team then estimated the transparency of the Milky Way to antihelium-3 nuclei, that is, the Galaxy’s ability to let the nuclei through without being absorbed. They did so by dividing the flux obtained with and without antinuclei disappearance.
For the dark-matter model, the ALICE researchers obtained a transparency of about 50%, whereas for the cosmic-ray model the transparency ranged from 25% to 90% depending on the energy of the antinucleus. These transparency values show that antihelium-3 nuclei originating from dark matter or cosmic-ray collisions can travel long distances – of several kiloparsecs2 – in the Milky Way without being absorbed.
“Our results show, for the first time on the basis of a direct absorption measurement, that antihelium-3 nuclei coming from as far as the centre of our Galaxy can reach near-Earth locations,” says ALICE physics coordinator Andrea Dainese.
“Our findings demonstrate that searches for light antimatter nuclei from outer space remain a powerful way to hunt for dark matter,” says ALICE spokesperson Luciano Musa.
First measurement of the absorption of anti-3He nuclei in matter and impact on their propagation in the galaxy. (Video: ORIGINS Cluster, Technical University Munich)Further information:
1 Antihelium-3 nuclei are made up of two antiprotons and one antineutron, the antimatter equivalents of the proton and the neutron, respectively.
2 One kiloparsec is a thousand parsecs. One parsec is about 31 trillion kilometres.
ssanchis Fri, 12/09/2022 - 16:16 Publication Date Mon, 12/12/2022 - 17:00
With the accelerator complex having shut down for the year on 28 November, now is a good moment to take stock of the healthy position of our research. This year saw a wealth of new results across the programme. At the LHC, Run 1 and 2 data continue to deliver a rich seam of results. There has also been much to whet the appetite from the new data that the first period of Run 3 has delivered, and the non-LHC programme continues to thrive.
Throughout the year, we saw important new results coming from the full LHC Run 2 dataset. These included a measurement of the mass of the top quark with unparalleled precision from CMS and a measurement of top quark production along with a photon from ATLAS. This is a rare phenomenon that offers a tool for exploring new physics.
A highlight from the ALICE experiment was the first direct observation of a phenomenon known as the dead cone effect, which gives access to the mass of the charm quark. LHCb, meanwhile, continued to enlarge its inventory of new exotic particles, adding a new pentaquark and the first two tetraquarks ever to be observed. Such observations strengthen our understanding of the strong force that binds quarks together. LHCb’s capacity for precision was also on display in the measurement of the largest matter-antimatter asymmetry so far observed in particle decays.
As we marked the 10th anniversary of the discovery of the Higgs boson, ATLAS and CMS both published comprehensive papers detailing all that we’ve learned about this intriguing particle so far. A good indicator of how far we’ve come is how precisely both ATLAS and CMS have measured the basic properties of the Higgs – we now know its mass to a precision of around 0.1% and its lifetime has been measured to be around 10-24 seconds, just as predicted by the Standard Model. For its part, the ALICE collaboration published a review of its journey to date through the quark–gluon plasma.
All of these results came from the analysis of existing data but, as the 2022 run came to an end, both the ATLAS and CMS experiments have published results based on Run 3 data. This was made possible thanks to the exceptional performance of the LHC this year, as described in the first of a series of Run 3 reports in the Bulletin.
These are just a handful of the results I could have chosen to highlight from the LHC this year, and the fact that I had to choose is testimony to the fantastic performance of the accelerator, the detectors and the computing infrastructures, along with the inventiveness of those who run them and analyse the data.
Of course, there is much more to the CERN programme than the LHC. Again, I have to be selective but, among the highlights of 2022, I could mention measurements of antiprotonic helium from the BASE experiment at the AD or important work at ISOLDE on thorium-229, which could pave the way for ultra-precise “nuclear clocks”.
We’ve called time on 2022 running earlier than planned as a consequence of the energy crisis, and running next year will also be curtailed. This is important, necessary and not without pain for the experimental programme of this laboratory. However, what this year has shown is that the CERN community is able to rise to the challenge. Whatever 2023 has in store, I’m confident that more good physics will be a strong part of the mix.
thortala Tue, 12/06/2022 - 13:37 Byline Joachim Mnich Publication Date Tue, 12/06/2022 - 13:34Hypothetical particles called axions could solve two enigmas at once. They could account for dark matter, the mysterious substance that is thought to make up most of the matter in the Universe, and they could also explain the puzzling symmetry properties of the strong force that holds protons and neutrons together in atomic nuclei.
But the theoretical space of possibilities for axions is vast, both in terms of their mass and the strength of their interaction with other particles. Axion searches are therefore targeting different regions of this space, each search bringing with it the possibility of discovery and its results guiding future searches.
In a new paper published in Nature Communications, a team of researchers working on the CAST experiment at CERN report how they have repurposed part of the experiment to target a previously uncharted region of the axion space.
CAST was originally designed to hunt for axions originating from the Sun. In their new study, the CAST team placed a resonator consisting of four cavities inside one of the two bores of the experiment’s magnet in order to build an axion detector that looks instead for axions from the Milky Way’s “halo” of dark matter – an axion haloscope, which they named CAST-CAPP.
In a strong magnetic field, such as the one provided by CAST’s magnet, axions should convert into photons. An axion haloscope’s resonator is basically a radio that researchers can tune to find the frequency of these axion-converted photons. But the frequency of the axion “radio station” is not known, so the researchers must slowly scan a band of frequencies to try to identify the frequency of the axion signal.
The CAST-CAPP resonator can be tuned to pick up axion signals ranging from 4.774 to 5.434 GHz, corresponding to axion masses of between 19.74 and 22.47 microelectronvolts.
The CAST researchers scanned this 660 MHz band of frequencies in steps of 200 kHz for 4124 hours, from12 September 2019 to 21 June 2021, and isolated known background signals such as the 5 GHz Wireless Local Area Network (WLAN), but did not pick up any signal coming from axions. However, the CAST-CAPP data places new bounds on the maximum strength of the interaction of axions with photons for axion masses of 19.74 to 22.47 microelectronvolts, narrowing down the space in which to look for axion dark matter.
The new bounds are complementary to results from previous axion searches, including those from another CAST haloscope, RADES, which took data in 2018.
The hunt for dark matter continues. Tune in to this station again to check for updates from CAST-CAPP or from other dark-matter investigations taking place at CERN, such as searches for dark matter that may be produced at the Large Hadron Collider.
abelchio Fri, 12/02/2022 - 12:04 Byline Ana Lopes Publication Date Fri, 12/02/2022 - 11:57Since discovering the Higgs boson 10 years ago, the ATLAS and CMS collaborations have been carrying out precision measurements of its properties and its interactions with other particles, which have been consistent with predictions from the Standard Model. The Higgs boson’s mass, for instance, has been measured to be 125 billion electronvolts (GeV), with a precision of 0.1%. However, one property that remains inaccessible via direct measurements is the particle’s “width”, which determines its lifetime and, if found to deviate from its predicted value, would indicate the presence of new physics. At the recent Higgs 2022 conference and at a CERN seminar this week, the ATLAS collaboration presented the results of its latest study of this property.
Width is a fundamental parameter of any unstable particle with a finite lifetime – the shorter the lifetime, the broader the width. The Higgs boson's width, which represents the range of possible masses around the particle’s nominal mass of 125 GeV, is predicted to be 4.1 MeV – too small to be directly measured. However, its value can be determined by comparing the rate of Higgs boson production at the particle’s nominal mass (“on-shell” production) with that at much larger masses (“off-shell” production). This relies on the fact that the on-shell Higgs boson production rate depends not only on the Higgs boson’s interactions with other particles, but also on its width. By contrast, the off-shell rate is independent of the width.
In its new study, the ATLAS collaboration looked for off-shell Higgs boson production using proton–proton collision data collected during Run 2 of the Large Hadron Collider (LHC) from 2015 to 2018. In particular, ATLAS physicists searched for collision events where the Higgs boson transforms, or “decays”, into two Z bosons, which in turn decay into four charged leptons or two charged leptons plus two neutrinos, as thesedecay channels provided the highest sensitivity to the off-shell signal.
After isolating these events from those of background processes that resemble them but do not involve the Higgs boson, the researchers combined the results from both channels to measure the ratio of the off-shell Higgs boson production rate to its Standard-Model prediction. The data were found to be consistent with Standard Model predictions, rejecting the background-only hypothesis, which assumes no off-shell Higgs boson production, with an observed (expected) statistical significance of 3.2 (2.4) standard deviations. This result provides experimental evidence of off-shell Higgs boson production.
By combining these results with their previous on-shell Higgs boson measurements, the ATLAS researchers obtained a Higgs boson width of 4.6 ± 2.6 MeV, which is in agreement with the Standard Model expectation and corresponds to a particle lifetime of 180 yoctoseconds (1 yoctosecond is 10-24 seconds).
The results are compatible with those from a recent study by the CMS collaboration, which also found evidence of off-shell Higgs boson production and measured the particle’s width. With the increased collision energy and greater accumulated data expected from Run 3 of the LHC, more precise measurements of both the production process and the particle’s width are anticipated.
Read more on the ATLAS website.
abelchio Fri, 11/18/2022 - 11:18 Byline ATLAS collaboration Publication Date Fri, 11/18/2022 - 11:179 November, 2022
The ALICE collaboration takes stock of its first decade of quantum chromodynamics studies at the Large Hadron Collider
Quantum chromodynamics (QCD) is one of the pillars of the Standard Model of particle physics. It describes the strong interaction – one of the four fundamental forces of nature. This force holds quarks and gluons – collectively known as partons – together in hadrons such as the proton, and protons and neutrons together in atomic nuclei. Two hallmarks of QCD are chiral symmetry breaking and asymptotic freedom. Chiral symmetry breaking explains how quarks generate the masses of hadrons and therefore the vast majority of visible mass in the universe. Asymptotic freedom states that the strong force between quarks and gluons decreases with increasing energy. The discovery of these two QCD effects garnered two Nobel prizes in physics, in 2008 and 2004, respectively.
High-energy collisions of lead nuclei at the Large Hadron Collider (LHC) explore QCD under the most extreme conditions on Earth. These heavy-ion collisions recreate the quark–gluon plasma (QGP): the hottest and densest fluid ever studied in the laboratory. In contrast to normal nuclear matter, the QGP is a state where quarks and gluons are not confined inside hadrons. It is speculated that the universe was in a QGP state around one millionth of a second after the Big Bang.
Different views of a lead–lead collision event recorded by ALICE in 2015. (Image: CERN)The ALICE experiment was designed to study the QGP at LHC energies. It was operated during LHC Runs 1 and 2, and has carried out a broad range of measurements to characterise the QGP and to study several other aspects of the strong interaction. In a recent review, highlights of which are described below, the ALICE collaboration takes stock of its first decade of QCD studies at the LHC. The results from these studies include a suite of observables that reveal a complex evolution of the near-perfect QGP liquid that emerges in high-temperature QCD. ALICE measurements also demonstrate that charm quarks equilibrate extremely quickly within this liquid, and are able to regenerate QGP-melted “charmonium” particle states. ALICE has extensively mapped the QGP opaqueness with high-energy probes, and has directly observed the QCD dead-cone effect in proton–proton collisions. Surprising QGP-like signatures have also been observed in rare proton–proton and proton–lead collisions. Finally, ALICE measurements of interactions of produced hadrons have also revealed novel features that have broad implications for nuclear physics and astrophysics.
Probing the QGP at various scalesThe QGP can be inspected with various levels of spatial and energy resolution (scale) using particles produced in heavy-ion collisions. High-energy quarks and gluons rapidly cross the QGP and interact with it as they evolve to a spray, or “jet”, of partons that eventually form hadrons, or “hadronise”. The interaction with the QGP reduces the jet’s energy and modifies its structure. A jet with an energy of 20 gigaelectronvolts, for example, can probe distances of 0.01 femtometres (1 femtometre is 10-15 metres), well below the roughly 10-fm size of the QGP. The jet modification, known as jet quenching, results in several distinct effects that ALICE has seen, including significant energy loss for jets and a smaller energy loss for beauty quarks compared to charm quarks.
Lower-energy charm quarks also probe the QGP microscopically, and undergo Brownian motion – a random motion famously studied by Albert Einstein. ALICE has provided evidence that these lower-energy charm quarks participate in the thermalisation process by which the QGP reaches thermal equilibrium.
Bound states of a heavy quark and its antimatter counterpart, or “quarkonia”, such as the J/ψ (charmonium) and Υ(1S) (bottomonium), are spatially extended particles and have sizes of about 0.2 fm. They therefore probe the QGP at larger scales compared to high-energy partons. The QGP interferes with the quark–antiquark force and suppresses quarkonia production. For quarkonia made up of charm quarks, ALICE has shown that this suppression, which is stronger for more weakly bound states and thus “hierarchical”, is counterbalanced by charm quark–charm antiquark binding.
Animation of the quark–gluon plasma formed in collisions between heavy ions. (Video: CERN)This recombination effect has been revealed for the first time at the LHC, where about one hundred charm quarks and antiquarks are produced in each head-on lead–lead collision. It constitutes a proof of quark deconfinement, as it implies that quarks can move freely over distances much larger than the hadron size. The hierarchical suppression can be explained assuming a QGP initial temperature roughly four times higher than the temperature at which the transition from ordinary hadronic matter to the QGP can occur (about two trillion degrees kelvin). An assessment of the QGP temperature was also obtained from the ALICE measurement of photons that are radiated by the plasma during its expansion, yielding an average temperature from the entire temporal evolution of the collision of about twice the QGP transition temperature.
Regarding the large-scale spatial evolution of the collision, ALICE has demonstrated that the QGP formed at LHC energies undergoes the most rapid expansion ever observed for a many-body system in the laboratory. The velocities of the particles that fly out of the QGP in a collective flow approach about 70% of the speed of light, and direction-dependent, or “anisotropic”, flow has been observed for almost all measured hadron species, including light nuclei made of two or three protons and neutrons. Small variations seen in some specific flow patterns of hadrons with opposite electric charge are influenced by the huge electromagnetic fields produced in non-head-on heavy-ion collisions.
Calculations based on hydrodynamics, originally conceived to describe liquids at a few hundred degrees kelvin, describe all of the flow observables, and demonstrate that this theoretical framework is a good description of many-body QCD interactions at trillions of degrees kelvin. Such a description is achieved with the crucial inclusion of a small QGP viscosity, which is the smallest ever determined and thus establishes the QGP as the most perfect liquid.
Hadron formation at high temperaturesDuring the evolution of a heavy-ion collision, the QGP cools below the transition temperature and hadronises. After this hadronisation, the energy density may be large enough to allow for inelastic (hadron-creating) interactions, which change the medium’s “chemical” composition, in terms of particle species. Such interactions cease at the chemical freeze-out temperature, at which the particle composition is fixed. Elastic (non-hadron creating) interactions can still continue, and halt at the kinetic freeze-out temperature, at which the particle momenta are fixed.
ALICE measurements of hadron production over all momenta have provided an extensive mapping of this hadron chemistry, and they show that hadrons with low momentum form by recombination of quarks from the QGP. Theoretical models, in which a hadron “gas” is in chemical equilibrium after the QGP phase, describe the relative abundances of hadron species using only two properties: the chemical freeze-out temperature, which is very close to the transition temperature predicted by QCD, and a “baryochemical potential” of zero within uncertainties, which demonstrates the matter–antimatter symmetry of the QGP produced at the LHC.
In addition, ALICE investigations into the hadron-gas phase indicate that this phase is prolonged, and that the decoupling of particles from the expanding hadron gas is likely to be a continuous process.
What are the limits of QGP formation? As the number of particles produced in proton–proton collisions increases (blue lines), the more particles containing at least one strange quark are measured (orange to red squares). (Image: CERN)Studying how observables such as the particle production yields and multi-particle correlations change with multiplicity – the total number of particles produced – for proton–proton and proton–lead collisions provides a means to explore the thresholds required to form a QGP. A suite of ALICE measurements of high-multiplicity proton–proton and proton–lead collisions exhibit features similar to those observed in lead–lead collisions, where these are associated with QGP formation. The effects include the enhancement of yields of particles with strange quarks, the anisotropic flow determined from particle correlations, and the reduction of the yield of the feebly bound charmonium state ψ(2S) in proton–lead collisions. These observations were among the most surprising and unexpected from the first ten years of LHC running.
The ability of the hydrodynamic framework and of theoretical models of a strongly interacting system to describe many of the observed features, even at low multiplicities, suggests that there is no apparent spatial limit to QGP formation. However, alternative models that do not require the presence of a QGP can also explain a limited number of these features. These models challenge the idea of QGP formation, and this might be supported by the fact that jet quenching has not been observed to date in the small proton–lead colliding system. However, such absence could also be caused by the small spatial extent of a possible QGP droplet, which would decrease the jet quenching. Therefore, the quest for the smallest collision system that leads to QGP formation remains open.
Exploring few-body interactionsALICE investigations of few-body QCD interactions, such as those that take place in proton–proton collisions or in heavy-ion collisions in which the colliding nuclei only graze past each other, have provided a wide range of measurements. Examples include precise measurements showing that in these collisions the formation of hadrons from charm quarks differs from expectations based on electron-collider measurements, and the first direct observation of the dead-cone effect, which consists of a suppression of the gluons radiated by a massive quark in a forward cone around its direction of flight.
Grazing collisions, known as ultra-peripheral collisions, provide a means of exploring the internal structure of nucleons (protons or neutrons) via the emission of a photon from one nucleus that interacts with the other nucleus. ALICE studies of these collisions show clear evidence that the internal structure of nucleons bound in a nucleus is different from that of free protons.
The large data samples of proton–proton and proton–lead collisions recorded by ALICE have allowed studies of the strong interaction between protons and hyperons – unstable particles that contain strange quarks and may be present in the core of neutron stars. ALICE has shown that the interactions between a proton and Lambda, Xi or Omega hyperon are attractive. These interactions may play a part in the stability of the observed large-mass neutron stars. In addition, ALICE measurements of the lifetime and binding energy of hypertriton – an unstable nucleus composed of a proton, a neutron and a Lambda – are the most accurate to date and shed light on the strong interaction that binds hypernuclei together.
The present and future of ALICEAfter a major upgrade, the ALICE experiment started to record Run 3 proton–proton collisions in July 2022. The next full-scale data-taking of lead–lead collisions is planned for 2023, with a proposed pilot run expected in late 2022. The upgraded detector will reconstruct particle trajectories much more precisely and record lead–lead collisions at a higher rate. With the resulting, much larger Run 3 and then Run 4 data sets, rare probes of the QGP that were already used in the past decade, such as heavy quarks and jets, will become high-precision tools to study the QGP. ALICE will also continue to use the small colliding systems to investigate, among other things, the smallest QGP droplet that can be formed and the proton’s inner structure.
Besides further smaller-scale but highly innovative upgrades for the next LHC long shutdown, the ALICE collaboration has prepared a proposal for a completely new detector to be operated in the 2030s. The new detector will open up even more new avenues of exploration, including the study of correlations between charm particles, of chiral-symmetry restoration in the QGP, and of the time-evolution of the QGP temperature.
On 5 July, the LHC roared to life for its third run after three years of continual improvements to the machine as well as to the experiments’ detectors and analysis tools, and immediately reached a record energy of 13.6 TeV. Just three weeks later, the CMS collaboration was ready for its physics data-taking period.
The CMS collaboration recently presented its first Run 3 physics results of the production rate of pairs of the heaviest elementary particle, the top quark. In just one week, from 28 July to 3 August, the CMS collaboration collected data equivalent to almost 12% of the data set that had been required for the Higgs boson discovery in 2012.
Before Run 3 began, it was hoped – and has now been confirmed — that it would be possible to gather such a vast amount of data in a very short time. It took physicists two years to collect the data used to announce the Higgs boson discovery in 2012. But now, thanks to developments in data acquisition and selection systems and to the unprecedented speed of the analyses, the Run 3 data can now be analysed in almost real time.
Due to the high number of top-quark pairs created at the LHC, physics analysis can start with even a small amount of data. The production rate of this heavy system of particles has been enhanced by about 10% thanks to the collision energy increase from 13 TeV in Run 2 to 13.6 TeV in Run 3. The CMS results, which agree with the Standard Model prediction, are important because precise measurements of top-quark properties provide, among other things, crucial input for various searches for new phenomena in Run 3. Because of its high mass, the top quark decays immediately to a b quark and a W boson, which is also an unstable particle. The decay products leave traces as they pass through the detector, making it possible to observe them and to test the detector performance.
Precision measurements of the Standard Model are an essential part of the Run 3 programme, as any significant deviation could hint at new physics. The measurement of top-quark pair production rate is only the first step into the unexplored territory of the new energy regime, where answers to fundamental physics questions may be found.
kbernhar Fri, 11/04/2022 - 10:47 Byline Kristiane Bernhard-Novotny Publication Date Fri, 11/04/2022 - 16:42Quark–gluon plasma is an extremely hot and dense state of matter in which the elementary constituents – quarks and gluons – are not confined inside composite particles called hadrons, as they are in the protons and neutrons that make up the nuclei of atoms. Thought to have existed in the early universe, this special phase of matter can be recreated at the Large Hadron Collider (LHC) in collisions between lead nuclei.
A new analysis from the international ALICE collaboration at the LHC investigates how different bound states of a charm quark and its antimatter counterpart, also produced in these collisions, are affected by quark–gluon plasma. The results open new avenues for studying the strong interaction – one of the four fundamental forces of nature – in the extreme temperature and density conditions of quark–gluon plasma.
Bound states of a charm quark and a charm antiquark, known as charmonia or hidden-charm particles, are held together by the strong interaction and are excellent probes of quark–gluon plasma. In the plasma, their production is suppressed due to “screening” by the large number of quarks and gluons present in this form of matter. The screening, and thus the suppression, increases with the temperature of the plasma (see illustration below) and is expected to affect different charmonia to varying degrees. For example, the production of the ψ(2S) state, which is ten times more weakly bound and 20% more massive than the J/ψ state, is expected to be more suppressed than that of the J/ψ state.
This hierarchical suppression is not the only fate of charmonia in quark–gluon plasma. The large number of charm quarks and antiquarks in the plasma – up to about a hundred in head-on collisions – also gives rise to a mechanism, called recombination, that forms new charmonia and counters the suppression to a certain extent (see illustration). This process is expected to depend on the type and momentum of the charmonia, with the more weakly bound charmonia possibly being produced through recombination later in the evolution of the plasma, and charmonia with the lowest (transverse) momentum having the highest recombination rate.
Previous studies, which used data from CERN’s Super Proton Synchrotron and subsequently from the LHC, have shown that the production of the ψ(2S) state is indeed more suppressed than that of the J/ψ. ALICE has also previously provided evidence of the recombination mechanism in J/ψ production. But, until now, no studies of ψ(2S) production at low particle momentum had been precise enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.
The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018.
The measurements show that, regardless of particle momentum, the ψ(2S) state is suppressed about two times more than the J/ψ. This is the first time that a clear hierarchy in suppression has been observed for the total production of charmonia at the LHC. A similar observation was previously reported by the LHC collaborations for bound states of a bottom quark and its antiquark.
When further studied as a function of particle momentum, the ψ(2S) suppression is seen to be reduced towards lower momentum. This feature, which was previously observed by ALICE for the J/ψ state, is a signature of the recombination process.
Future higher-precision studies of these and other charmonia using data from LHC Run 3, which started in July, may lead to a definitive understanding of the modification of hidden-charm particles and, as a result, of the strong interaction that holds them together, in the extreme environment of quark–gluon plasma.
Illustration of the effect of quark–gluon plasma on the formation of charmonia in lead-nuclei collisions. When the plasma temperature increases, the more weakly bound ψ(2S) state is more likely to be “screened”, and thus not form, due to the larger number of quarks and gluons in the plasma (the coloured circles). The increase in the number of charm quarks and antiquarks (c and c̄) can lead to the formation of additional charmonia by quark recombination. (Image: ALICE collaboration) abelchio Wed, 10/19/2022 - 13:56 Byline ALICE collaboration Publication Date Thu, 10/20/2022 - 10:05
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