Jobs – Page 31 of 521

Bruce Marsh 1980–2023

Bruce Marsh, a recognised international leader in laser technologies for radioactive ion-beam production, tragically passed away on 30 December 2023 at the age of 43. Bruce was an invaluable member of the sources, targets and interactions group at CERN, where he was leader of the lasers and photocathodes section. Across CERN, he contributed to shared endeavours with his vast expertise, knowledge, attention to detail and kindness towards those around him. Outside CERN, he was an outstanding figure in the scientific and technical domains of laser ion-source technology, revolutionising its application across various fields.

Bruce completed his PhD at the University of Manchester, and his thesis research included work at the ISOLDE facility at CERN, which he later joined as a fellow in 2006 before taking up a staff position in 2010. In his different roles, Bruce contributed greatly to the development of the resonance ionisation laser ion source (RILIS) techniques dedicated to the production of radioisotopes for fundamental research and medicine at ISOLDE. The ISOLDE RILIS system has become a reference for all radioactive ion-beam facilities worldwide. Furthermore, Bruce worked closely with his team to contribute to the advancement of future accelerators and colliders at CERN, with a particular focus on CLIC, AWAKE and the Gamma Factory.

His work extended beyond CERN, influencing the global landscape of nuclear-structure and laser-physics research. He was a leading expert in the development and applications of RILIS-based in-source resonance ionisation spectroscopy. The technique allowed studies of nuclear ground- and isomer-state properties of radioactive isotopes, including recent investigations in mercury and bismuth isotopes that gained wide attention outside nuclear physics. Bruce authored articles in many high-impact journals and was invited to present his work at numerous international conferences, workshops and schools, many of which he helped to organise.

Bruce also achieved international recognition through his role as the coordinator of the European Union-funded Marie Skłodowska-Curie training network Laser Ionisation and Spectroscopy of Actinides (LISA), which comprises a dozen laboratories in Europe, several other international partners and 15 doctoral students, who explore the structure of the actinide elements.

Bruce will be fondly remembered for his warm and welcoming spirit – always ready with a smile for everyone – and for his strong sense of justice. An unwavering champion of diversity in the workplace, Bruce fostered an inclusive environment where every voice was valued, and every individual felt empowered to contribute to shared goals with their unique perspectives. He provided invaluable opportunities for integration, learning and professional growth for his colleagues and was always available for advice on professional and private matters. Despite his wealth of knowledge and accomplishments, he remained humble in all situations, leaving a lasting impact on all who knew him.

Probing resonant production of Higgs bosons

CMS figure 1 — **Fig. 1.** Observed (solid lines) and expected (dotted lines) upper limits on the cross section of a spin-0 resonance times its branching fraction into a pair of Higgs bosons, as a function of its mass, for six analysis signatures and their combination (red). Source: CMS collab.

Besides being a cornerstone of the Standard Model (SM), the Higgs boson (H) opens a very powerful path to search for physics beyond the SM. In particular, in the SM there are no particles that are sufficiently heavy to decay into two Higgs bosons. Therefore, if we observe the resonant production of HH pairs, for example, we have clear evidence for the existence of new physics, as predicted by models with an extended Higgs sector.

The CMS collaboration recently conducted a search for the resonant production of Higgs-boson pairs. The analysis combines six different analyses and five HH final states, targeting H decays into b quarks, photons, τ leptons and W bosons. As figure 1 shows for a spin-0 resonance (denoted X), the combination of the decay modes covers a wide mass range, from 280 GeV to 4 TeV. While no resonant signal is observed, stringent upper limits on the pp → X → HH cross section are obtained, which reach values of about 0.2 fb at the highest masses. These are the strongest observed limits to date for a scalar mass below 320 GeV or above 800 GeV.

CMS figure 2 — **Fig. 2.** Interpretation of the HH combination measurement in a MSSM benchmark model, showing exclusion limits in the tanβ versus m_A plane and comparisons to other CMS results. Source: CMS collab.

One possible candidate for such a resonance is a heavy scalar from an extended Higgs sector, as predicted in the Minimal Supersymmetric Standard Model (MSSM), which features three neutral and two charged Higgs bosons. Figure 2 shows the excluded region of the model parameter tanβ (the ratio of vacuum expectation values of the two underlying Higgs doublets) as a function of the mass of the CP-odd Higgs boson, m_A. The HH combination is sensitive up to well beyond tanβ = 6, just above the HH threshold, and its exclusion extends up to beyond 600 GeV, outperforming the lower limits from the (also shown) searches of single heavy Higgs-boson production in this mass range. Compared to other direct searches, there is unique sensitivity for m_A > 450 GeV and tanβ < 5.

This result is part of a recent comprehensive review article on resonant Higgs-boson production searches by the CMS collaboration, covering the VH, HH and YH final states, with V denoting a W or Z boson and Y representing an additional new boson.

The coolest job in physics

Surviving long polar nights — **Light in the dark** (clockwise from top left) The Aurora Australis, a music room and a greenhouse help IceCube winterovers such as Marc Jacquart survive long polar nights. Credit: M Jacquart

IceCube’s 5160 optical sensors positioned deep within the Antarctic ice detect around 100,000 neutrinos per year, some of which are the most energetic events ever recorded. To make sure that the detector is operational throughout the year, people are required to spend extended periods at the South Pole, where temperatures are on average around –60°C during the winter.

Marc Jacquart was one of two “winterovers” for IceCube during the season November 2022 to November 2023. Having completed his master’s degree, during which he analysed IceCube data, he saw an internal email about the position and applied: “It was a long-time dream-come-true. I had wanted to go to the South Pole since I heard about IceCube six years earlier.” First he had to pass medical tests, a routine requirement for winterovers because it is difficult to evacuate people during the winter. His next stop was the University of Wisconsin–Madison, the lead institution for the IceCube collaboration, where he and his colleague Hrvoje Dujmović received three months’ training on how to operate, troubleshoot, calibrate and repair IceCube’s hardware and software components using a small replica of the data centre. “Our job is to ensure the highest detector uptime, so we need to know how to fix a problem immediately if something breaks.”

The pair made their way to McMurdo Station on the shores of Antarctica closest to New Zealand in early November 2022. From there, a plane took them 1350 km to the Amundsen–Scott station, located 2835 m above sea level and only 150 m from the geographic South Pole. During the summer, up to 150 people stay at the station to make major repairs and upgrades to the research facilities, which also include the South Pole Telescope, BICEP and an atmospheric research observatory. By mid-February, most people leave. “We were only 43 winterovers left, and that’s when you can help each other and busy yourself with all kinds of things,” says Marc.

Part of station life is volunteering for teams, which in Marc’s case included the fire fighters, amongst others. To bide their time during a nearly six-month-long night, the inhabitants can go to the library, music room or grow vegetables in a repurposed biology experiment to freshen up the preserved foods. While winter in the Antarctic Circle is harsh outside, says Marc, it has one major highlight: the southern lights. “I remember one time, they were just dancing, moving and very bright. We stayed outside for a full hour packed in layers and layers of clothes!”

The only real downtime for the detector is when operators perform a full restart every 32 hours

As a winterover, Marc ensured that the IceCube detector worked 24/7 and recorded every incoming neutrino. “Usually, we have 99.9% uptime. If there is something wrong, we have a pager that pings us, even in the middle of the night.” To ensure that the rarest high-energy neutrinos are recorded, the only real downtime for the detector, he says, is when operators perform a full restart every 32 hours. For such events, which could point to high-energy phenomena in the universe, IceCube sends a real-time alert to other experiments. About 200 machines are located in the data centre and collect 1 TB of data per day, only 10% of which are sent north to a data centre in the US due to satellite-bandwidth limitations. The remaining data gets stored on hard drives, which must be swapped manually by the winterovers every two weeks. During the summer, when aircraft can reach the South Pole on a regular basis, boxes stashed with hard drives are taken back for thorough data analysis and archiving.

Since returning home to Switzerland, Marc is considering his next steps. “I have the opportunity to work on a radio observatory in the US next year. After a year operating the IceCube detector, I’m interested to work with hardware more. And I am definitely considering a PhD with IceCube afterwards, as there is a lot coming up.” Currently, the IceCube collaboration is working towards IceCube-Gen2, with the first step being to add seven strings with improved optical modules to the existing underground complex. In a second step, 120 further cables with refined light sensors will optimise the detector, and two radio detectors as well as an extended array will be placed on the surface. The upgrades will enlarge IceCube’s coverage from one to eight cubic kilometres, offering more than enough tasks for future winterovers during the decade . “Maybe in a few years I would be keen to return to the South Pole. It’s a very special place.”

New pentaquark searches in beauty decays

LHCb figure 1 — **Fig. 1.** Invariant-mass distributions of (left) Λ_c⁺ D_s^– and (right) Λ_c⁺ D⁰ K^– candidates with the results of the fit overlaid. The exclusive Λ_b⁰ decays to Λ_c⁺ D_s^– and Λ_c⁺ D⁰ K^– peak around the nominal Λ_b⁰ mass of 5620 MeV. The partially reconstructed decays Λ_b⁰ → Λ_c⁺ D_s^*– and Λ_b⁰ → Λ_c⁺ D^*0 K^– are shown in orange and green, while nuisance components are shown in different colours. Source: LHCb-PAPER-2023-034

Pentaquarks, bound states of five quarks predicted in the first formulation of the quark model in 1964, have had a troubled history. Following disputed claims of the discovery of light-flavour species over 20 years ago, pentaquarks with hidden charm are now well-established members of the hadronic spectrum. The breakthrough was achieved by the LHCb experiment in 2015 with the observation of P_c⁺ states in the J/ψ p system.

The P_c⁺ quark content (uudcc) implies that decays to two open-charm hadrons, such as Λ_c⁺ D⁰ or Λ_c⁺ D*⁰, are possible. The rates of such decays are important for understanding more about the nature of the P_c⁺ states, as different models predict rates that differ by orders of magnitude. Distinguishing between the proposed mechanisms by which pentaquarks, and excited hadrons in general, are produced and bound allows a better understanding of the dynamics of the strong interaction in the non-perturbative regime.

A new analysis by LHCb of the open-charm hadrons in Λ_b decays was presented at the International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, held in Mainz in October. It concerns the first observation and measurement of the branching fractions of Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– and Λ_b⁰ → Λ_c⁺ D_s^*^– decays using proton–proton collision data collected during LHC Run 2.

All branching fractions are measured relative to the known Λ_b⁰ → Λ_c⁺ D_s^– decay mode, which is reconstructed with the same set of six final-state hadrons: p K^– π⁺ K⁺ π^– K^–. Many systematic uncertainties in the measured ratios therefore cancel out, making the precision on the relative branching fraction of Λ_b⁰ → Λ_c⁺ D⁰ K^– statistically limited. For Λ_b⁰ → Λ_c⁺ D^0* K^– and Λ_b⁰ → Λ_c⁺ D*^– the resulting branching fractions are systematically limited. This is because either a photon or neutral pion is not reconstructed, so their shape in the invariant mass spectrum of the reconstructed particles is more difficult to describe and more affected by the backgrounds (see figure 1, where the components with a missing photon for which a branching fraction is calculated are shown in orange and those with a missing neutral pion in green).

The partially reconstructed Λ_b⁰ → Λ_c⁺ D_s^*^– decay cannot be used directly to search for pentaquarks, but it is an important input to model calculations. In addition, as a two-body decay, it is a powerful test of factorisation assumptions in heavy-quark effective theory.

In the Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– decay, the production process of the P_c⁺ pentaquarks is the same as in the discovery channel, Λ_b⁰ → J/ψ p K^–. A comparison between the measured branching fractions and observed signal yields can thus be used to estimate the expected sensitivity for observing P_c⁺ signals in the open-charm channels. In particular, the rate of a Λ_b⁰ decay to Λ_c⁺ D⁰ K^– is about six times greater than to J/ψ p K^–; however, more than 60 times as much data would be needed to match the currently available Λ_b⁰ → J/ψ p K^– signal yield.

A factor of about 24 in this calculation comes from the branching fractions ratio of J/ψ and open-charm hadrons, given their reconstructed decay modes. The rest is from reconstruction and selection inefficiencies, which favour the four-prong μ⁺μ^– p K^– over the fully hadronic six-body final state. With the upgraded Run 3 detector and now triggerless detector readout, a large part of the inefficiency for fully hadronic final states is recoverable, making pentaquark searches in double open-charm final states more favourable compared to the situation in Run 2.

Advances in cosmology

On the 30th anniversary of the discovery of weak neutral currents, the architects of the Standard Model of strong and electroweak interactions met in the CERN main auditorium on 16 September 2003 to debate the future of high-energy physics. During the panel discussion, Steven Weinberg repeatedly propounded the idea that cosmology is part of the future of high-energy physics, since cosmology “is now a science” as opposed to a mere theoretical framework characterised by diverging schools of thought. Twenty years later, this viewpoint may serve as a summary of the collection of articles in Advances in Cosmology.

The papers assembled in this volume encompass the themes that are today associated with the broad domain of cosmology. After a swift theoretical section, the contributions range from dark-matter searches (both at the LHC and in space) to gravitational waves and optical astronomy. The last two sections even explore the boundaries between cosmology, philosophy and artistic intuition. Indeed, as former CERN Director-General Rolf Heuer correctly puts it in his thoughtful foreword, the birth of quantum mechanics was also a philosophical enterprise: both Wolfgang Pauli and Werner Heisenberg never denied their Platonic inspiration and reading Timaeus (the famous Plato dialogue dealing with the origin and purpose of the universe) was essential for physicists of that generation to develop their notion of symmetry (see, for instance, Heisenberg’s 1969 book Physics and Beyond).

In around 370 pages, the editors of Advances in Cosmology manage to squeeze in more than two millennia of developments ranging from Pythagoras to the LHC, and for this reason the various contributions clearly follow different registers. Interested readers will not only find specific technical accounts but also the wisdom of science communicators and even artists. This is why the complementary parts of the monograph share the same common goals, even if they are not part of the same logical line of thinking.

Advances in Cosmology appeals to those who cherish an inclusive and eclectic approach to cosmology and, more generally, to modern science. While in the mid 1930s Edwin Hubble qualified the frontier of astronomy as the “realm of the nebulae”, modern cosmology combines the microscopic phenomena of quantum mechanics with the macroscopic effects of general relativity. As this monograph concretely demonstrates, the boundaries between particle phenomenology and the universe’s sciences are progressively fading away. Will the next 20 years witness only major theoretical and experimental breakthroughs, or more radical changes of paradigm? From the diverse contributions collected in this book, we could say, a posteriori, that scientific revolutions are never isolated as they need environmental selection rules that come from cultural, technological and even religious boundary conditions that cannot be artificially manufactured. This is why paradigm shifts are often difficult to predict and only recognised well after their appearance.

Tango for two: LHCb and theory

The 13th annual “Implications of LHCb measurements and future prospects” workshop, held at CERN on 25–27 October 2023, drew substantial interest with 231 participants. This collaborative event between LHCb and the theoretical community showcased the mutual enthusiasm for LHCb’s physics advances. The workshop featured five streams highlighting the latest experimental and theoretical developments in mixing and CP violation, heavy ions and fixed-target results, flavour-changing charged currents, QCD spectroscopy and exotics, and flavour-changing neutral currents.

The opening talk by Monica Pepe Altarelli underscored LHCb’s diverse physics programme, solidifying its role as a highly versatile forward detector. While celebrating successes, her talk candidly addressed setbacks, notably the new results in tests of lepton-flavour universality. LHCb detector and computing upgrades for Run 3 include a fully software-based trigger using graphics processing units. The collaboration is also working towards an Upgrade II programme for Long Shutdown 4 (2033–2034) that would position LHCb as a potentially unique global flavour facility.

On mixing and CP violation, the October workshop unveiled intriguing insights in both the beauty and charm sectors. In the beauty sector, notable highlights encompass measurements of the mixing parameter ΔΓ_s and of CP-violating phases such as ϕ_s,d, ϕ_s^ss^s and γ. CP asymmetries were further scrutinised in B → DD decays, accounting for SU(3) breaking and re-scattering effects. In the charm sector, the estimated CP asymmetries considering final-state interactions were found to be small compared to the experimental values related to D⁰ → π^–π⁺ and D⁰ → K^–K⁺ decays. Novel measurements of CP violation in three-body charm hadron decays were also presented.

Unique capabilities

On the theoretical front, discussions delved into the current status of bottom-baryon lifetimes. Recent lattice predictions on the ε_K parameter were also showcased, offering refined constraints on the unitarity triangle. The LHCb experiment’s unique capabilities were discussed in the heavy ions and fixed-target session. Operating in fixed-target mode, LHCb collected data pertaining to proton–ion and lead–ion interactions during LHC Run 2 using the SMOG system. Key highlights included measurements impacting theoretical models of charm hadronisation, global analyses of nuclear parton density functions, and the identification of helium nuclei and deuterons. The first Run 3 data with the SMOG2 upgrade showed promising results in proton–argon and proton–hydrogen collisions, opening a path to measurements with implications for heavy-ion physics and astrophysics.

The session on flavour-changing charged currents unveiled a recent measurement concerning the longitudinal polarisation of D^*⁻ mesons in B⁰ → D^*⁻τ^–ν_τ decays, aligning with Standard Model (SM) expectations. Discussions delved into lepton-flavour-universality tests that showed a 3.3σ tension with predictions in the combined R(D⁽^*⁾) measurement. Noteworthy were new lattice-QCD predictions for charged current decays, especially R(D⁽^*⁾), showcasing disparities in the SM prediction across different lattice groups. Updates on the CKM matrix elements |V_ub| and |V_cb| lead to a reduced tension between inclusive and exclusive determinations. The session also discussed the impact of high-energy constraints of Wilson coefficients on charged-current decays and Bayesian inference of form-factor parameters, regulated by unitarity and analyticity. The QCD spectroscopy and exotics session also featured important findings, including the discovery of novel baryon states, notably Ξ_b(6087)⁰ and Ξ_b(6095)⁰. Pentaquark exploration involved diverse charm–hadron combinations, alongside precision measurements of the Ω⁰_c mass and first observations of b-hadron decays with potential exotic-state contributions. Charmonia-associated production provided fresh insights for testing QCD predictions, and an approach based on effective field theory (EFT) interpreting pentaquarks as hadronic molecules was presented. A new model-independent Born–Oppenheimer EFT framework for the interpretation of doubly heavy tetraquarks, utilising lattice QCD predictions, was introduced. Scrutinising charm–tetraquark decays and the interpretation of newly discovered hadron states at the LHC were also discussed.

During the flavour-changing neutral-current session a new analysis of B⁰ → K^*⁰μ⁺μ^– decays was presented, showing consistency with SM expectations. Stringent limits on branching fractions of rare charm decays and precise differential branching fraction measurements of b-baryon decays were also highlighted. Challenges in SM predictions for b → sℓℓ and rare charm decays were discussed, underscoring the imperative for a deeper comprehension of underlying hadronic processes, particularly leveraging LHCb data. Global analyses of b → dℓℓ and b → sℓℓ decays were presented, alongside future prospects for these decays in Run 3 and beyond. The session also explored strategies to enhance sensitivity to new physics in B^± → π^±μ⁺μ^– decays.

The keynote talk, delivered by Svjetlana Fajfer, offered a comprehensive summary and highlighted existing anomalies that demand further consideration. Tackling these challenges necessitates precise measurements at both low and high energies, with the collaborative efforts of LHCb, Belle II, CMS and ATLAS. Additionally, advancements in lattice QCD and other novel theoretical approaches are needed for precise theoretical predictions in tandem with experimental efforts.

Giuseppe Fidecaro 1926–2024

Experimental physicist Giuseppe Fidecaro, who joined CERN in 1956 and continued there long into his retirement, passed away on 28 March.

Born in Messina, Italy in 1926, Giuseppe studied physics at the University of Rome, graduating in 1947 under the supervision of Edoardo Amaldi. Amaldi had become interested in cosmic rays and asked young “Pippo” to help him build a large detector to study the scattering of mesons on an iron target to explore the nuclear force. Between 1952 and 1954, Giuseppe continued to work on cosmic rays at the Tête Grise laboratory, 3500 m above Cervinia, where Maria Cervasi, whom he had met during his studies at Rome, also worked.

In 1953 Amaldi, who had become secretary general of the provisional CERN, suggested that Giuseppe spend time at the University of Liverpool to learn from the new synchrocyclotron being built there. He went to CERN with Maria, by then his wife, in 1956 and began preparing experiments for the 600 MeV Synchrocyclotron (SC), which came into operation in August 1957.

In January 1958, during a conference in New York, Giuseppe attended a presentation by Feynman describing the universal “V–A” theory of weak interactions. He heard that the theory lacked a key experimental ingredient: the decay of a pion into an electron and neutrino, predicted to occur 10 000 times less frequently than to a muon and a neutrino, which had not been observed in two experiments performed by well-known physicists. Upon his return to CERN, Pippo decided with the other members of the SC group that this would be the target of the next experiment. A device was immediately designed and built, and 40 events in perfect agreement with the V–A prediction were presented by Pippo in September 1958. The news put the newly born CERN on the map of the world of particle physics and laid the groundwork for the future discoveries of neutral currents, the W and Z bosons, and the Higgs field.

In 1960, with the start-up of the PS, Giuseppe led his group to measure – using a system of precision scintillators – the antiproton–proton cross section. The following year, he became professor at the University of Trieste and established a group that carried out a series of important scattering measurements at the PS and the SPS, in particular using polarised targets, during the 1970s. Following the proposal and execution of an experiment at the ILL in Grenoble searching for possible neutron–antineutron oscillations, in 1990 he presented an article “Fixed target B-physics at the Large Hadron Collider” at the LHC workshop in Aachen, which proposed, among other things, the use of a very intense proton beam extracted from the accelerator with a crystal, similar to what had been envisaged for the Superconducting Super Collider. This, and discussions with Giovanni Carboni and Walter Scandale, were at the origin of the RD22 collaboration, which for the first time proved the possibility of high-efficiency proton extraction from an accelerator using a bent crystal – a technique that is now used in LHC beam collimation.

Outside physics, Giuseppe made numerous contributions to CERN. In the early 1960s he was a member of the founding committee of the International Center for Theoretical Physics. In 1975 he was appointed as co-chair of a joint scientific committee set up under a collaboration agreement between CERN and the former USSR concerning the use of atomic energy, a responsibility he held until 1986. He was also tasked with coordinating cooperation with JINR in Dubna.

Giuseppe officially retired in 1991 but, together with Maria, continued his work at CERN as an honorary member of the personnel until as recently as 2020, during which time he devoted himself to research in the history of physics. He produced reports of rare beauty and precision, notably three well-documented articles on the contributions of Bruno Pontecorvo, whose friend he became in Dubna in 1989. Giuseppe was also known to CERN visitors, featuring prominently in the film shown in the Synchrocyclotron exhibition. Maria Fidecaro, with whom his rich human and scientific journey was deeply entwined, passed away in September 2023.

Education and outreach in particle physics

The imposing structure of CERN Science Gateway has been likened to a space station. In fact, it was CERN’s technical buildings and underground tunnels that were the inspiration for chief architect Renzo Piano. Its three pavilions and two tubes house exhibitions, hands-on laboratories, artworks, a 900-seat auditorium, a shop and a restaurant – all connected by a 220 m-long bridge and nestled amongst 400 trees and 13,000 shrubs. It has a net-zero carbon footprint, with 2000 m² of solar panels on the pavilion roofs providing all the energy needed, while feeding 40% back into the CERN grid. The Gateway is free to enter and open all year, every day except Mondays, offering the capacity to welcome up to 500,000 visitors of all ages per year.

We look at the importance of reaching out as far and wide as possible

The following articles of expert exposition and opinion lift the lid on CERN Science Gateway. In addition to hearing from the teams behind its content, we explore the broader issues surrounding the theory and practice of education, communication and outreach in particle physics – beginning with what these three terms mean today (From the cosmos to the classroom). Exploring the Gateway’s exhibition spaces, authors reflect on four stunning art installations (Beautiful minds collide), the secrets of success for an interactive exhibit (Interactive exhibits: theory and practice) and the simple power of objects (The power of objects). Following a deep-dive into the new educational labs (Hands on, minds on, goggles on!), learn about CERN’s physics-education research (Why research education?), the impact of its hugely popular teacher programmes (Inspiring the inspirers), and how particle physics is or is not integrated in school curricula (Particle physics in school curricula). From empowering children to aspire to science (Empowering children to aspire to science) to taking physics to festivals (Going where the crowd is), and transcending physical and neurological boundaries (Expanding the senses), three articles emphasise the importance of reaching out as far and wide as possible. Last but certainly not least, we consider the invaluable role played by physicists (Physicists go direct and Time for an upgrade) and weave the rich experiences of CERN guides throughout these articles. Feel inspired? Your nifty red Science Gateway vest awaits!

Pushing accelerator frontiers in Bern

Novel accelerator concepts will play an important role in future accelerators for high-energy physics. Two relevant scenarios being explored in the framework of the European Union I.FAST project are the generation of relativistic single electrons with gigahertz repetition rate for dark-matter searches, and the rapid acceleration of muons with GV/m accelerating fields for experiments at the energy frontier. The topical workshop “Gigahertz Rate and Rapid Muon Acceleration”, held in Bern from 10 to 13 December 2023, addressed the latest developments in these and related topics.

The first part of the workshop was devoted to dark-matter searches and dielectric laser acceleration (DLA). For dark-matter searches, multiple experiments are proposed across different classes (muons vs electrons and positrons, appearance vs disappearance experiments, etc), and an adequate background rejection is important. Promising advanced accelerator technologies are DLA for single electrons – perhaps also muons – and plasma-wakefield accelerators for muons and pions.

Some dark matter-experiments look for an appearance that requires a high flux of incoming particles. For electrons, the standard is set by BDX at JLab, for protons by the proposed SHiP experiment at CERN, and for photons by the proposed Gamma Factory at CERN. In addition, appearances could be seen at existing collider experiments such as the LHC. Other dark-matter experiments search for disappearance. They rely on DC-like electron beams, with prominent examples being LDMX at SLAC and the newly proposed DLA–DMX at PSI. A DC-like muon beam could be explored by the M3 experiment at Fermilab.

Paolo Crivelli (ETH Zürich) described the NA64 experiment as one of the most prominent examples of ongoing accelerator-based dark-matter searches, and presented the first results using a high-energy muon beam. The proposed LDMX experiment at SLAC, presented by Silke Möbius (University of Bern), may set a new standard for indirect dark-matter searches, while advanced concepts employing dielectric laser acceleration, in particular when integrating the accelerating structure with laser oscillator, could achieve many orders of magnitude higher rates of single high-energy electrons entering into an LDMX-type detector.

Uwe Niedermayer (TU Darmstadt), Stefanie Kraus (University Erlangen-Nürnberg) and Raziyeh Dadashi (PSI/EPFL) reviewed the state of the art in DLA plus future plans. Yves Bellouard (EPFL) discussed advances in high-repetition-rate lasers and micro/nano-structures, which suggests that the proposed combined laser-accelerator structures are within reach. Of course, the detector time resolution would also need to be improved tremendously to keep pace with the higher rate of the accelerator.

Acceleration and decay

The second part of the workshop was devoted to the plasma acceleration of non-ultra relativistic and rapidly decaying particles, such as muons and pions. Vladimir Shiltsev (Fermilab) and Daniel Schulte (CERN) presented tentative parameters and ongoing R&D efforts towards a muon collider. Shiltsev also discussed the intriguing possibility of low-emittance muon sources based on plasma-wakefield accelerators, while Alexander Pukhov (Heinrich Heine University Düsseldorf) and Chiara Badiali (IST Lisbon) discussed how plasma acceleration could bring slow particles, such as muons, to relativistic velocities.

The workshop fostered numerous heated discussions and uncovered unresolved issues, which included the “Bern controversy” regarding the ultimate limits of luminosity for PeV energies. Muons are considered particles of choice for future accelerators at the energy frontier. Both low- and high-energy muons have useful applications. Is there an Angstrom limit to the beam diameter? Are tiny beta functions possible? Can plasmas help to overcome such limitations? Understanding and modelling non-point-like particle luminosity is another important topic, also relevant for the Gamma Factory.

The workshop showed how advanced accelerator concepts can jump-start dark-sector searches

The final part of the workshop assembled a roadmap and perspective. DLA studies are to be maintained and, if possible, accelerated. A reasonable target is achieving a gradient of 500 MeV/m and an energy gain of 0.05 GeV in five years on a single wafer, while an integrated DLA laser oscillator could be foreseen five to seven years from now. Plasma-wakefield acceleration of muons could conceivably be tested either at CERN–AWAKE or PSI. It was proposed, as a first step, to put a solid target or tape into the AWAKE set up.

The gamma factory, presented by Witek Krasny (LPNHE), was recognised as an intense source of polarised muons and positrons. For muon-acceleration studies, the dephasing issue, linked to the muons’ non-ultrarelativistic energy, seems to be resolved. A demonstrator experiment for muon plasma acceleration is called for. Open questions include when and where?

Overall, the Bern workshop showed how advanced accelerator concepts can jump start dark-sector searches and muon/pion acceleration. High-repetition-rate acceleration of single electrons for dark-matter searches, using dielectric laser accelerators, and applying high-gradient plasma acceleration to muon and/or pion beams, are intriguing and far-forward looking topics.

A global forum for high-energy physics

The International Committee for Future Accelerators (ICFA) was formally founded in 1977 as a working group in IUPAP’s commission 11 (C11, Particles and Fields). Today it remains the place for discussions on all aspects of particle physics, in particular on the large accelerators that are at the heart of the field, and on the strategic deliberations in the various regions of the world. Although ICFA has no means of ensuring that any of its resolutions are carried out, it can act as the “conscience” of the field, and its recommendations can also influence national or regional activities. Among the currently 16 members, which include directors of CERN, Fermilab, IHEP, KEK and DESY, three are from Europe, three from the US, two from Russia, two from Japan, and one each from China and Canada. Three further members collectively represent smaller countries and regions, and the functions of chair and secretary rotate through the Americas, Europe and Asia, usually every three years.

A significant fraction of ICFA’s work is carried out within a set of seven panels, which meet regularly and assemble expertise on more technical or detailed aspects of particular importance to the field. One is devoted to the International Linear Collider (ILC). For more than two decades, ICFA has promoted the realisation of the ILC, for which a global design effort was put in place in 2005. In parallel, an international collaboration under CERN’s leadership had been working on the Compact Linear Collider (CLIC). Recognising the synergies between the two concepts, ICFA established a single coordinating structure, the Linear Collider Collaboration (LCC), in 2012. Also that year, the Japanese high-energy physics community proposed to host the ILC in Japan as a global project.

The LCC mandate came to an end in 2020, when ICFA put in place the ILC International Development Team (IDT) and its working groups. In June 2021 the IDT developed a proposal for the “preparatory laboratory” as a first step towards the realisation of the ILC in Japan.

Evolving landscape

While the IDT is continuing its work, the global Higgs-factory landscape has evolved since the early days of the ILC: more – linear and circular – studies and proposals are on the table, not least as demonstrated by the P5 report in the US. ICFA will soon discuss in what way its discussions and structures need to be adapted to better reflect this evolving landscape.

In November 2023 ICFA established a new panel devoted to the “data lifecycle”, which involves everything from data acquisition, processing, distribution, storage, access, analysis, simulation and preservation, to management, software, workflows, computing and networking. The panel, which replaces two previous ones on related topics, was created in response to the growing importance of data management and open science in recent years. Its membership is currently being put together with the aim to develop ideas and strategies for workforce development and professional recognition mechanisms.

For more than two decades, ICFA has promoted the realisation of the ILC

ICFA’s farthest-reaching and most visible activity is the ICFA Seminar. The 13th ICFA seminar on “Future Perspectives in High-Energy Physics” took place at DESY from 28 November to 1 December 2023. For the first time in six years (the prior ICFA seminar had taken place in 2017 in Ottawa, Canada), this select crowd of scientists, lab directors and funding agency representatives could come together in person for updates and discussions. One highlight was the panel discussion between the directors of KEK, CERN, Fermilab and IHEP, in which views on a future global strategy were discussed. The seminar concluded on a festive note with the formal passing of the ICFA chair baton from Stuart Henderson (JLAB) to Pierluigi Campana (INFN), who will lead ICFA for the next three years.

ICFA is the only global representation of the particle-physics community, and the ideal discussion forum for global strategic developments, especially large international collider projects. In view of the current situation with numerous opportunities for future facilities – not least a future Higgs factory, but also smaller and more diverse projects – the committee and its panels look forward to serving the field of particle physics through continued advocacy, exploration, discussion and facilitation.

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Evolving landscape