Jobs – Page 29 of 505

Stavros Katsanevas 1953–2022

Stavros Katsanevas, who shaped the field of astroparticle physics in Europe, died on 27 November 2022. He had just become professor emeritus of Université Paris Cité and was preparing his return to the Astroparticle and Cosmology (APC) laboratory.

Born in Athens in 1953, Stavros pursued physics at the University of Athens. In 1979 he obtained his speciality doctorate from École polytechnique in Paris. He obtained his PhD at Athens in 1985, and later became an associate professor there (1989–1996). From 1979 to 1982 he spent three years as a postdoc at Fermilab. He also worked at CERN, as a research fellow (1983–1986), research associate (1991–1992) and corresponding fellow (1996). He was then appointed professor at the University Claude Bernard Lyon 1, and in 2004 became a professor at the University Paris VII Denis Diderot (now Université Paris Cité).

From 2002 to 2012 Stavros was deputy scientific director of IN2P3, during which he steered the institute to a leading position in astroparticle physics. He was particularly active in the emerging field of multi-messenger astronomy and in instrumentation. In this context, he played a key role in the creation of the APC laboratory in Paris, of which he was director from 2014 to 2017. Until his death, he led the French–Italian European Gravitational Observatory consortium, coordinating projects related to the detection of gravitational waves with the Virgo observatory.

Stavros’s scientific career was extremely rich, as evidenced by hundreds of publications on topics related to research collaborations, experimental techniques, or the conception and design of new research infrastructures. At CERN, he distinguished himself by developing software for simulating particle interactions, which later became a standard used at LEP. He also played an essential role in federating teams in several large international collaborative projects. One example is his involvement in the OPERA experiment at Gran Sasso laboratory; another is his leading role in the development of underwater neutrino telescopes, starting with the NESTOR project, which led to ANTARES and KM3NeT.

Over the past 15 years, Stavros played a central role in defining a global strategy in astroparticle physics. With the support of the European Commission, he created ASPERA, followed by the AstroParticle Physics European Consortium, which today gathers about 20 European countries. He was also involved in interdisciplinary research projects, mainly in the field of geosciences. He was co-director of the Laboratory of Excellence UnivEarthS from 2014 to 2018 and at the forefront of a seismometer project to be installed on the Moon.

Stavros was keen to promote science to a wide audience. Since 2015, he was a member of the jury for the Daniel and Nina Carasso Foundation, and in 2019 he organised an exhibition “The Rhythm of Space” at the museo della Grafica in Pisa. He was also coordinator of the European Horizon 2020 project REINFORCE, which intends to support more than 100,000 citizens to increase their awareness of and attitude towards science.

Stravros was driven by an inexhaustible desire to contribute to the advancement of science by serving, stimulating and animating the community. Steeped in philosophy, literature and poetry, he was also remarkably kind and generous. His thought, his vision, his driving force, will continue to accompany us.

Stanley Deser 1931–2023

Theoretical physicist Stanley Deser, a co-inventor of supergravity, passed away in Pasadena, California, on 21 April.

Stanley was born to middle-class Jewish parents in Rovno, then in Poland. In 1935 the family emigrated first to Palestine and then to France. After the Second World War broke out they fled to the US via Portugal (they were one of the families saved by Aristides de Sousa Mendes), eventually settling in Brooklyn. Stanley graduated Summa Cum Laude from Brooklyn College in 1949, and received his PhD at Harvard in 1953 under the supervision of Julian Schwinger. After postdocs at the Institute for Advanced Study in Princeton, NJ (1953–1955) and the Niels Bohr Institute in Copenhagen (1955–1957), and a lectureship at Harvard University (1957–1958), he joined the faculty of the physics department at Brandeis in 1958, where he remained until he retired in 2005. After moving to Pasadena, he remained an emeritus professor at Brandeis, and continued to publish physics papers until this year (as well as his autobiography, Forks in the Road, in 2021).

Stanley was a towering figure in theoretical high-energy physics, classical gravity and quantum gravity. His work cuts through mathematical complexity with deep physical insight. His first signature work, the Arnowitt–Deser–Misner (ADM) formalism, gave a Hamiltonian initial-value formalism for general relativity. This work is the foundation of precise calculations in inflationary cosmology, needed to match cosmic microwave background observations; and in numerical relativity calculations needed to interpret the results of gravitational-wave experiments. He leaves behind a lifetime of work in theoretical physics that remains foundational, including co-inventing supergravity (contemporaneously with Ferrara, Freedman and van Nieuwenhuizen) and formulating the dynamics of the superstring with Zumino; showing that general theories with massive gravity are inconsistent; and developing topologically massive gauge theories and gravity with Jackiw and Templeton.

Stanley was an important member of the scientific community. As Rainer Weiss, who shared the 2017 Nobel Prize in Physics for the observation of gravitational waves, related, he played an important role in convincing the National Science Foundation to fund the LIGO gravitational-wave detector. He was a fellow of the National Academy of Sciences (NAS) and the American Academy of Arts and Sciences; a foreign member of the Royal Society and the Torino Academy of Sciences; he was awarded the Dannie Heineman Prize in Mathematical Physics and the Einstein Medal, along with the Guggenheim and Fulbright awards; and held honorary doctorates from Stockholm University and the Chalmers Institute of Technology.

Stanley will be remembered for his wisdom and ready wit; emails and talks in which every sentence had multiple meanings and were packed with allusions and jokes; his delight and skill in acquiring languages; a love of travel; and a deep appreciation for art and literature.

Stanley was preceded in death by his wife, the artist Elsbeth Deser (daughter of Oskar Klein), and his daughter Eva. He leaves behind three daughters – retired linguist Toni Deser; theatre director Abigail Deser; and atmospheric scientist (and fellow NAS member) Clara Deser – and four grandchildren, Ursula, Oscar, Louise and Simon.

Kitty Wakley 1928–2023

A pillar of CERN is no more. Kitty Wakley, originally from Liverpool, UK started working at CERN in around 1960 and was the beloved leader of the document typing service (“typing pool”) until it was dissolved more than 30 years later. Back in the days before physicists and engineers became familiar with word-processing systems and LaTeX, they would present her with their scruffy, hand-written manuscripts for preprints and technical reports. The (occasionally approximate) English would be polished and typed to the highest standards by her team, following the CERN publication rules that her service had established.

Kitty presided over a close-knit team assembled from diverse backgrounds. She was a rather strict boss, in keeping with the usual unwritten standards of the time, but her team members still remember her fondly over 30 years later. Throughout her career at CERN, Kitty was unfailingly kind, cheerful and helpful towards all those who called on her services, from early-career researchers and technicians to Nobel prize-winners. Her mission was to help them disseminate their science in the best possible way, such as by working through the weekend with her team on the presentation of the discovery of the W boson.

Kitty was a much-loved institution of CERN. A lover of Italian opera, following her retirement from CERN she settled in Spain, where she lived for many years before passing away on 13 May, just four days before her 95th birthday. She is remembered fondly by many scientists who have passed through CERN over the decades.

A soft spot for heavy metal

Welding is the technique of fusing two materials, often metals, by heating them to their melting points, creating a seamless union. Mastery of the materials involved, meticulous caution and remarkable steadiness are integral elements to a proficient welder’s skillset. The ability to adjust to various situations, such as mechanised or manual welding, is also essential. Audrey Vichard’s role as a welding engineer in CERN’s mechanical and materials engineering group (MME) encompasses comprehensive technical guidance in the realm of welding. She evaluates methodologies, improves the welding process, develops innovative solutions, and ensures compliance with global standards and procedures. This amalgamation of tasks allows for the effective execution of complex projects for CERN’s accelerators and experiments. “It’s a kind of art,” says Audrey. “Years of training are required to achieve high-quality welds.”

Audrey is one of the newest additions to the MME group, which provides specific engineering solutions combining mechanical design, fabrication and material sciences for accelerator components and physics detectors to the CERN community. She joined the forming and welding section as a fellow in January 2023, having previously studied metallurgy in the engineering school at Polytech Nantes in France. “While in school, I did an internship in Toulon, where they build submarines for the army. I was in a group with a welder, who passed on his passion for welding to me – especially when applied in demanding applications.”

Extreme conditions

What sets welding at CERN apart are the variety of materials used and the environments the finished parts have to withstand. Radioactivity, high pressure to ultra-high vacuum and cryogenic temperatures are all factors to which the materials are exposed. Stainless steel is the most frequently used material, says Audrey, but rarer ones like niobium also come into play. “You don’t really find niobium for welding outside CERN – it is very specific, so it’s interesting and challenging to study niobium welds. To keep the purity of this material in particular, we have to apply a special vacuum welding process using an electron beam.” The same is true for titanium, which is a material of choice for its low density and high mechanical properties. It is currently under study for the next-generation HL-LHC beam dump. Whether it’s steel, titanium, copper, niobium or aluminium, each material has a unique metallurgical behaviour that will greatly influence the welding process. To meet the strict operating conditions over the lifetime of the components, the welding parameters are developed consequently, and rigorous control of the quality and traceability are essential.

“Although it is the job of the physicists at CERN to come up with the innovative machines they need to push knowledge further, it is an interesting exchange to learn from each other, juggling between ideal objects and industrial realities,” explains Audrey. “It is a matter of adaptation. The physicists come here and explain what they need and then we see if it’s feasible with our machines. If not, we can adapt the design or material, and the physicists are usually quite open to the change.”

Touring the main CERN workshop – which was one of CERN’s first buildings and has been in service since 1957 – Audrey is one of the few women present. “We are a handful of women graduating as International Welding Engineers (IWE). I am proud to be part of the greater scientific community and to promote my job in this domain, historically dominated by men.”

The physicists come here and explain what they need and then we see if it’s feasible with our machines

In the main workshop at CERN, Audrey is, along with her colleagues, a member of the welding experts’ team. “My daily task is to support welding activities for current fabrication projects CERN-wide. On a typical day, I can go from performing visual inspections of welds in the workshop to overseeing the welding quality, advising the CERN community according to the most recent standards, participating in large R&D projects and, as a welding expert, advising the CERN community in areas such as the framework of the pressure equipment directive.”

Together with colleagues from CERN’s vacuum, surfaces and coatings group (TE-VSC), and MME, Audrey is currently working on R&D for the Einstein Telescope – a proposed next-generation gravitational-wave observatory in Europe. It is part of a new collaboration between CERN, Nikhef and the INFN to design the telescope’s colossal vacuum system – the largest ever attempted (see CERN shares beampipe know-how for gravitational-wave observatories). To undertake this task, the collaboration is initially investigating different materials to find the best candidate combining ultra-high vacuum compatibility, weldability and cost efficiency. So far, one fully prototyped beampipe has been finished using stainless steel and another is in production with common steel; the third is yet to be done. The next main step will then be to go from the current 3 m-long prototype to a 50 m version, which will take about a year and a half. Audrey’s task is to work with the welders to optimise the welding parameters and ultimately provide a robust industrial solution to manufacture this giant vacuum chamber. “The design is unusual; it has not been used in any industrial application, at least not at this quality. I am very excited to work on the Einstein Telescope. Gravitational waves have always interested me, and it is great to be part of the next big experiment at such an early stage.”

A carnival of ideas in Kolkata

A one-of-a-kind conference MMAP (Macrocosmos, Microcosmos, Accelerator and Philosophy) 2020 was held in May last year in Kolkata, India, attracting 200 participants in person and remotely. An unusual format for an international conference, it combined the voyage from the microcosmos of elementary particles to the macrocosmos of our universe up to the horizon and beyond with accelerator physics and philosophy through the medium of poetry and songs, as inspired by the Indian poet Rabindranath Tagore and the creative giant Satyajit Ray.

The first presentation was by Roger Penrose, who talked about black holes, singularities and conformal cyclic cosmology. He discussed the cosmology of dark matter and dark energy, and inspired participants with the fascinating idea of one aeon going over to another aeon endlessly with no beginning or end of time and space.

Larry McLerran’s talk “Quarkyonic matter and neutron stars” provided an intuitive understanding of the origin of the equation of state of neutron stars at very high density, followed by Debadesh Bandyopadhyay’s talk on unlocking the mysteries of neutron stars. Jean-Paul Blaziot talked about the emergence of hydrodynamics in expanding quark–gluon plasma, whereas Edward Shuryak discussed the role of sphaleron explosions and baryogengesis in the cosmological electroweak phase transition. Subir Sarkar’s talk “Testing the cosmological principle” was provocative, as usual, and Sunil Mukhi and Aninda Sinha described the prospects for string theory. Sumit Som, Chandana Bhattacharya, Nabanita Naskar and Arup Bandyopadhyay discussed the low- and medium-energy physics possible using cyclotrons at Kolkata.

Moving to extreme nuclear matter, Barbara Jacak talked about experimental studies of transport in dense gluon matter. Jurgen Schukraft, Federico Antinori, Tapan Nayak, Bedangadas Mohanty and Subhasis Chattopadhyay spoke on signatures for the early-universe quark-gluon plasma and described the experimental programme of the ALICE experiment at the LHC, and Dinesh Srivastava focussed on the electromagnetic signatures of quark-gluon plasma.

A carnival of ideas, a mixture of low- to high-energy physics on the one hand and the cosmology of the creation of the universe on the other

Amanda Cooper-Sarkar emphasised the role of parton distribution functions in searches for new physics at colliders such as the LHC. Shoji Nagamiya presented the physics prospects of the J-PARC facility in Japan, Paolo Giubellino described the evolution of the latest FAIR accelerator at GSI, and Horst Stöcker discussed how to observe strangelets using fluctuation tools. In his presentation on the history of CERN, former Director-General Rolf Heuer talked about the marvels of large-scale collaboration capturing the thrill of a big discovery.

The MMAP 2020 conference witnessed a carnival of ideas, a mixture of low- to high-energy physics on the one hand and the cosmology of the creation of the universe on the other.

Magnificent CEvNS in Munich

Coherent elastic neutrino–nucleus scattering (CEvNS) is a new neutrino-detection channel with the potential to test the Standard Model (SM) at low-momentum transfer and to search for new physics beyond the SM (BSM). It also has applications in nuclear physics, such as measurements of nuclear form factors, and the detection of solar and supernova neutrinos. In the SM, neutrinos interact with the nucleus as a whole, enhancing the cross section by approximately the neutron number squared. However, detection is challenging as the observable is the tiny recoil of the nucleus, which has an energy ranging from sub-keV to a few tens of keV depending on the nucleus and neutrino source. Several decades after its prediction, CEvNS was measured for the first time in 2017 by the COHERENT experiment and the field has grown rapidly since.

The aims of the Magnificent CEvNS workshop, named after the Hollywood Western, are to bring together the broad community of researchers working on CEvNS and promote student engagement and connection among experimentalists, theorists and phenomenologists in this new field. The first workshop was held in 2018 in Chicago, and the most recent in Munich from 22 to 24 March with 96 participants.

Examining CEvNS opens a multitude of promising ways to look for BSM interactions. Improved limits on generalised neutrino interactions, new light mediators and sterile neutrinos derived from the complete COHERENT dataset were presented. These data enable the nuclear radius to be probed in a new way. More physics potential was highlighted in talks showing limits on the Weinberg angle and dark matter (axion-like particles). Notable advances by reactor experiments include new limits on CEvNS on germanium by the CONUS and NuGen experiments, which disagree with the previously published Dresden-II results.

The talks underlined the large experimental effort toward a complete mapping of the neutron and energy dependence of the CEvNS cross section. The observation of CEvNS on CsI and Ar by the COHERENT experiment will be complemented with future measurements on targets ranging from light (sodium) to heavy (tungsten) elements in COHERENT and new facilities such as NUCLEUS and Ricochet. Precision will be achieved by increasing statistics in CEvNS events with larger target masses, lower detection thresholds and increased neutrino flux. Reducing systematic effects by characterising backgrounds and detector responses is also critical. The growing precision will trigger studies on BSM physics in the near future, complementing high-energy experimental efforts.

A half-day satellite workshop “Into the Blue Sky” was dedicated to new ideas related to the CEvNS community. These included measurements of neutrino-induced fission, and detector concepts based on latent damage to the crystalline structure of minerals and superconducting crystals. The workshop was followed by a school organised by the Collaborative Research Center “Neutrinos and Dark Matter in Astro- and Particle Physics” at TU Munich from 27 to 29 March. Six lectures covered the fundamentals of low-energy neutrino physics with a focus on CEvNS, backgrounds, neutrino sources and detectors. The 40 participants then applied this knowledge by creating a fictional micro-CEvNS experiment.

Half a century since it was proposed theoretically, the physics accessible with CEvNS is proving to be extensive. The next Magnificent CEvNS workshop will take place next year at a new location and the participants are looking forward to further exploration of the CEvNS frontier.

Probing for periodic signals

ATLAS figure 1 — **Fig. 1.** The measured diphoton invariant mass distribution (grey), respective background-only fit parametrisation (red), analytical clockwork signal with k = 500 GeV and M₅ = 10,000 GeV, close to the sensitivity limit (green), and the background-plus-signal parametrisation (blue). Source: arXiv:2305.10894

New physics may come at us in unexpected ways that may be completely hidden to conventional search methods. One unique example of this is the narrowly spaced, semi-periodic spectra of heavy gravitons predicted by the clockwork gravity model. Similar to models with extra dimensions, the clockwork model addresses the hierarchy problem between the weak and Planck scales, not by stabilising the weak scale (as in supersymmetry, for example), but by bringing the fundamental higher dimensional Planck scale down to accessible energies. The mass spectrum of the resulting graviton tower in the clockwork model is described by two parameters: k, a mass parameter that determines the onset of the tower, and M₅, the five-dimensional reduced Planck mass that controls the overall cross-section of the tower’s spectrum.

At the LHC, these gravitons would be observed via their decay into two light Standard Model particles. However, conventional bump/tail hunts are largely insensitive to this type of signal, particularly when its cross section is small. A recent ATLAS analysis approaches the problem from a completely new angle by exploiting the underlying approximate periodicity feature of the two-particle invariant mass spectrum.

Graviton decays with dielectron or diphoton final states are an ideal testbed for this search due to the excellent energy resolution of the ATLAS detector. After convolving the mass spectrum of the graviton tower with the ATLAS detector resolution corresponding to these final states, it resembles a wave-packet (like the representation of a free particle propagating in space as a pulse of plane-wave superposition with a finite momenta range). This implies that a transformation exploiting the periodic nature of the signal may be helpful.

ATLAS figure 2 — **Fig. 2.** The scalograms resulting from the continuous wavelet transformation of the blue line of figure 1 in the scale-versus-mass space without (left) and with (right) realistic fluctuations. The localised blob at low scales (high frequencies) corresponds to the signal contribution, while the solid continuum at high scales (low frequencies) corresponds to the falling background shape. Source: arXiv:2305.10894

Figure 1 shows how a particularly faint clockwork signal would emerge in ATLAS for the diphoton final state. It is compared with the data and the background-only fit obtained from an earlier (full Run 2) ATLAS search for resonances with the same final state. As an illustration, the signal shape is given without realistic statistical fluctuations. The tiny “bumps” or the shape’s integral over the falling background cannot be detected with conventional bump/tail-hunting methods. Instead, for the first time, a continuous wavelet transformation is applied to the mass distribution. The problem is therefore transformed to the “scalogram” space, i.e. the mass versus scale (or inverse frequency) space, as shown in figure 2 (left). The large red area at high scales (low frequencies) represents the falling shape of the background, while the signal from figure 1 now appears as a clear, distinct local “blob” above m_γγ = k and at low scales (high frequencies).

The strongest exclusion contours to date are placed in the clockwork parameter space

With realistic statistical fluctuations and uncertainties, these distinct “blobs” may partially wash out, as shown in figure 2 (right). To counteract this effect, the analysis uses multiple background-only and background-plus-signal scalograms to train a binary convolutional neural-network classifier. This network is very powerful in distinguishing between scalograms belonging to the two classes, but it is also model-specific. Therefore, another search for possible periodic signals is performed independently from the clockwork model hypothesis. This is done in an “anomaly detection” mode using an autoencoder neural-network. Since the autoencoder is trained on multiple background-only scalograms (unlabelled data) to learn the features of the background (unsupervised learning), it can predict the compatibility of a given scalogram with the background-only hypothesis. A statistical test based on the two networks’ scores is derived to check the data compatibility with the background-only or the background+signal hypotheses.

Applying these novel procedures to the dielectron and diphoton full Run 2 data, ATLAS sees no significant deviation from the background-only hypothesis in either the clockwork-model search or in the model-independent one. The strongest exclusion contours to date are placed in the clockwork parameter space, pushing the sensitivity to beyond 11 TeV in M₅. Despite the large systematic uncertainties in the background model, these do not exhibit any periodic structure in the mass space and their impact is naturally reduced when transforming to the scalogram space. The sensitivity of this analysis is therefore mostly limited by statistics and is expected to improve with the full Run 3 dataset.

Inclusive photon production at forward rapidities

ALICE figure 1 — **Fig. 1.** The pseudorapidity density distributions of inclusive photons and predictions from Monte Carlo event generators compared with charged-particle measurements at midrapidity in pp, pPb and Pbp collisions (left), and for different multiplicity classes (right). Source: arXiv:2303.00590

The primary goal of high-energy heavy-ion physics is the study of a new state of nuclear matter, quark–gluon plasma, a thermalised system of quarks and gluons. The study of proton–proton (pp) and proton–nucleus (pA) collisions provides the baseline for the interpretation of results from heavy-ion collisions. The study of pA collisions also helps researchers understand the effects of cold nuclear matter on the production of final-state particles.

Global observables, such as the number of produced particles (particle multiplicity) and their distribution in pseudorapidity (η), provide key information about particle-production mechanisms in these collisions. The total multiplicity is mostly determined by soft interactions, i.e. processes with small momentum transfer, which cannot be calculated using perturbative techniques and are instead modelled using non-perturbative phenomenological descriptions. For example, the distribution of the number of produced particles can be used to disentangle relative contributions to particle production from hard and soft processes using a two-component model.

ALICE has recently completed the measurement of the multiplicity and pseudorapidity density distributions of inclusive photons at forward rapidity, spanning the range η = 2.3 to 3.9, by using the photon multiplicity detector (PMD) in pp, pPb and Pbp collisions at a centre-of-mass energy of 5.02 TeV per nucleon pair using LHC Run 1 and 2 data. Since photons mostly originate from decays of neutral pions, this result complements existing measurements of charged-particle production. A comparative study of charged particles and inclusive photons can reveal possible similarities and differences in the underlying production mechanisms for charged and neutral particles.

The PMD uses the preshower technique, where a three-radiation-length-thick lead converter is sandwiched between two planes comprising an array of 184,320 gas-filled proportional counters. Photons are distinguished from hadrons in the PMD’s preshower plane by applying suitable thresholds on the number of detector cells and the energy deposited in reconstructed clusters.

The measured distributions are corrected for instrumental effects using a Bayesian unfolding method. This is the first time that the dependence of the inclusive photon production on the number of nucleons participating in the pPb collision and its scaling behaviour has been studied at the LHC.

Figure 1 (left) compares the pseudorapidity density distribution of inclusive photons in minimum bias pp, pPb and Pbp collisions measured at forward rapidity to that of charged particles at midrapidity. The pseudorapidity distribution of inclusive photons at forward rapidity smoothly matches that of charged particles at midrapidity, indicating that the production mechanisms for charged and neutral pions are similar. Figure 1 (right) shows the pseudorapidity density distribution of inclusive photons in pPb collisions for different multiplicity classes as estimated using the energy deposited in the zero-degree calorimeter (ZNA) at beam rapidity. The multiplicity in the most central collisions reaches values twice as large as those in minimum bias events. The data and model agree within one sigma of the measurement uncertainties.

These results of inclusive photon production in pp, pPb and Pbp collisions provide valuable input for the development of theoretical models and Monte Carlo event generators, and help to establish the baseline measurements for the interpretation of PbPb collision data.

Charm production in proton–lead collisions

LHCb figure 1 — **Fig. 1.** Production ratio of prompt Ξ⁺_c to Λ⁺_c baryons times the branching ratio for Ξ⁺_c → pK^–π⁺. Source: arXiv:2305.06711

A crucial missing piece in our understanding of quantum chromodynamics (QCD) is a complete description of hadronisation in hard scattering processes with a large momentum transfer, which has now been investigated by the LHCb collaboration in proton–lead (pPb) collisions. While perturbative QCD describes reasonably well the transverse momentum (p_T) dependence of heavy-quark production in proton–proton (pp) collisions, the situation is different in heavy-ion collisions due to the formation of quark–gluon plasma (QGP), which affects the behaviour of particles traversing the medium. In particular, hadronisation can be affected, modifying the relative abundance of hadrons compared to pp collisions. Several models predict an enhanced strange-quark production. Thus an abundance of strange baryons is seen as a signature of QGP formation.

The role that QGP may play in pPb collisions is currently unclear. Some models predict the formation of “QGP droplets”, which could partially induce the same behaviour, albeit less pronounced, as in PbPb collisions. In addition, in pPb interactions, “cold nuclear matter” (CNM) effects are also present that can mimic the behaviour caused by QGP but via different mechanisms. For all these reasons, a strangeness enhancement in pPb collisions would strongly indicate the formation of a deconfined medium in small systems, providing crucial information about QGP properties and formation once the CNM effects are under control.

The LHCb collaboration recently analysed pPb data for QGP effects with the twofold purpose of searching for strangeness enhancement and providing a precise understanding of the CNM effects. This search was performed by measuring the production ratio of the strange baryon Ξ⁺_c, which has never been observed in pPb collisions before, to the strangeless baryon Λ⁺_c. Using an earlier pPb sample, LHCb has also studied the ratios of the D⁺_s, D⁺ and D⁰, the first being measured for the first time down to zero p_T in the forward region, precisely addressing CNM effects. All measurements are performed differentially in p_T and the rapidity of the produced particle, and compared to the latest theory predictions. The Ξ⁺_c cross section has been measured for the first time in pPb collisions, giving strong indications on the factorisation scale μ₀of the theory model. This result allows to set the absolute scale of the theoretical computations in terms of strangeness production, a trend confirmed with even higher precision by comparing the measurement to the Λ⁺_c production-cross section evaluated in the same decay mode. Moreover, the ratio is roughly constant as a function of p_Tand behaves in the same way at positive (pPb) and negative (Pbp) rapidities (see figure 1). The measurement is consistent with models incorporating initial-state effects due to gluon-shadowing in nuclei, suggesting that QGP formation and the resulting strangeness enhancement have little or no effect on Ξ⁺_c production in pPb collisions.

This interpretation is confirmed by the measurement of the D⁺_s, D⁺ and D⁰cross sections and corresponding ratios in different rapidity regions. While the ratios show little enhancement within the statistical uncertainty, a large asymmetry is observed in the forward-backward production. This strongly indicates CNM effects and provides detailed constraints on models of nuclear parton distribution functions and hadron production in a very wide range of Bjorken-x (10^–2– 10^–5). A strong suppression is observed for the D mesons, giving insight into the nature of the CNM effects involved. An explanation via additional final-state effects is challenged by the Ξ⁺_c data that are well described by models not including them. The production ratios of Ξ⁺_c, D⁺_s, D⁺ and D⁰measured as a function of p_T in pPb collisions confirm these findings. All these studies will profit from the increased statistics in pPb collisions that are expected from future LHC runs.

A novel search for inelastic dark matter

CMS figure 1 — **Fig. 1.** Left: the efficiency of the displaced muon reconstruction algorithm (red) remains high for large displacements from the collision point, so that dimuons can be identified, even if produced outside the tracker volume. Right: two-dimensional observed limits on the product of IDM production cross-section and χ₂ decay branching fraction as a function of the χ₁ mass and y, for a 10% mass splitting scenario and m_Aʹ = 3m₁. The solid (dashed) lines denote the observed (expected) exclusion regions at 95% CL, with 68% uncertainty bands. Regions above the line are excluded depending on the choice of fine-structure constant in the dark sector (α_D). Source: arXiv:2305.11649

As dark matter (DM) search experiments increasingly constrain minimal models, more complex ones have gained importance, featuring a rich “dark sector” with additional particle states and often involving forces that cannot be directly felt by Standard Model (SM) particles. Nevertheless, the SM and dark sector are typically connected by a “portal” that can be experimentally probed.

The CMS collaboration recently presented the first dedicated collider search for inelastic dark matter (IDM) using the LHC Run 2 dataset. In IDM models, a small Majorana mass component is combined with a Dirac fermion field corresponding to the DM and added to the SM Lagrangian, resulting in two new DM mass eigenstates with a predominantly off-diagonal (inelastic) coupling and a small mass splitting. In addition, a dark photon (a gauge boson similar to the ordinary photon) serves as the portal to the SM. This means that at the LHC, the lighter (χ₁) and heavier (χ₂) DM states are simultaneously produced via a dark photon (A′). While the lighter state is stable and escapes the detector, the heavier one can travel a macroscopic distance before decaying to the lighter one and a pair of muons, which are produced away from the collision point.

This process can be probed by exploiting a striking signature: a pair of almost collinear, low-momentum and displaced muons from the χ₂decay; significant missing transverse momentum (MET) from the χ₁; and an initial-state radiation jet that can be used for trigger purposes. The MET-dimuon system recoils against the high-momentum jet, so that the muons and MET are also almost collinear. This unique topology presents challenges, including the reconstruction of the displaced muons. This problem was addressed by using a dedicated reconstruction algorithm, which remains efficient even for muons produced several metres away from the collision point (figure 1, left).

The first dedicated collider search for IDM using the full dataset collected during LHC Run 2

After applying event-selection criteria targeting the expected IDM signal, the number of events is compared to the data-driven background prediction: no excess is observed. Upper limits are set on the product of the pp → A′ → χ₂χ₁ production cross-section and the branching fraction of the χ₂ → χ₁ μ⁺μ^– decay; they are shown in figure 1 (right) for a scenario with 10% mass splitting between the χ₁ and χ₂ states. The y variable is roughly proportional to the interaction strength between the SM and the DM sector. Values of y > 10^–7 to 10^–9 are excluded for masses between 3 and 80 GeV, when assuming that the fine structure constant has the same value in the dark sector and in the SM.

CMS physicists are looking forward to probing more complex and well-motivated DM models with novel and creative uses of the existing detector.

Topics

Reset your password

Registration complete

Extreme conditions