Your shopping cart is empty.
In 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.
Further material:
Infographic
Photo collection
Video collection
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
_____
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.
----
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
___
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 - 11:08In an article recently published in Physical Review X, the ALICE collaboration presented its studies of correlations in the kaon–deuteron[1] and proton–deuteron systems, opening the door to precise studies of the forces in three-body nuclear systems.
A fundamental force is typically described as an interaction between two objects. Extending this to more complicated systems is not always trivial. The description of strongly interacting three-hadron systems is key to understanding many phenomena in modern nuclear physics, such as the structure of nuclei, properties of high-density nuclear matter and the composition of neutron star cores.
Proton–proton collisions at the LHC produce a large number of particles that are emitted very close to each other, at distances of about 10-15 m (a femtometre). It is interesting to explore whether they influence each other in any way before spraying off in all directions. If two particles are produced close to each other and with similar momenta and direction, the pair can be subject to quantum statistics, Coulomb force and strong interaction. If one of the pair is a deuteron, then a system with a deuteron and another hadron, like a proton or a kaon, is effectively a three-body system. Thus, the measurement of correlations between deuterons and kaons or protons is expected to reveal the interactions of three-body systems.
The ALICE collaboration utilises its excellent particle identification capabilities to study these correlations in high-multiplicity proton–proton collisions at a centre-of-mass energy of 13 TeV. The result is a correlation function that measures how the probability of finding two particles with certain relative momenta differs from what would be expected if their momenta were completely independent or uncorrelated. In the absence of correlation, the value of the function is unity. A value above one indicates attractive interaction, whereas a value below one indicates repulsive interaction.
The correlation functions for both the kaon–deuteron and proton–deuteron systems are below unity for low relative transverse momenta (as seen in the figure below), indicating an overall repulsive interaction. The analysis of the kaon–deuteron correlation shows that the relative distances at which deuterons and protons or kaons are produced are quite small, around 2 fm.
The kaon–deuteron correlations are well described with an effective two-body model that incorporates both the Coulomb interaction and strong interaction between the kaon and the deuteron. In contrast, the same effective two-body approach fails to describe the proton–deuteron correlations, necessitating a full three-body calculation that accounts for the structure of the deuteron. An excellent data description is achieved using theoretical calculations that account for both two- and three-body strong interactions. This demonstrates the sensitivity of the correlation function to the short-range dynamics of the three-nucleon system.
The correlation measurements at short distances constitute an innovative method to study three-body systems at the LHC, with the potential to extend such studies to other hadrons. It is envisaged to apply a similar approach to data from LHC Runs 3 and 4 to investigate three-baryon systems in the strange and charm sectors, which are otherwise experimentally inaccessible.
Measured correlation function as a function of relative momentum in (left) kaon–deuteron fitted with effective two-body calculation and (right) proton–deuteron fitted with full three-body calculation. (Image: ALICE/CERN)[1] Deuteron is the atomic nucleus of deuterium, composed of a neutron and a proton held together by the strong interaction.
ptraczyk Wed, 09/25/2024 - 14:29 Byline ALICE collaboration Publication Date Wed, 09/25/2024 - 14:26Geneva, 25 September 2024. At a seminar held at CERN this week, the NA62 collaboration reported the unequivocal confirmation of the ultra-rare decay of a positively charged kaon into a positively charged pion and a neutrino–antineutrino pair. Experiments including NA62 have previously measured and seen evidence of this process, but this is the first time it has been measured with a statistical significance of five standard deviations, crossing the threshold traditionally required to claim a discovery in particle physics.
Denoted by K+→π+νν, this decay is among the rarest particle processes ever observed: in the Standard Model of particle physics, less than one in 10 billion positively charged kaons are predicted to decay in this way.
“This observation is the culmination of a project that started more than a decade ago,” says NA62 spokesperson Giuseppe Ruggiero. “Looking for effects in nature that have probabilities of happening of the order of 10-11 is both fascinating and challenging. After rigorous and painstaking work, we have finally seen the process NA62 was designed and built to observe.”
But why are physicists looking for a process that occurs so rarely? The reason is that theoretical models suggest that the K+→π+νν decay is extremely sensitive to deviations from the Standard Model prediction, making it one of the most interesting processes to search for evidence of new physics beyond the Standard Model.
Analysing data collected by the NA62 detector between 2016 and 2022, the NA62 researchers measured the fraction of K+ that decay in this way to be 13.0 +3.3
-2.9 × 10-11. With a relative precision of 25%, this is the most precise measurement of the K+→π+νν decay to date.
The result is about 50% larger than the Standard Model prediction but is compatible with it given the overall uncertainty. With data taking ongoing, NA62 is set to be able to test the possibility of new physics in this decay within the next few years.
“Searching for hints of new physics in this decay requires more data, but this result is a leap forward and further strengthens the strong interest in this line of research,” says NA62 physics coordinator Karim Massri.
In the NA62 experiment, kaons are produced by slamming a high-intensity proton beam from CERN’s Super Proton Synchrotron into a stationary target. As a result, almost a billion secondary particles are produced each second, and these fly into the NA62 detector. Of these particles, about 6% are positively charged kaons. NA62 precisely detects the decay products of kaons, identifying and measuring all the particles produced except the neutrinos, whose presence is deduced from their missing energy.
Crucial to this result was the data from 2021 and 2022, which was taken following the completion of detector upgrades that enabled NA62 to operate at 30% higher beam intensities. Combined with improvements to data-analysis techniques, these hardware upgrades enabled the collection of signal candidates 50% faster than before, while adding new tools to suppress the background processes that could mimic the K+→π+νν decay.
“This measurement relies on identifying the one-in-10-billion K+ decay that is our signal and making sure it is not one of the other 9 999 999 999 decays that can mimic the signal,” says lead data analyst Joel Swallow. “The whole NA62 collaboration has made this almost impossible result possible.”
ssanchis Wed, 09/25/2024 - 08:30 Publication Date Wed, 09/25/2024 - 11:28The CMS experiment at CERN is the latest to weigh in on the mass of the W boson – an elementary particle that, along with the Z boson, mediates the weak force, which is responsible for a form of radioactivity and initiates the nuclear fusion reaction that powers the Sun.
At a seminar held at CERN today, the CMS collaboration reported how it has analysed proton–proton collision data from the second run of the Large Hadron Collider, the Laboratory’s flagship particle accelerator, to make its first mass measurement of this fundamental particle.
The result is the most precise measurement of the W mass made so far at the LHC, and is in line with the prediction from the Standard Model of particle physics and with all previous measurements, except the measurement from the CDF experiment at the former proton–antiproton Tevatron collider at Fermilab.
In the Standard Model, the W mass is related closely to the strength of the interaction unifying the electromagnetic and weak forces and to the masses of the Higgs boson and the top quark, which constrain its value to 80353 million electronvolts (MeV) within an uncertainty of 6 MeV.
Measuring the W boson mass with high precision therefore makes it possible to test whether or not these properties all align in a way that is consistent with the Standard Model. If they don’t, the cause could be new physics phenomena such as new particles or interactions.
Since its discovery at CERN about 40 years ago, the W boson has had its mass measured ever more precisely by several collider experiments, at CERN and elsewhere. In 2022, a surprisingly high value of its mass measured by the CDF experiment plunged the particle into a “midlife crisis”. The CDF W boson mass, 80433.5 MeV with an uncertainty of 9.4 MeV, differed significantly from the Standard Model prediction and from the other experimental results, calling for more studies.
In 2023, the ATLAS collaboration, which provided its first W boson mass measurement in 2017, released an improved measurement based on a reanalysis of proton–proton collision data from the first run of the LHC. This improved result, 80366.5 MeV with an uncertainty of 15.9 MeV, lined up with all previous measurements except the CDF measurement, which remains the most precise to date, with a precision of 0.01%.
The CMS experiment has now contributed to this global endeavour with its first W boson mass measurement. The keenly anticipated result, 80360.2 with an uncertainty of 9.9 MeV, has a precision comparable to that of the CDF measurement and is in line with all previous measurements except the CDF result.
“The wait for the CMS result is now over. After carefully analysing data collected in 2016 and going through all the cross checks, the CMS W mass result is ready,” says outgoing CMS spokesperson Patricia McBride. “This analysis is the first attempt to measure the W mass in the harsh collision environment of the second running period of the LHC. And all the hard work from the team has resulted in an extremely precise W mass measurement and the most precise measurement at the LHC.”
“W mass measurements are very challenging, involving delicate measurements and theoretical modelling of the production of the W boson and its decay into a lepton (here, a muon) and a neutrino that escapes detection,” explains incoming CMS spokesperson Gautier Hamel de Monchenault. “By exploiting the ability of the CMS detector to measure muons with high precision and using the latest and most advanced theoretical ingredients, some of which were tested by a cross-checking analysis, we attained this record level of precision.”
The result showcases once again the exceptional performances of the LHC and its detectors, which continue to push the precision frontier and put the Standard Model and its extensions to ever more stringent tests. Further data from the ongoing third run of the collider and from its upgrade, the High-Luminosity LHC, is poised to push this frontier further.
Measurements of the W boson mass reported over the years by experiments worldwide. The CMS measurement is the most precise one made at the LHC so far, and has a precision comparable to that of the CDF measurement. (Image: CMS/CERN)
sandrika Mon, 09/16/2024 - 16:49 Publication Date Tue, 09/17/2024 - 16:43Quantum entanglement is a fascinating feature of quantum physics – the theory of the very small. If two particles are quantum-entangled, the state of one particle is tied to that of the other, no matter how far apart the particles are. This mind-bending phenomenon, which has no analogue in classical physics, has been observed in a wide variety of systems and has found several important applications, such as quantum cryptography and quantum computing. In 2022, the Nobel Prize in Physics was awarded to Alain Aspect, John F. Clauser and Anton Zeilinger for groundbreaking experiments with entangled photons. These experiments confirmed the predictions for the manifestation of entanglement made by the late CERN theorist John Bell and pioneered quantum information science.
Entanglement has remained largely unexplored at the high energies accessible at particle colliders such as the Large Hadron Collider (LHC). In an article published today in Nature, the ATLAS collaboration reports how it succeeded in observing quantum entanglement at the LHC for the first time, between fundamental particles called top quarks and at the highest energies yet. First reported by ATLAS in September 2023 and since confirmed by two observations made by the CMS collaboration, this result has opened up a new perspective on the complex world of quantum physics.
"While particle physics is deeply rooted in quantum mechanics, the observation of quantum entanglement in a new particle system and at much higher energy than previously possible is remarkable,” says ATLAS spokesperson Andreas Hoecker. “It paves the way for new investigations into this fascinating phenomenon, opening up a rich menu of exploration as our data samples continue to grow."
The ATLAS and CMS teams observed quantum entanglement between a top quark and its antimatter counterpart. The observations are based on a recently proposed method to use pairs of top quarks produced at the LHC as a new system to study entanglement.
The top quark is the heaviest known fundamental particle. It normally decays into other particles before it has time to combine with other quarks, transferring its spin and other quantum traits to its decay particles. Physicists observe and use these decay products to infer the top quark’s spin orientation.
To observe entanglement between top quarks, the ATLAS and CMS collaborations selected pairs of top quarks from data from proton–proton collisions that took place at an energy of 13 teraelectronvolts during the second run of the LHC, between 2015 and 2018. In particular, they looked for pairs in which the two quarks are simultaneously produced with low particle momentum relative to each other. This is where the spins of the two quarks are expected to be strongly entangled.
The existence and degree of spin entanglement can be inferred from the angle between the directions in which the electrically charged decay products of the two quarks are emitted. By measuring these angular separations and correcting for experimental effects that could alter the measured values, the ATLAS and CMS teams each observed spin entanglement between top quarks with a statistical significance larger than five standard deviations.
In its second study, the CMS collaboration also looked for pairs of top quarks in which the two quarks are simultaneously produced with high momentum relative to each other. In this domain, for a large fraction of top quark pairs, the relative positions and times of the two top quark decays are predicted to be such that classical exchange of information by particles traveling at no more than the speed of light is excluded, and CMS observed spin entanglement between top quarks also in this case.
“With measurements of entanglement and other quantum concepts in a new particle system and at an energy range beyond what was previously accessible, we can test the Standard Model of particle physics in new ways and look for signs of new physics that may lie beyond it.” says CMS spokesperson Patricia McBride.
Read more:
ldragu Fri, 09/06/2024 - 14:28 Publication Date Wed, 09/18/2024 - 16:00A central aim of the ATLAS Higgs physics programme is to measure, with increasing precision, the strength of interactions of the Higgs boson with elementary fermions and bosons. According to the theory of electroweak symmetry breaking, these interactions are responsible for generating the masses of the particles. The interaction strengths can be determined by precisely measuring the Higgs boson’s production via and decay into the relevant particles.
At the recent International Conference on High-Energy Physics (ICHEP) 2024, the ATLAS collaboration presented improved measurements of the strength of Higgs boson interactions with the three heaviest quarks: top, bottom and charm. The new results are based on a reanalysis of LHC Run 2 data taken in the years 2015–2018 with significantly enhanced analysis methods, including improved jet tagging.
But what are jets and why do they need to be tagged? When the Higgs boson decays into a pair of quarks, each quark fragments, creating a collimated spray of particles (mostly hadrons) that can be observed in the detector. The aim of jet tagging is to determine which type (or “flavour”) of quark produced a given jet through detailed analysis of the jet’s properties. With new bespoke jet (or “flavour”) tagging techniques for charm and bottom quarks, ATLAS researchers managed to significantly increase the sensitivity of their analyses. Together with other analysis improvements, they increased sensitivity to H→bb and H→cc decays by 15% and a factor of three, respectively.
Updated measurements of Higgs boson production in association with a W or Z boson and decays into a pair of bottom or charm quarks yielded the first observation of the WH, H→bb process with 5.3σ significance and a measurement of ZH, H→bb with 4.9σ significance. The Higgs boson decay into c quarks is suppressed by a mass factor of 20 relative to the decay into b quarks and thus is still too rare to be observed. ATLAS sets an upper limit on the rate of the VH, H→cc process of 11.3 times the Standard Model prediction. These results are the most precise probes of these processes to date, and they are compatible with the Standard Model.
A new measurement of Higgs boson interaction with the top quark focused on Higgs production in association with two top quarks and its subsequent decay into a pair of bottom quarks. This challenging process features a very complex final state and suffers from large backgrounds. The new analysis, which benefits from a refined understanding of the dominant background processes involving top quarks, improved the sensitivity by a factor of two and measured a signal strength for ttH, H→bb production of 0.81 ± 0.21, relative to the Standard Model prediction.
Further improved analysis techniques and new data from the ongoing Run 3 hold the promise of measuring these interactions with even greater precision. These advancements in the search for H→cc heighten anticipation for the High-Luminosity LHC (HL-LHC), where detecting this process enters the realm of feasibility.
Read more in the ATLAS briefings here and here.
ptraczyk Wed, 09/04/2024 - 11:14 Byline Piotr Traczyk Publication Date Wed, 09/04/2024 - 11:133 September 2024 · Voir en français
Part 16 of the CERN70 feature series. Find out more: cern70.cern
Walter Oelert led the team of researchers who produced the world’s first atoms of antihydrogen in 1995
For each particle, there exists an antiparticle with opposite properties, in particular electric charge. This has been well established, ever since Paul Dirac's theoretical predictions in the late 1920s. Over the three decades that followed, scientists discovered the constituents that would make up an antimatter atom: the antielectron (or positron), the antiproton and the antineutron. But it was not until 1995 that a CERN collaboration succeeded in making, for the first time in the world, a few atoms of antihydrogen, each consisting of one positron and one antiproton.
These first antihydrogen atoms were observed by the PS210 experiment, in the Low Energy Antiproton Ring (LEAR). This experiment was a real challenge, both for the experimenters and the LEAR operation team, but they succeeded, although the antimatter atoms were produced with nearly the speed of light and could not be used for further investigations of the properties of antimatter. On 4 January 1996, CERN and the institutes participating in the experiment issued a press release announcing the observations. The news, which seemed to come straight out of a science fiction film, went round the world.
This success, and the considerable interest shown by scientists and the public, opened up a new field of study and launched the development of a new machine. The Antiproton Decelerator (AD) supplied several experiments with antiprotons from 2000 onwards. Just two years later, exactly 100 years after the birth of Paul Dirac, two collaborations announced the successful production of many “cold” antihydrogen atoms. Since then, experiments on antimatter have been increasingly successful, measuring the fundamental parameters of antiprotons and antihydrogen with ever greater precision. A second decelerator, ELENA, which came into service in 2020, further slows down the antiprotons to make it easier to study them.
Recollections No one in that small new collaboration will ever forget those exciting days in 1995. Every part of the experimental set-up would have to work; there would be no second chance.Walter Oelert, then a researcher at the Jülich Research Centre in Germany, led the team who worked on the PS210 experiment. This experiment was rapidly assembled in 1995 to take advantage of the last weeks of the LEAR machine, in order to produce the world’s first atoms of antihydrogen. He then took part in one of the pioneering experiments on antimatter in the early 2000s and was one of the initiators of the new ELENA antimatter decelerator.
“I’d like to quote Werner Heisenberg from 1972: “the discovery of antimatter was perhaps the biggest jump of all the big jumps in physics in our century”. He refers to the trendsetting predictions of Dirac, followed by the discovery and observation of the positron and antiproton in 1932 and 1955 respectively, with antihydrogen still awaiting its first observation.
During the early 1990s, the JETSET collaboration was running an experiment with antiprotons at LEAR. At a coffee break in autumn 1993, three accelerator scientists, Michel Chanel, Pierre Lefèvre and Dieter Möhl, reported that they had heard about an idea of how antihydrogen could be produced with just the experimental set-up that the scientists were using on JETSET, with some additional detection systems. The JETSET collaboration was not interested in joining this enterprise, but after a few parasitic tests some of us were convinced that it must work and finally the LEAR programme committee allocated 48 hours of beam time for the experiment PS210 for producing antihydrogen atoms in flight.
No one in that small new collaboration will ever forget those exciting days in 1995. Every part of the experimental set-up would have to work; there would be no second chance. To add to the pressure, the press had already got hold of what was planned before we had any interactions that could possibly create the first antihydrogen atoms observed on Earth.
We finally took data during a period of four weeks, two hours a day. Then it became really hectic. Offers from journals to publish the successful production of antihydrogen atoms were made even before the final results were available! A press release was prepared although the referee of the scientific paper was not willing to accept proof of the observation of antihydrogen atoms based only on the coincident registration of the annihilation signals from a positron and an antiproton, the constituents of an antihydrogen atom. However, further signals of energy and momentum determined during the experiment convinced both the referee and the editor, and the first observation of the production of atoms formed from antimatter particles could be announced. The public interest was enormous, the result made it to headlines in many newspapers including the “New York Times” and “Der Spiegel”.
One of the many interviews should be quoted:
In the final article, the journalist wrote something like: "Oelert thinks about an antimatter-bomb". But that is totally unrealistic, crazy science fiction and by no means my business. For basic science however, it was a great step towards the real work of producing ample amounts of cold antihydrogen and testing of fundamental symmetries. Seven years after this first observation, two experiments at the new AD complex successfully produced large numbers of cold antihydrogen atoms for the first time ever.
The ELENA decelerator, photographed in 2018. (Image: CERN)Approval for the ELENA upgrade was challenging. Luckily, after my presentation to the CERN Scientific Council, the Director-General rested a hand on my shoulder and said: “This was exactly the talk I wanted, thank you”. ELENA began in 2020 and now decelerates antiprotons from the AD even further, allowing many more to be trapped by the experiments and ushering a new, exciting era of antimatter research.
Now, almost 30 years after the fabrication of the first antihydrogen atoms, comparing hydrogen to antihydrogen atoms constitutes one of the best ways to make precise tests of differences between matter and antimatter. Their spectra are predicted to be identical, so any tiny differences would open a window to new physics.”
----
This interview is adapted from the 2004 book “Infinitely CERN”, published to celebrate CERN’s 50th anniversary, updated with the help of Walter Oelert in 2024. Heisenberg’s quote appears in The Physicist's Concept of Nature (1973), Vol. 1972, 271.
Walter Oelert led the team of researchers who produced the world’s first atoms of antihydrogen in 1995Born in Berlin, Max earned his Diploma in Physics in 1973 from Humboldt University of Berlin (HUB, East Germany, GDR), with a thesis on low-energy heavy-ion physics. He received his PhD in 1977 from the Institute for High Energy Physics (IHEP) of the Academy of Sciences of the GDR in Zeuthen (now part of DESY) on the subject of multiparticle production, and his “habilitation” degree in 1984 from HUB.
From 1973 to 1991, he conducted research at IHEP Zeuthen, spending several years from 1977 at the Joint Institute for Nuclear Research (JINR) in Dubna, and from the 1980s at DESY and CERN. He was awarded the Max von Laue Medal by the Academy of Sciences of the GDR in 1985 for his role in determining the asymmetric interaction of polarised positive and negative muons with the NA4 muon spectrometer at CERN’s SPS M2 muon beam.
Max worked at DESY from 1992 to 2006. As a member of the H1 experiment at the lepton–proton collider HERA from 1985, his research focused on investigating the internal structure of protons using deep inelastic scattering, and he served as H1 spokesperson from 2002 to 2006.
He became a Professor in the Department of Physics at the University of Liverpool in 2006 and joined the ATLAS collaboration in 2007. Within ATLAS, he made key contributions to data analysis, notably on the high-precision 7 TeV inclusive W and Z boson production cross sections and associated properties. He led the Liverpool ATLAS team for eight years, from 2009 to 2017. Under his guidance, the 30-strong group contributed to the maintenance of the SCT detector, as well as to ATLAS data preparation, precision SM measurements, Higgs physics studies, new physics searches and more. His contributions to understanding the proton structure using deep-inelastic scattering led to the award of the Max Born Prize by the Deutsche Physikalische Gesellschaft (DPG) and the Institute of Physics (IOP) in 2013.
Max had a unique ability to form collaborations, bringing together people from different backgrounds to work towards a common goal. From 2008 to 2022, he led the LHeC study, an electron beam upgrade of the LHC. Following the Higgs boson discovery in 2012, the CERN Management gave him the official mandate to develop this study and included LHeC in the European Strategy of Particle Physics discussions and as an integral part of the FCC study. Max was also a strong advocate for developing energy recovery linear (ERL) accelerators and was influential in the development of the PERLE ERL demonstrator accelerator at IJCLab, for which he acted as spokesperson up to 2023.
Besides being a distinguished scientist, he was a man of unwavering principles, grounded in his selfless interactions with others and his deep sense of humanity. His visions were rooted in realism. Drawing from his experience as a bridge between East and West, he was a strong advocate for international scientific collaboration and the responsibility of scientists toward their societies.
The many of us who were fortunate enough to have worked alongside Max over the years know how profound his knowledge of physics was and how dedicated he was to experimental particle physics. We admired his ability to mentor and support students, postdocs and early-career researchers, as well as his wise and calm approach to solving problems. Throughout his long and exceptional career, Max worked with passion on theory, detectors, accelerators and data analysis.
Max’s passing is a profound loss for ATLAS and the entire high-energy physics community, but his legacy will endure. Our deepest condolences go to his family, especially to his beloved wife and our close colleague, Uta.
His friends and colleagues
katebrad Wed, 08/28/2024 - 09:49 Publication Date Wed, 08/28/2024 - 09:54
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.
