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In the first study of its kind at the Large Hadron Collider (LHC), the CMS collaboration has tested whether top quarks adhere to Einstein’s special theory of relativity.
Along with quantum mechanics, Einstein’s special theory of relativity serves as the basis of the Standard Model of particle physics. At its heart is a concept called Lorentz symmetry: experimental results are independent of the orientation or the speed of the experiment with which they are taken.
Special relativity has stood the test of time. However, some theories, including particular models of string theory, predict that, at very high energies, special relativity will no longer work and experimental observations will depend on the orientation of the experiment in space-time. Remnants of such Lorentz symmetry breaking could be observable at lower energies, such as at the energies of the LHC, but despite previous efforts, they have not been found at the LHC or other colliders.
In its recent study, the CMS collaboration searched for Lorentz symmetry breaking at the LHC using pairs of top quarks – the most massive elementary particles known. In this case, a dependence on the orientation of the experiment would mean that the rate at which top-quark pairs are produced in proton–proton collisions at the LHC would vary with time.
More precisely, since Earth rotates around its axis, the direction of the LHC proton beams and the average direction of top quarks produced in collisions at the centre of the CMS experiment also change depending on the time of the day. As a consequence, and if there is a preferential direction in space-time, the top-quark-pair production rate would vary with the time of the day. Hence, finding a deviation from a constant rate would amount to discovering a preferential direction in space-time.
The new CMS result, which is based on data from the second run of the LHC, agrees with a constant rate, meaning that Lorentz symmetry is not broken and Einstein’s special relativity remains valid. The CMS researchers used the result to set limits on the magnitude of parameters that are predicted to be null when the symmetry holds. The limits obtained improve by up to a factor of 100 upon results from a previous search for Lorentz symmetry breaking at the former Tevatron accelerator.
The results pave the way for future searches for Lorentz symmetry breaking based on top-quark data from the third run of the LHC. They also open the door to scrutiny of processes involving other heavy particles that can only be investigated at the LHC, such as the Higgs boson and the W and Z bosons.
abelchio Wed, 01/22/2025 - 14:57 Byline CMS collaboration Publication Date Thu, 01/23/2025 - 08:15The ATLAS collaboration at the Large Hadron Collider (LHC) has released a new high-precision measurement of the lifetime of the electrically neutral beauty (B0) meson – a hadron composed of a bottom antiquark and a down quark.
Beauty (B) mesons are made up of two quarks, one of which is a bottom quark. Over the past decades, by studying B mesons, physicists have been able to examine rare and precisely predicted phenomena to gain insights into interactions mediated by the weak force and into the dynamics of heavy-quark bound states. The precise measurement of the B0 meson lifetime – the average time it exists before decaying into other particles – is of critical importance in this context.
The new ATLAS study of the B0 meson looked for the particle’s decay into an excited neutral kaon (K*0) and a J/ψ meson. The J/ψ meson subsequently decays into a pair of muons while the K*0 meson is studied through its decay into a charged pion and a charged kaon. The analysis is based on proton–proton collision data collected by the ATLAS detector during Run 2 of the LHC (2015–2018), amounting to an impressive data set of 140 inverse femtobarns (1 inverse femtobarn corresponds to approximately 100 trillion proton–proton collisions).
The ATLAS researchers measured the B0 meson lifetime to be 1.5053 picoseconds (1 picosecond (ps) is a trillionth (10-12) of a second), with a statistical uncertainty of 0.0012 ps and a systematic uncertainty of 0.0035 ps. This result is the most precise to date and significantly improves upon previous measurements, including a previous ATLAS result (see figure below).
To achieve such precision, many experimental challenges needed to be overcome, including minimising systematic uncertainties, performing precise modelling and improving detector alignment.
In addition to measuring the lifetime of the B0 meson, the ATLAS team calculated its decay "width”. Width is a fundamental parameter of any unstable particle with a finite lifetime. The shorter the lifetime, the broader the decay width – a direct consequence of Heisenberg’s uncertainty relation in quantum mechanics. The decay width of the B0 meson was measured to be 0.664 inverse picoseconds (ps-1), with a total uncertainty of 0.004 ps-1.
The researchers then compared this result to a previous measurement of the decay width of the Bs0 meson (composed of a bottom quark and a strange quark). They found that the ratio of the decay widths was consistent with unity, reflecting the close values of the measured widths. These results are in line with heavy-quark model predictions and can be used to further tune these predictions.
The new ATLAS precision measurements enhance the understanding of weak-force-mediated decays in the Standard Model and provide valuable data for future theoretical developments.
A comparison of the new ATLAS result for the B0 lifetime with the previous ATLAS result, as well as results from the LHCb, CMS, D0, CDF, Belle II, BaBar and Belle experiments. (Image: ATLAS/CERN) abelchio Thu, 12/19/2024 - 10:49 Byline ATLAS collaboration Publication Date Thu, 12/19/2024 - 10:47In the Big Bang, matter and antimatter should have been created in equal amounts. But fast forward 13.8 billion years to the present day and the Universe is made almost entirely of matter, so something must have happened to create this imbalance.
The Standard Model of particle physics predicts an asymmetry between matter and antimatter known as charge–parity (CP) violation. But the size of this asymmetry in the Standard Model is not large enough to account for the imbalance and the asymmetry has so far been observed only in certain decays of particles called mesons, which are made of a quark and an antiquark. It remains to be seen in other meson decays and in decays of other types of particles, such as three-quark particles called baryons.
In two new articles, the LHCb collaboration at the Large Hadron Collider (LHC) reports seeing evidence of CP violation in decays of baryons and in decays of beauty hadrons into charmonium particles, shedding light on these two pieces of the matter–antimatter puzzle.
Experiments, including LHCb, have previously conducted searches for CP violation in baryons, by looking for differences in the way matter and antimatter baryons decay into other particles. But these searches have essentially come up empty handed so far. Although one LHCb study has delivered evidence of the process in a particular decay of the bottom lambda baryon, that evidence has not increased in a subsequent study that analysed a larger sample of such decays.
In its first new study, the LHCb team sifted through proton–proton collision data taken during the first and second runs of the LHC to search for different decay modes of the bottom lambda baryon, including its decay into a lambda baryon and two kaons. It then looked for CP violation in each decay mode by basically counting the number of decays of the bottom lambda baryon and of its antimatter partner and taking the difference between the two. For the decay into a lambda baryon and two kaons, this difference showed evidence of CP violation with a significance of 3.2 standard deviations.
In its second new study, the LHCb team turned its attention to the decay of the electrically charged beauty meson into a J/psi and a charged pion. The J/psi is a charmonium particle – a meson consisting of a charm quark and a charm antiquark. Performing a similar analysis to that of the bottom lambda baryon study, and also using data from the first and second runs of the LHC, the LHCb researchers found evidence of CP violation in this decay mode of the charged beauty meson, again with a significance of 3.2 standard deviations. The finding represents evidence of CP violation in decays of beauty hadrons to charmonium particles.
The two studies mark significant steps towards establishing whether or not CP violation exists in these types of decays. Data from the third run of the LHC and from the collider’s planned upgrade, the High-Luminosity LHC, are set to shed further light on these and other pieces of the matter–antimatter puzzle.
abelchio Mon, 12/16/2024 - 12:41 Byline Ana Lopes Publication Date Mon, 12/16/2024 - 12:38Since the CMS and ATLAS experiments announced the discovery of the Higgs boson in 2012, they have been measuring its mass and interaction with other particles with ever-increasing precision. Now, researchers are setting their sights on the Higgs boson’s interaction with itself, which could provide physicists with clues to the stability of the Universe. To do this, physicists search for a much rarer phenomenon than the production of one Higgs boson: the production of Higgs boson pairs, known as di-Higgs. In a new study, using data from high-energy proton–proton collisions in Run 2 of the Large Hadron Collider (LHC), the CMS experiment has released its latest search for di-Higgs production and provided constraints on their production rate.
Higgs boson pairs can be created in two main ways. The first is called gluon–gluon fusion, where gluons – particles inside the colliding protons – interact to produce the Higgs bosons. This process allows scientists to study the interaction between one so-called intermediate-state and two final-state Higgs bosons. The second method involves quarks, also inside the colliding protons, that radiate two vector bosons. These vector bosons then interact to form Higgs bosons, enabling the study of interactions between two Higgs bosons and two vector bosons.
Physicists at CMS performed their most recent analysis by searching for multiple ways that di-Higgs could decay. These final states resulted from Higgs boson pairs decaying to bottom quarks, W bosons, tau leptons and photons. By combining these searches and analysing all the data simultaneously using sophisticated analysis techniques – including boosted decision trees and deep neural networks – the collaboration was able to extract more information than ever before.
The study allowed physicists to establish upper limits on the rates of Higgs boson pair production with a 95% confidence level. The measured limits are currently at 3.5 times the Standard Model expectation for the total production of di-Higgs, and 79 times the Standard Model expectation for its production by fusing vector bosons.
With the Run 3 data-taking era of the LHC in progress, the CMS experiment has already doubled the amount of data collected, and CMS researchers are in the process of analysing it. One of the most interesting prospects for measuring the Higgs boson self-interaction is the upcoming High-Luminosity LHC (HL-LHC), scheduled to start operation in 2030. In this new phase, the accelerator will deliver to CMS the highest luminosity ever reached at a collider. Considering projections of luminosity and systematic uncertainties, scientists have estimated that they might start to see the first evidence for di-Higgs production with around half of the HL-LHC data. The collaboration looks forward to exploring this rare and exciting phenomenon further.
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ndinmore Wed, 12/11/2024 - 09:42 Byline CMS collaboration Publication Date Wed, 12/11/2024 - 11:3612 December 2024 · Voir en français
Part 23, the final part of the CERN70 feature series. Find out more: cern70.cern
Gian Giudice is Head of CERN’s Theoretical Physics department
Advancements in fundamental knowledge have always been driven by a dialogue between theory and experiment. Theory sometimes opens up new avenues of exploration, which experimentalists then venture into and either validate them or, on the contrary, find a new direction for their research. Experimentalists sometimes produce a completely unexpected result that requires an explanation, a theoretical framework.
From CERN’s earliest days, its founders recognised the importance of theory for experimental physics. The Theory group was one of the the first to be created. It was set up in May 1952, under the leadership of the Danish Nobel laureate in physics Niels Bohr, one of the founding fathers of quantum mechanics. Initially located at Bohr’s Institute of Theoretical Physics in Copenhagen, it moved to Geneva in 1957.
At that time, the landscape of fundamental physics was evolving rapidly. Experiments were detecting exotic particles in cosmic rays and in collisions at brand new accelerators in the United States, and the resulting menagerie seemed to defy all logic. Based on these successive discoveries, theorists established the quark model, which explained the existence of this multitude of exotic particles. Building on the theoretical advances of previous decades, they developed the Standard Model, providing a theoretical framework for particles and the forces that bind them together. This model proved to be extraordinarily effective, enabling physicists to make precise predictions about the interactions between particles, which were subsequently validated by experiments at accelerators.
The CERN Theory group and CERN’s experiments made many contributions to building and verifying the Standard Model throughout the 1970s, for example with the discovery of neutral currents in 1973. They helped to consolidate the model in the 1980s, notably with the discovery of the intermediate W and Z bosons and the numerous results from the Large Electron–Positron Collider (LEP). At the end of the 1990s, the particle landscape appeared to be well organised, with a coherent classification system and established and verified equations.
The inside of a High-Luminosity Large Hadron Collider (HL-LHC) perforated beam screen. (Image: CERN)However, the Brout-Englert-Higgs mechanism, on which the Standard Model is based and through which particles acquire their mass, had not yet been validated. When the Higgs boson was discovered at the Large Hadron Collider (LHC) in 2012, the final piece of the Standard Model puzzle fell into place.
Was the quest over? The LHC had answered the crucial question of the Brout-Englert-Higgs mechanism and made incredibly precise measurements. But the Standard Model theory, powerful as it was, had many gaps. The community hoped that the unprecedented energy of the LHC would reveal other particles that were predicted by theories beyond the Standard Model or, at least, that anomalies would indicate the existence of new physics. Some of these theories have since been ruled out and measurements continue.
In the absence of new clues, particle physics is at a crossroads, and there are many possible paths to be followed. The High-Luminosity LHC, which will start up in 2030, will continue the exploration, providing experimentalists with a very large volume of data. For the longer term, the community is designing instruments that could reach higher energies and thus enable unknown areas to be explored. For example, the FCC study is examining the feasibility of a more powerful collider. All of the options are being studied in the framework of the European Strategy for Particle Physics. The quest to understand the Universe at the smallest and largest scales therefore continues.
Recollections When I arrived at CERN, I had high hopes for a revolutionary breakthrough to take place at LEP or the LHC. In the ensuing years, the LHC indeed triggered a revolution in particle physics, but not of the kind I was expecting.Gian Giudice is Head of CERN’s Theoretical Physics department.
“In her speech at the CERN70 ceremony, Ursula von der Leyen said: "Every physicist in the world wants to work at CERN." After spending nearly 32 years at this Lab, I know exactly what she meant. My time at CERN has been the most fantastic experience of my life, from both a scientific and a human point of view.
At the time I joined CERN as a fellow in 1993, particle physics had just gone through a heroic period. In the previous 30 years, knowledge of fundamental physics had been transformed from a confused zoology of particles into a precise and successful description based on a single mathematical principle: gauge symmetry in quantum field theory. LEP had just conclusively established this principle as the ultimate law in particle physics. Notwithstanding this impressive result, the Standard Model (SM) had limitations. The agreement with experimental data was perfect, but there were too many unanswered questions, mostly related to the structural aspects of the theory and to cosmological puzzles. The general feeling was that many of these questions would find their answers in the next layer of physical reality, ready to be discovered in the following 30 years.
When I arrived at CERN, I had high hopes for a revolutionary breakthrough to take place at LEP or the LHC. In the ensuing years, the LHC indeed triggered a revolution in particle physics, but not of the kind I was expecting. On one hand, the discovery of the Higgs boson proved that the spontaneous breaking of electroweak symmetry – already identified by LEP – is completed at short distances by a single spin-zero particle. On the other hand, the lack of any other particles or unexpected phenomena at the LHC showed that there must exist an energy gap between the SM and the next layer of microscopic reality. These two results represent the legacy of the LHC, whose consequence was to completely reshuffle the cards in particle physics, leaving us today even more puzzled about the SM than we were before the project started.
The conceptual revolution triggered by the LHC is a measure of the success of the scientific method. Research in fundamental physics is a combination of theoretical speculations and experimental exploration. More often than not, it leads to surprises, and this has certainly been the case for the LHC results.
The LHC discoveries of the Higgs boson and the new-physics energy gap exemplify the power of the scientific method. The precision reached in these measurements was astounding and, in many cases, exceeded anything that could have been imagined at the beginning of the project. The secret behind this success was not only the superb performance of the CERN accelerator complex and the LHC detectors, but also the advances in the theoretical calculations of the SM backgrounds. Only with all these accomplishments put together was it possible to extract precision measurements out of hadron-collider data.
The interplay between theory and experiment has always been the key to CERN's success. Most of the LEP achievements would not have been possible without an intense theory/experiment collaboration. The LHC has continued on LEP's path, bringing the symbiotic theory/experiment relationship to an unprecedented level of sophistication, as I have witnessed during my 30 years at CERN.
What lies ahead in the next 30 years? Many of the questions that faced particle physics when I arrived at CERN are still unanswered, and the discoveries of the Higgs boson and the energy gap have only sharpened the meaning of those questions and made more urgent the need to find answers. In the meantime, the landscape of research in fundamental physics has broadened beyond traditional boundaries. To tackle the open questions in fundamental physics we need today a diverse experimental programme, which includes large and small projects, involving different goals and techniques and bridging fields. A post-LHC collider project is essential to spearhead research towards shorter distances, and CERN must fulfil its leadership role in particle physics by developing and operating a new ambitious high-energy project during the next 30 years.
Although theory and experiment are always intricately connected in the advancement of fundamental physics, in recent decades experiments had a predominant role, thanks to the impact of the LHC results. However, my expectation is that the pendulum will swing back, and theory will be the driving force of the next 30 years.
Theorist Freeman Dyson claimed that concept-driven scientific revolutions are highly celebrated by historians but are actually relatively rare. Along the path of scientific progress, the most common revolutions – Dyson argued – are tool-driven. Obvious examples are the discovery of the Medicean stars orbiting around Jupiter, made possible by Galileo's telescope, and the discovery of the Higgs boson, which could not have been made without the LHC. But mathematical tools are as important as scientific instruments to trigger revolutions. Examples are infinitesimal calculus, which provided the necessary tool for Newton to understand that elliptical planetary orbits are compatible with a spherically-symmetric central force, and differential geometry, which provided the right set-up to formulate general relativity.
My bet is that the tool to profoundly change how theoretical physics develops in the next 30 years will be artificial intelligence (AI). AI is already revolutionising all aspects of experimental data analysis but it could also transform how theoretical physics operates. Moreover, understanding how AI solves physics problems may reveal new logical frameworks, well beyond the methods currently used by humans. The whole meaning and definition of theoretical physics is likely to change in the next 30 years and this will open new horizons to scientific exploration.
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Gian was a speaker at the first CERN70 public event “Unveiling the Universe”, and additional CERN70 events explored the case of the (still) mysterious Universe and exploring farther with machines for new knowledge.
Gian Giudice is Head of CERN’s Theoretical Physics departmentCollisions between heavy ions at the Large Hadron Collider (LHC) create quark–gluon plasma, a hot and dense state of matter that is thought to have filled the Universe around one millionth of a second after the Big Bang. Heavy-ion collisions also create suitable conditions for the production of atomic nuclei and exotic hypernuclei, as well as their antimatter counterparts, antinuclei and antihypernuclei. Measurements of these forms of matter are important for various purposes, including helping to understand the formation of hadrons from the plasma’s constituent quarks and gluons and the matter–antimatter asymmetry seen in the present-day Universe.
Hypernuclei are exotic nuclei formed by a mix of protons, neutrons and hyperons, the latter being unstable particles containing one or more quarks of the strange type. More than 70 years since their discovery in cosmic rays, hypernuclei remain a source of fascination for physicists because they are rarely found in nature and it’s challenging to create and study them in the laboratory.
In heavy-ion collisions, hypernuclei are created in significant quantities, but until recently only the lightest hypernucleus, hypertriton, and its antimatter partner, antihypertriton, have been observed. A hypertriton is composed of a proton, a neutron and a lambda (a hyperon containing one strange quark). An antihypertriton is made up of an antiproton, an antineutron and an antilambda.
Following hot on the heels of an observation of antihyperhydrogen-4 (a bound state of an antiproton, two antineutrons and an antilambda), reported earlier this year by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), the ALICE collaboration at the LHC has now seen the first ever evidence of antihyperhelium-4, which is composed of twoantiprotons, an antineutron and an antilambda. The result has a significance of 3.5 standard deviations and also represents the first evidence of the heaviest antimatter hypernucleus yet at the LHC.
The ALICE measurement is based on lead–lead collision data taken in 2018 at an energy of 5.02 teraelectronvolts (TeV) for each colliding pair of nucleons (protons and neutrons). Using a machine-learning technique that outperforms conventional hypernuclei search techniques, the ALICE researchers looked at the data for signals of hyperhydrogen-4, hyperhelium-4 and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by looking for the (anti)helium-4 nucleus and the charged pion into which it decays, whereas candidates for (anti)hyperhelium-4 were identified via its decay into an (anti)helium-3 nucleus, an (anti)proton and a charged pion.
In addition to finding evidence of antihyperhelium-4 with a significance of 3.5 standard deviations, as well as evidence of antihyperhydrogen-4 with a significance of 4.5 standard deviations, the ALICE team measured the production yields and masses of both hypernuclei.
For both hypernuclei, the measured masses are compatible with the current world-average values. The measured production yields were compared with predictions from the statistical hadronisation model, which provides a good description of the formation of hadrons and nuclei in heavy-ion collisions. This comparison shows that the model’s predictions agree closely with the data if both excited hypernuclear states and ground states are included in the predictions. The results confirm that the statistical hadronisation model can also provide a good description of the production of hypernuclei, which are compact objects with sizes of around 2 femtometres (1 femtometre is 10-15 metres).
The researchers also determined the antiparticle-to-particle yield ratios for both hypernuclei and found that they agree with unity within the experimental uncertainties. This agreement is consistent with ALICE’s observation of the equal production of matter and antimatter at LHC energies and adds to the ongoing research into the matter–antimatter imbalance in the Universe.
abelchio Mon, 12/09/2024 - 09:33 Byline ALICE collaboration Publication Date Mon, 12/09/2024 - 09:27In a paper published today in the journal Nature, the CLOUD collaboration at CERN reveals a new source of atmospheric aerosol particles that could help scientists to refine climate models.
Aerosols are microscopic particles suspended in the atmosphere that arise from both natural sources and human activities. They play an important role in Earth’s climate system because they seed clouds and influence their reflectivity and coverage. Most aerosols arise from the spontaneous condensation of molecules that are present in the atmosphere only in minute concentrations. However, the vapours responsible for their formation are not well understood, particularly in the remote upper troposphere.
The CLOUD (Cosmics Leaving Outdoor Droplets) experiment at CERN is designed to investigate the formation and growth of atmospheric aerosol particles in a controlled laboratory environment. CLOUD comprises a 26 m3 ultra-clean chamber and a suite of advanced instruments that continuously analyse its contents. The chamber contains a precisely selected mixture of gases under atmospheric conditions, into which beams of charged pions are fired from CERN’s Proton Synchrotron to mimic the influence of galactic cosmic rays.
“High concentrations of aerosol particles have been observed high over the Amazon rainforest for the past twenty years, but their source has remained a puzzle until now,” says CLOUD spokesperson Jasper Kirkby. “Our latest study shows that the source is isoprene emitted by the rainforest and lofted in deep convective clouds to high altitudes, where it is oxidised to form highly condensable vapours. Isoprene represents a vast source of biogenic particles in both the present-day and pre-industrial atmospheres that is currently missing in atmospheric chemistry and climate models.”
Isoprene is a hydrocarbon containing five carbon atoms and eight hydrogen atoms. It is emitted by broad-leaved trees and other vegetation and is the most abundant non-methane hydrocarbon released into the atmosphere. Until now, isoprene’s ability to form new particles has been considered negligible.
The CLOUD results change this picture. By studying the reaction of hydroxyl radicals with isoprene at upper tropospheric temperatures of −30 °C and −50 °C, the collaboration discovered that isoprene oxidation products form copious particles at ambient isoprene concentrations. This new source of aerosol particles does not require any additional vapours. However, when minute concentrations of sulphuric acid or iodine oxoacids were introduced into the CLOUD chamber, a 100-fold increase in the aerosol formation rate was observed. Although sulphuric acid derives mainly from anthropogenic sulphur dioxide emissions, the acid concentrations used in CLOUD can also arise from natural sources.
In addition, the team found that isoprene oxidation products drive rapid growth of particles to sizes at which they can seed clouds and influence the climate – a behaviour that persists in the presence of nitrogen oxides produced by lightning at upper-tropospheric concentrations. After continued growth and descent to lower altitudes, these particles may provide a globally important source for seeding shallow continental and marine clouds, which influence Earth’s radiative balance (the amount of incoming solar radiation compared to outgoing longwave radiation).
“This new source of biogenic particles in the upper troposphere may impact estimates of Earth's climate sensitivity, since it implies that more aerosol particles were produced in the pristine pre-industrial atmosphere than previously thought,” adds Kirkby. “However, until our findings have been evaluated in global climate models, it’s not possible to quantify the effect.”
The CLOUD findings are consistent with aircraft observations over the Amazon, as reported in an accompanying paper in the same issue of Nature. Together, the two papers provide a compelling picture of the importance of isoprene-driven aerosol formation and its relevance for the atmosphere.
Since it began operation in 2009, the CLOUD experiment has unearthed several mechanisms by which aerosol particles form and grow in different regions of Earth’s atmosphere.
“In addition to helping climate researchers to understand the critical role of aerosols in Earth’s climate, the new CLOUD result demonstrates the rich diversity of CERN’s scientific programme and the power of accelerator-based science to address societal challenges,” says CERN Director for Research and Computing, Joachim Mnich.
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At a talk held at CERN this week, the ATLAS collaboration at the Large Hadron Collider (LHC) reported observing top quarks in collisions between lead ions, marking the first observation of this process in interactions between atomic nuclei. This observation represents a significant step forward in heavy-ion collision physics, paving the way for new measurements of the quark–gluon plasma (QGP) that is created in these collisions and delivering fresh insights into the nature of the strong force that binds protons, neutrons and other composite particles together.
In QGP, the fundamental components of protons and neutrons – quarks (matter particles) and gluons (strong force carriers) – are not bound within particles, but instead exist in a “deconfined” state of matter, forming an almost perfect dense fluid. Scientists believe that QGP filled the Universe briefly after the Big Bang and its study offers a glimpse into the conditions of that early epoch in the history of our Universe. However, QGP’s extremely short lifetime when created in heavy-ion collisions – around 10−23 seconds – means it cannot be observed directly. Instead, physicists study particles that are produced in these collisions and pass through the QGP, using them as probes of QGP’s properties.
The top quark, in particular, is a very promising probe of QGP’s evolution over time. As the heaviest known elementary particle, the top quark decays into other particles an order of magnitude faster than the time needed to form QGP. The delay between the collision and the top quark’s decay products interacting with the QGP could serve as a “time marker”, offering a unique opportunity to study the QGP’s temporal dynamics. Additionally, physicists could extract new information on nuclear parton distribution functions, which describe how the momentum of a nucleon (proton or neutron) is distributed among its constituent quarks and gluons.
In their new result, ATLAS physicists studied collisions of lead ions that took place at a collision energy of 5.02 teraelectronvolts (TeV) per nucleon pair during Run 2 of the LHC. They observed top-quark production in the “dilepton channel”, where the top quarks decay into a bottom quark and a W boson, which subsequently decays into either an electron or a muon and an associated neutrino. The result has a statistical significance of 5.0 standard deviations, making it the first observation of top-quark-pair production in nucleus–nucleus collisions. The CMS collaboration had previously reported evidence of this process in lead–lead collisions.
The observation was made possible by the ATLAS experiment's precise lepton reconstruction capabilities, coupled with a few other elements. These include the high statistics of the full Run-2 lead–lead data set, data-driven estimations of background processes that could mimic the signal, new simulations of top-quark events and dedicated jet calibration methods. Notably, the analysis does not rely on techniques that “tag” the jet originating from the bottom quark. This opens the possibility for the analysis to be used for the notoriously difficult bottom-tagging calibration in heavy-ion collisions, which would improve future measurements of the top quarks produced during these collisions.
ATLAS physicists measured the top-quark-pair production rate, or “cross section”, with a relative uncertainty of 35%. The total uncertainty is primarily driven by the data set size, meaning that new heavy-ion data from the ongoing Run 3 will enhance the precision of the measurement.
The new ATLAS result opens a window into the study of QGP. In future studies, ATLAS scientists will also consider the “semi-leptonic” decay channel of top-quark pairs in heavy-ion collisions, which may allow them to get a first glimpse of the evolution of QGP over time.
abelchio Fri, 11/15/2024 - 11:27 Byline ATLAS collaboration Publication Date Fri, 11/15/2024 - 11:16Experimental particle physicist Ian Shipsey, a remarkable leader and individual, passed away suddenly and unexpectedly in Oxford on 7 October.
Ian was educated at Queen Mary University of London and the University of Edinburgh, where he earned his PhD in 1986 for his work on the NA31 experiment at CERN. Moving to the US, he joined Syracuse as a post-doc and then became a faculty member at Purdue, where, in 2007, he was elected Julian Schwinger Distinguished Professor of Physics. In 2013 he was appointed the Henry Moseley Centenary Professor of Experimental Physics at the University of Oxford.
Ian was a central figure behind the success of the CLEO experiment at Cornell, which was for many years the world’s pre-eminent detector in flavour physics. He led many analyses, most notably in semi-leptonic decays, from which he measured four different CKM matrix elements, and oversaw the construction of the silicon vertex detector for the CLEO III phase of the experiment. He served as co-spokesperson between 2001 and 2004 and was one of the intellectual leaders that saw the opportunity to re-configure the detector and the CESR accelerator as a facility for the precise exploration of physics at the charm threshold. The resulting CLEO-c programme yielded many important measurements in the charm system and enabled critical experimental validations of lattice-QCD predictions.
Within the CMS collaboration, Ian played a leading role in the construction of the forward-pixel detector, exploiting the silicon laboratory he had established at Purdue. His contributions to CMS physics analyses were no less significant. These included the observation of upsilon suppression in heavy-ion collisions (a smoking gun for the production of quark–gluon plasma) and the discovery, reported in a joint Nature paper with the LHCb collaboration, of the ultra-rare decay Bs→μ+μ-. He was also an influential voice as CMS Collaboration Board chair (2013–2014).
After moving to the University of Oxford and, in 2015, joining the ATLAS collaboration, Ian became Oxford’s ATLAS team leader and established state-of-the-art clean rooms, which are used for the construction of the future inner tracker (ITk) pixel end-cap modules. Together with his students, he contributed to measurements of the Higgs boson mass and width, and to the search for its rare di-muon decay. Ian also led the UK’s involvement in LSST (now the Vera Rubin Observatory), where Oxford is providing deep expertise for the CCD cameras.
Following his tenure as the dynamic head of the particle physics sub-department, Ian was elected head of Oxford Physics in 2018 and re-elected in 2023. Among his many successful initiatives, he played a leading role in establishing the UKRI “Quantum Technologies for Fundamental Physics” programme, which is advancing quantum-based applications across various areas of physics. With the support of this programme, he led the development of novel atom interferometers for light dark matter searches and gravitational wave detection.
Ian took a central role in establishing roadmaps for detector R&D, both in the US and (via ECFA) in Europe. He was one of the coordinators of the ECFA R&D Roadmap Panel and a driving force behind it, as well as being co-chair of the US effort to define the basic research needs in this area. As chair of the ICFA Instrumentation, Innovation and Development Panel, he promoted R&D in instrumentation for particle physics and the recognition of excellence in this field.
Among his many prestigious honours, Ian was elected a Fellow of the Royal Society in 2022 and received the James Chadwick Medal and Prize from the Institute of Physics in 2019. He served on numerous collaboration boards, panels and advisory and decision-making committees shaping national and international science strategies.
The success of Ian’s career is even more remarkable given that he lost his hearing in 1989. He received a cochlear implant, which restored limited auditory ability. He gave unforgettable talks on this subject, explaining the technology and its impact on his life.
Ian was an outstanding physicist and also a remarkable individual. His legacy is not only an extensive body of transformative scientific results, but also the impact that he had on all who met him. He was equally charming whether speaking to graduate students or lab directors. Everyone felt better after talking to Ian. His success derived from a remarkable combination of optimism and limitless energy. Once he had identified the correct course of action, he would not allow himself to be dissuaded by cautious pessimists who worried about the challenges ahead. His colleagues and many graduate students will continue to benefit for many years from the projects he initiated. The example he set as a physicist, and the memories he leaves as a friend, will endure still longer.
Many of Ian’s scientific successes were achieved in professional collaboration with his wife, Daniela Bortoletto, who survives him, together with their daughter, Francesca.
His friends and colleagues
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This obituary will also appear in the CERN Courier.
Read the interview with Ian Shipsey published in the CERN Courier in 2020.
anschaef Tue, 11/12/2024 - 18:42 Publication Date Thu, 11/14/2024 - 08:3731 October 2024 · Voir en français
Candidate Higgs boson events from collisions between protons in the LHC, in the CMS experiment (left) and in the ATLAS experiment (right). (Images: ATLAS/CMS/CERN)Ask any member of the particle physics community where they were on 4 July 2012 and they won’t need to think too hard about it. The discovery of the Higgs boson was a milestone in the history of science, as evidenced by the media tsunami that it caused.
At 9.00 a.m. on 4 July 2012, Joe Incandela and Fabiola Gianotti, the spokespersons for the CMS and ATLAS experiments, took the floor one after the other in front of an excited audience to present the latest data from their experiments. No news had yet been released, but all of the experts were anticipating a spectacular announcement. The conference was broadcast in dozens of institutes around the world. In the United States, scientists woke up in the middle of the night to attend the event, on a bank holiday no less. At 10.40 a.m., thunderous applause erupted in CERN’s Main Auditorium, Peter Higgs shed a tear and the Director-General, Rolf Heuer, declared: “As a layman, I would say: now we have it!”. The results were unequivocal: they revealed the presence of a particle whose properties matched those of the Higgs boson, a particle that had been predicted 48 years earlier.
The story starts in the 1960s, when physicists began to realise that the electromagnetic and weak interactions (two of nature’s four fundamental forces) might be described by the same mathematical structure. A major challenge in building this unified “electroweak” theory, which is now a pillar of the Standard Model of particle physics, was that its equations demanded that the mediating particles of the interactions have zero mass, which would imply that they have an infinite range. But while the photon, the vector of the electromagnetic force, is indeed devoid of mass, this can’t be the case for the carriers of the weak interaction, as this interaction has a very small range, on an atomic scale.
Belgian theorist, François Englert (left), and British theorist, Peter Higgs (right), in CERN's Main Auditorium, speaking after the announcement of the discovery on 4 July 2012. They were awarded the Nobel Prize in Physics the following year. (Image: CERN)The solution to this problem lay in work published earlier by the Belgian theorists Robert Brout and François Englert and, independently, the British theorist Peter Higgs. Taking inspiration from extensive research work – in particular, work on superconductivity – they had proposed a mechanism that, when it was applied by Steven Weinberg to the electroweak theory, enabled the mediators of the weak force to gain mass while ensuring that the photon of electromagnetism did not. The mechanism had two profound implications: the existence of the Z boson (a further carrier of weak interactions, in addition to the W boson), and the existence of an invisible field that permeates the entire Universe, with which more familiar particles such as electrons interact to gain their all-important masses. Indirect evidence for the existence of the Z boson was obtained at CERN in 1973, and both the W and Z bosons were finally discovered at CERN a decade later, their masses being as predicted by the electroweak theory. The existence of the Brout-Englert-Higgs field remained to be proven and, to do that, the only hope was to detect the associated particle, known as the Higgs boson.
After the attempts of the experiments at the Large Electron-Positron Collider (LEP) at CERN and at the Tevatron at Fermilab in the United States, the boson hunters placed all their hope in the Large Hadron Collider (LHC), which has been generating higher-energy collisions since 2010. At the end of 2011, the two general-purpose LHC experiments, ATLAS and CMS, presented promising early results that were nonetheless still inconclusive. The LHC restarted in April 2012 at a slightly higher energy after a technical maintenance stop in the winter. Data quickly revealed the presence of a particle with properties that matched those of the long-sought Higgs boson, which culminated in the announcement of 4 July. One year later, the Nobel Prize in Physics was awarded jointly to François Englert and Peter Higgs. The Nobel academy mentioned CERN and the ATLAS and CMS experiments in the statement accompanying the prize.
Since the discovery, the two experiments have carried out a lot of work to define the new particle. For the Higgs boson is an exotic item in the particle zoo. As the only known elementary particle with zero “spin”, it could potentially shed light on profound open questions in fundamental physics – ranging from the decoupling of the electromagnetic and weak forces immediately after the Big Bang to the ultimate stability of the Universe. That’s why 4 July 2012 marked the start of a new adventure for particle physics.
Recollections Those weeks in June 2012 were memorable: lots of emotion, lots of pressure, people working day and night. There was such an electricity, such an exciting atmosphere!Fabiola Gianotti came to CERN as a research physicist in 1994. She was spokesperson of the ATLAS experiment at the Large Hadron Collider (LHC) from March 2009 to February 2013. On 4 July 2012, alongside the CMS spokesperson at the time, Joe Incandela, she announced the discovery of the Higgs boson. She has been CERN Director-General since 2016.
On 4 July 2012, in front of a packed amphitheatre, Fabiola Gianotti, spokesperson for the ATLAS collaboration, presents the ATLAS results on the search for the Higgs boson. (Image: CERN)“It was 2012 and, as we usually do in our field to avoid biases, we had “blinded” a relatively small mass region that had not been excluded by previous searches for the Higgs boson. All of our analysis optimisation, Monte Carlo tuning, etc., was done outside this region. At the beginning of June 2012, we were ready to unblind our results for the Higgs to gamma–gamma channel.
I was at Fermilab in Chicago the day of the unblinding and I emailed one of the conveners, asking “Did you unblind? What did you find? Send me a plot.” He did and there it was, an excess at a mass of around 125 GeV, at exactly the same position as the hints from the 2011 data! I replied “my goodness”, to which he responded “indeed”. It was a very short exchange, just three words: we both knew that the Higgs boson was there.
Results in only one channel weren’t enough for a robust discovery claim. If it really was the Higgs boson, we had to see a handful of events in the four-lepton final state. We had nothing at the beginning of June. But all of a sudden, in the second half of June, candidates in the four-lepton channel began to pop up. The CMS collaboration had promising signals in the same mass region and, as spokespersons of the CMS and ATLAS collaborations, Joe Incandela and I shared our preliminary results with the then Director-General, Rolf Heuer, and began to prepare for an announcement. Joe and I were in contact daily. I knew what CMS had, he knew what ATLAS had, but we never disclosed this to our collaborations to avoid increasing the pressure and introducing any bias.
Those weeks in June 2012 were memorable: lots of emotion, lots of pressure, people working day and night, eating pizza at 3 a.m. at CERN. There was such an electricity, such an exciting atmosphere! The LHC was working fantastically well, delivering large amounts of data daily, and we had put in place fast-track procedures to calibrate and analyse data almost immediately. Despite the emotion and pressure, we were extremely focused, and we did zillions of checks and cross checks. It was an immense amount of work, done in a very short time. What really surprised me was that, despite the result at stake, no-one in the ATLAS or CMS collaborations – almost 6000 people – disclosed our findings. It was a strong sign of our responsibility, commitment and integrity.
On the morning of 4 July, I got to CERN and saw the crowds of people trying to enter the Main Auditorium. The room was so packed.
Joe Incandela spoke first. I remember that while he was showing his slides, I thought with gratitude of the thousands of colleagues that had worked on ATLAS, CMS and the LHC over the decades and all the people that had made the LHC and its experiments possible. Some of them were no longer with us to see such a great accomplishment.
CERN Director-General, Rolf Heuer (left), and ATLAS and CMS experiment spokespersons Fabiola Gianotti (centre) and Joe Incandela (right), following the announcement of the results on the search for the Higgs boson on 4 July 2012. (Image: CERN)When I started to speak, I looked at the audience and spotted a few ATLAS colleagues who became my reference points during the talk. They were looking at me with such intensity, and they really gave me strength and energy. The room exploded into a big round of applause when I announced the five-sigma significance. Straight after the presentations came the press conference. This all happened on a Wednesday, and Wednesday meant the LHC Machine Committee (LMC) meeting, which I always attended. So, I was there, of course, as on every Wednesday. I remember Mike Lamont, who was machine coordinator at the time, was very surprised to see me on that special day. He started his report with a slide that said: “Status report from the Higgs factory”.
Finally, after a long and exhausting day, I went home and packed for the International Conference on High Energy Physics (ICHEP) in Melbourne. My flight was early in the morning on 5 July. At 3 a.m., I woke up suddenly, realising that Australia is on the other side of the globe and I’d packed for summer not winter. I quickly repacked in the middle of the night! The next morning, I walked onto the plane, past the table of newspapers, and all the front pages showed the Higgs boson discovery. I thought, “oh my goodness, we are everywhere” and then fell fast asleep for most of the flight! The adrenaline had evaporated all of a sudden.
At ICHEP, everybody was excited, but very quickly we got back to business discussing the next steps. The 4 July announcement was just the beginning of the huge work that followed to understand a very special particle, which is still ongoing today.
It was an immense privilege to be the spokesperson of an experiment at the time of a monumental discovery. A discovery is teamwork, the result of decades of hard work by physicists, technicians, engineers and other personnel. I was representing this great community. I could feel that strength on 4 July 2012.
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More information about the search for the Higgs boson and the research carried out since its discovery can be found in the Higgs10 series published in 2022, on the tenth anniversary of the discovery.
Fabiola Gianotti, CERN Director-General, was spokesperson of the ATLAS experiment at the Large Hadron Collider from 2009 to 2013 and responsible for announcing the discovery of the Higgs boson, alongside the CMS spokespersonAt the Large Hadron Collider (LHC), heavy ions are accelerated to extremely high energies, which creates strong electromagnetic fields. As a result, photons from the oncoming lead-ion beams can interact with each other or with the nuclei; these interactions are known as ultraperipheral collisions. Photon–nucleus scatterings at the highest energy that can be achieved with existing particle accelerators are useful probes that allow physicists to investigate the structure of nuclei. While the common picture of nucleons is that they contain three quarks (up–up–down for protons and up–down–down for neutrons), in reality, a complex sea consisting of quark–antiquark pairs and gluons makes up a large fraction of the proton and neutron energies. Ultraperipheral collisions are an extraordinary tool to test the nature of nuclear matter.
The CMS experiment has recently released the first results using data from the first heavy-ion run of LHC Run 3. The results measure the production of D0 mesons (containing a charm quark and an up antiquark) and their antiparticles, D0 bar mesons (made of an up quark and a charm antiquark), in ultraperipheral collisions for the first time. D0 mesons are formed by charm quarks that are kicked out of the nuclei by the photons and carry information about parton distribution functions, which describe how quarks and gluons behave inside nuclei.
To measure D0 production, the CMS detector first selects events in which photon and lead nuclei collisions have caused the latter to break up. When this happens, neutrons flow from the collision in parallel to the beam, whereas protons and intact nuclei will follow a curved path as their charge interacts with the LHC’s magnetic fields. Two calorimeters, at zero degrees to the beam and located 140 m away on either side of the interaction point, are able to detect such neutrons. If they are seen in one calorimeter and not the other, in a time window consistent with the collision, this event is selected for further investigation.
Then, the products of the D0 decay – oppositely charged kaon and pion pairs – are reconstructed in the CMS detector. Physicists consider all combinations of pion and kaon trajectories, with each track taking an assumed mass of the kaon and pion. They then filter these combinations using the data to identify tracks that match what they expect from a D0 meson. From this, they are able to measure the so-called production cross section, which is the rate at which D0 mesons are produced.
For CMS, the study of nuclear structure using D0 meson production is just one of many applications of ultraperipheral collisions. With time, as methods are refined and systematic uncertainties are reduced, this technique will be able to constrain the parton distribution functions, allowing physicists to understand the structure of nuclear matter more deeply.
The next heavy-ion run of LHC Run 3 will start at the beginning of November.
Read more:
ndinmore Fri, 10/25/2024 - 09:55 Byline CMS collaboration Publication Date Fri, 10/25/2024 - 09:50Our dear colleague and friend, Karel Šafařík, sadly passed away on Monday 7 October. Karel graduated in theoretical physics in Bratislava in 1976, and worked at JINR in Dubna for more than 11 years, participating in experiments on the phenomenology of particle production at high energies at Dubna and at Serpukhov.
In 1990, Karel joined the Collège de France and the heavy-ion programme at CERN and very soon became one of the most influential scientists in the OMEGA series of heavy-ion experiments at the CERN SPS (WA85, WA94, WA97, NA57). In 2002, Karel was awarded the Slovak Academy of Sciences Prize for his contributions to the observation of the enhancement of the production of multistrange particles in heavy-ion collisions at the SPS, one of the pillars of the CERN announcement of the observation of the quark–gluon plasma in February 2000. In 2013, he was also awarded the medal of the Czech Physical Society.
Karel was one of the founding members of the ALICE collaboration. As early as 1991, he was part of the small group working on the design of a dedicated heavy-ion detector for the LHC, which would then become the ALICE experiment. He had a central role in shaping ALICE, from the definition of physics topics and the detector layout, to the design of the data format, tracking, storage and analysis. He had a central role in convincing the collaboration to introduce two layers of pixel detectors, in order to reconstruct, amidst the thousands of tracks produced in central Pb–Pb collisions at the LHC, the topologies of charm hadrons decaying only a few tens of microns from the primary vertex. At the time, many people considered this to be impossible in heavy-ion collisions, and yet this has become today one of the pillars of the ALICE physics programme and long-term scientific strategy. He was the ALICE Physics Coordinator for many years leading up to and including the first ALICE data taking.
Over the years, Karel also made multiple contributions to ALICE Upgrade studies and became known as the “wise man” to be consulted for the trickiest questions. He had organised the ALICE Upgrade Week in Prague in 2022. He was remembered with a minute’s silence at the beginning of the first plenary session of the ALICE Upgrade Week in Krakow (which he had originally planned to attend), on the day of his passing.
Karel was a top-class physicist, with a sharp analytical mind, a legendary memory and a seemingly unlimited set of competencies ranging from higher mathematics to formal theory, from detector physics to high-performance computing. At the same time, he was a generous, caring and kind colleague, and over the years has variously supported, helped, mentored and guided a large number of ALICE collaborators.
Karel’s passing leaves a huge void. We shall miss him. Our thoughts go out to his family, friends and close colleagues.
His friends and colleagues in the ALICE collaboration
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An obituary will also appear in the CERN Courier.
ndinmore Thu, 10/17/2024 - 10:29 Publication Date Thu, 10/17/2024 - 10:25In collisions between protons at the Large Hadron Collider (LHC), pairs of top quarks – the heaviest known elementary particles – are frequently produced along with other heavy quarks, including bottom and charm quarks. These collision events can provide physicists with valuable insights into quantum chromodynamics (QCD), the theory that describes the strong force. Precisely determining the production rates (or “cross-sections”) of these processes also enables researchers to more effectively distinguish them from rarer phenomena.
In two recent studies, the ATLAS collaboration analysed proton–proton collision data from LHC Run 2 (2015–2018) to measure how often top-quark pairs are produced alongside bottom quarks or charm quarks and to explore the detailed dynamics of these processes.
The first ATLAS study focused on how often top-quark pairs are created along with “jets” of particles produced by bottom quarks (b-jets). To identify (or “tag”) these b-jets with high accuracy, researchers used tools called flavour-tagging algorithms, which look for specific patterns, such as how far through the ATLAS detector particles traverse before they decay.
By analysing events with opposite-charge electron–muon pairs and at least three or four b-jets, ATLAS physicists made the most precise measurements to date of the total cross-sections of this top-quark-pair process in these event categories. These measurements surpass the accuracy of current theoretical predictions, especially in events with higher numbers of b-jets. The results were compared to multiple theoretical predictions to assess the accuracy of the modelling of this process.
The researchers also examined motional (or “kinematic”) properties, such as the momenta of the b-jets and the total jet energy, to test how well simulations of top-quark-pair production match the data. Their findings point to areas where theoretical models need improvement to better capture the complexities of additional b-jet production.
The second ATLAS study, presented recently at the 17th International Workshop on Top Quark Physics, broke new ground by providing the first dedicated ATLAS measurement of how often top-quark pairs are produced along with jets originating from charm quarks (c-jets). ATLAS physicists analysed events with one or two leptons (electrons and muons), using a custom flavour-tagging algorithm developed specifically for this study to distinguish c-jets from b-jets and other jets. This algorithm was essential because c-jets are even more challenging to identify than b-jets, as they have shorter lifetimes and produce less distinct signatures in the ATLAS detector.
The study found that most theoretical models provided reasonable agreement with the data, though they generally underpredicted the production rates of c-jets. These results, which for the first time separately determined the cross-sections for single and multiple charm-quark production in top-quark-pair events, highlight the need for refined simulations of these processes to improve future measurements.
Taken together, these studies deepen the understanding of the top quark’s relationship with bottom and charm quarks within the framework of QCD. In addition, they pave the way for new explorations of rarer processes involving top quarks – such as the simultaneous production of four top quarks – that could push the boundaries of known physics.
Read more on the ATLAS website.
abelchio Wed, 10/16/2024 - 10:13 Byline ATLAS collaboration Publication Date Wed, 10/16/2024 - 10:05Geneva, 11 October 2024. During its two hundred-and-eighteenth session, the CERN Council formally finalised the organisation of the European Strategy process by appointing the Physics Preparatory Group, whose aim will be to prepare the scientific contribution based on the input from the community. The European Strategy is driven by the CERN Council, the highest authority of the Organization, composed of representatives of all Member States. During the next year and a half, the whole community will work to develop a common vision for the future of particle physics in Europe – including which major project should succeed the Large Hadron Collider at CERN. The process is expected to be concluded at the end of 2025, after which the European Strategy Group will submit its recommendations to the Council.
Particle physics, the study of the smallest constituents of matter and the laws of nature at the most fundamental level, is at a fascinating juncture. The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 confirmed the final predicted piece of the Standard Model of particle physics – a powerful quantum theory which describes three of nature’s four fundamental forces. Yet, despite its unparalleled success, the Standard Model explains only 5% of the Universe and is not able to answer other outstanding questions. It cannot account for the invisible “dark matter” that influences the motion of galaxies, nor what is causing the expansion of the Universe to speed up. It also falls short in explaining why matter and antimatter did not annihilate each other in the first moments of the Big Bang to leave a featureless sea of radiation. Studying the Higgs boson and other elementary particles in greater detail and exploring nature at higher energies using next-generation colliders will be key in tackling these and other profound open questions.
“The discoveries at the LHC of the Higgs particle and more than 70 new composite hadrons have been highlights of an amazing decade in particle physics dominated by experiments at CERN, which will be continued at the LHC and its high-luminosity upgrade until 2041. Researchers across the globe agree that deeper study of the Higgs particle is certain to lead to very valuable scientific results, and the CERN Council is united in its vision to have CERN continue to provide the most exciting experimental scientific programme in high energy physics with the aid of the best possible technology. The community and the Council therefore eagerly await the recommendations of the European Strategy Group on the best path to be taken", said CERN Council President, Eliezer Rabinovici, Hebrew University of Jerusalem.
The third update of the European Strategy for Particle Physics was launched by the CERN Council on 21 March. In June, the Council elected Professor Karl Jakobs (University of Freiburg) as Strategy Secretary and established the European Strategy Group, which will be responsible for submitting final recommendations to the Council for approval in early 2026. The European Strategy Group remit states that the aim of the Strategy is to develop “a visionary and concrete plan that greatly advances human knowledge in fundamental physics through the realisation of the next flagship project at CERN”.
The previous update of the European Strategy for Particle Physics, completed in 2020, recommended that Europe, together with its international partners, should investigate the technical and financial feasibility of a future hadron collider at CERN with a centre-of-mass energy of at least 100 TeV and with an electron-positron Higgs and electroweak factory as a possible first stage.
Since then significant progress has been made and an international consensus reached on the scientific case for a Higgs factory. A mid-term report on the feasibility study for the proposed multi-stage Future Circular Collider (FCC) at CERN was presented in March 2024, with a final report expected in spring 2025. In December 2023, a rigorous planning exercise in the US prioritised support for a Higgs factory based outside the US. It was followed in April 2024 by a joint statement of intent by CERN and the US government to continue collaboration on the feasibility study for the FCC Higgs factory (FCC-ee) and on its construction and physics exploitation, should the CERN Member States determine the FCC-ee to be CERN’s next world-leading research facility.
“Given the long timescales involved in building large colliders, it is vital that the community is united so as to enable the Council to take a decision on the next collider at CERN in 2027/2028. There is excellent progress with the LHC and no new indication that would change our physics priorities: understanding the Higgs boson much better and exploring further the energy frontier are key to the next project”, said Strategy Secretary, Karl Jakobs, University of Freiburg.
In addition to identifying the preferred option for the next collider at CERN, the Strategy update is expected to prioritise alternative options to be pursued if the chosen preferred plan turns out not to be feasible or competitive. It will also indicate areas of priority for scientific exploration complementary to colliders, as well as for other items identified as relevant to the field. These include accelerator, detector and computing research and development, theory developments, actions to minimise environmental impact and improve the sustainability of accelerator-based particle physics, initiatives to attract, train and retain early-career researchers, and public engagement.
During its September session the Council appointed members of the Physics Preparatory Group, which will prepare scientific input to the work of the European Strategy Group based on the views it gathers from the community. The Council also announced that the Strategy Open Symposium, at which researchers will be invited to debate the future orientation of European particle physics, will take place in Venice from 23 to 27 June 2025.
“This year we celebrate seven decades of groundbreaking progress in fundamental science and technology at CERN, achieved through international collaboration. This success is the result of CERN’s unique culture and the CERN community’s boldness in pursuing projects at the limit of human capabilities and has only been possible thanks to the support of our Member and Associate Member States and our partners from all over the world. Future projects will be even more challenging, but their expected physics and technological impact is immense. The future of CERN and the field is bright”, said CERN Director-General, Fabiola Gianotti.
Further information:
Strategy webpage: europeanstrategyupdate.web.cern.ch/welcome
2020 Strategy update: https://cds.cern.ch/record/2720129
About the European Strategy for Particle Physics
The European Strategy for Particle Physics is an open, inclusive and evidence-driven process and takes into account the worldwide particle physics landscape and developments in related fields. It was initiated by the CERN Council in 2005, when completing the Large Hadron Collider (LHC) was named as the top scientific priority. The first strategy update, adopted in 2013, continued to prioritise the LHC and its high-luminosity upgrade and stated that Europe needed to be in a position to propose an ambitious post-LHC accelerator project at CERN by the time of the next update. The second update, completed in 2020, recommended an electron-positron “Higgs factory” collider as the highest-priority next facility and, in the longer term, that Europe should have the ambition to operate a proton-proton collider at the highest achievable energies.
The third strategy update, launched on 21 March 2024, invites the community to submit written input by 31 March 2025. This will be followed by a scientific open symposium in Venice from 23 to 27 June 2025, where the community will be invited to debate the future orientation of European particle physics. A “briefing book” based on the input and discussions will then be prepared by the Physics Preparatory Group. The briefing book will be submitted to the European Strategy Group by the end of September 2025 for consideration during a five-day-long drafting session which is scheduled to take place from 1 to 5 December 2025. The European Strategy Group is expected to submit the proposed strategy update to the CERN Council by the end of January 2026.
sandrika Fri, 10/11/2024 - 09:09 Publication Date Fri, 10/11/2024 - 11:08
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.
