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XFELs join hunt for axion-like particles

Bounds on axion–photon coupling — **New territory** Bounds on the axion–photon coupling from the latest study (pink) compared with those from previous results from synchrotron X-ray facilities (purple) and other lab searches. Credit: arXiv:2404.17333

A first-of-its-kind experiment performed at the European X-Ray Free-Electron Laser (European XFEL) in Hamburg, Germany, has placed new constraints on axion-like particles in a mass range that is relatively unconstrained by laboratory searches. While similar searches have been performed at advanced storage ring-based synchrotron X-ray sources, the new study exploits the higher brightness of the European XFEL’s beams to improve the sensitivity of axion searches in the 10^–3–10⁴ eV mass range.

The axion is predicted to arise from the breaking of Peccei–Quinn symmetry, proposed in the mid-1970s to explain the observed absence of CP violation in strong interactions. Indeed, axion-like particles (ALPs) appear in any quantum field theory with a spontaneously broken global symmetry and arise naturally in many models based on string theory. They are also a promising candidate for dark matter. As such, ALPs are the target of a growing number and variety of experiments worldwide. While not yet able to reach the sensitivity of astrophysical experiments, lab-based searches are less model-dependent as they enable direct control of the axion production process.

Most laboratory searches for axions exploit the Primakoff effect: photons in the presence of a strong external electric field convert into axions, which then convert back into photons after passing through an opaque wall. This “light shining through a wall” technique has been employed in experiments with optical lasers and external magnetic fields, such as ALPS (and now ALPS II) at DESY and OSQAR at CERN. Stringent bounds on heavy axions have also been placed by the CERN Axion Solar Telescope, which looked for the conversion of photons to axions in the strong magnetic field of an LHC dipole magnet pointed at the Sun, and constraints have been set by accelerator experiments such as Belle II at KEK and NA64 at CERN.

The use of X-rays can increase the detection sensitivity by exploiting the strong electric fields (up to 10¹¹ V m^–1, which corresponds to magnetic field strengths of order 1 kT) present in crystalline materials. Gianluca Gregori of the University of Oxford and co-workers used the European XFEL’s HED/HiBEF instrument, in which axion production and photon regeneration are expected to take place via the electric field within a pair of germanium crystals. Orienting the crystals such that their lattice planes are parallel to one another leads to a coherent effect analogous to Bragg scattering, while the much shorter duration and higher brightness of photon pulses from the European XFEL compared to previous synchrotron X-ray experiments allows for a more accurate discrimination of the signal against background.

Using three days of beam time, the team was able to improve on previous lab-based searches at several discrete axion masses. For masses greater than about 200 eV, the team claims to have surpassed the sensitivity of bounds from all previous searches for lab-generated axions except those at NA64. Further improvements in sensitivity – for example by enabling a higher X-ray flux and bunch-number, and by cooling the first crystal to extend the data-acquisition time – are possible, says the team, perhaps bringing the estimated bounds close to the expectation for QCD axions to be dark matter.

“This study shows the power of XFELs, alongside their principal role in more applied domains, to probe fundamental physics mysteries,” says Gregori. “This experiment required a difficult interpretation of a non-standard measurement, and it is hoped that further work will improve on these first limits.”

Detectors in Particle Physics: A Modern Introduction

Progress in elementary particle physics is driven by the development of radiation-detection technologies. From early photographic emulsions to the gargantuan modern systems that are deployed at particle accelerators and astrophysics experiments, radiation detectors use extraordinary means to disclose the nature and fundamental interactions of elementary particles.

In Detectors in Particle Physics, Georg Viehhauser and Tony Weidberg offer an accessible and comprehensive introduction to this intricate world. Addressed to graduate students in particle and nuclear physics, and more advanced researchers, this book provides the knowledge needed to understand and appreciate these indispensable tools. Building on their personal contributions to the conception, construction and operation of major detector systems at the DELPHI and ATLAS detectors at CERN, the authors review basic physics principles to enable the reader to grasp the fundamental operating mechanisms of gaseous, liquid and semiconductor detectors, as well as systems for particle identification and calorimetry.

In addition to exploring core concepts in detector physics, another objective of the book is to introduce the reader to case studies of applications in particle physics and astrophysics. From the Large Hadron Collider to neutrino experiments, the University of Oxford-based authors connect theoretical physics to practical applications and present real-world examples of modern detectors, bridging the gap between theory and experimentation. The book describes key practical aspects of particle detectors, including electronics, alignment, calibration and simulation. These practical insights enhance the reader’s understanding of how detectors operate in experiments, and each chapter includes practical exercises to help further the reader’s understanding of the subject.

Detectors in Particle Physics offers a unique blend of theoretical foundations and practical considerations. Whether you’re fascinated by the mysteries of the universe or planning a career in experimental physics, Viehhauser and Weidberg will undoubtedly prove to be a valuable resource.

Herwig Schopper: Scientist and Diplomat in a Changing World

It is rare and inspiring to be able to read the memoirs of a person who has celebrated their 100th birthday and yet is still exceptionally inquisitive and reflective. Herwig Schopper, director-general of CERN from 1981 to 1988, is one such person. In Scientist and Diplomat in a Changing World, he takes stock of his personal life, the development of physics, the political challenges he has faced, and the human interactions that knit each of these subjects together.

Schopper told the story of his life to co-author James Gillies, a particle physicist and former head of communications at CERN. Their interviews shed a brilliant light on his life. Starting work as a physicist in the field of optics, he moved first to study beta decays and parity violation, and then to nuclear and particle physics, accelerator physics and detector development more generally. Initial chapters cover his early years in what is today the Czech Republic, the dark years of war, and his studies in Hamburg from 1945 to 1954.

But though he loved to work hands on, Schopper was soon asked to found and direct institutes. His impact on nuclear and particle physics in Germany becomes eminently clear when he describes his career at the universities of Erlangen, Mainz, Karlsruhe and Hamburg. The book therefore next turns to time spent as a university professor establishing and directing institutes from 1954 to 1973, including productive sabbaticals in Stockholm, Cambridge and Cornell, and his journey to DESY via CERN between 1973 and 1980. It then explores his time as director-general of CERN from 1981 to 1988, his transition from science to diplomacy, his travels east and his impact on LEP and the LHC. These chapters are captivating, as they describe not only the life story of a remarkably active and productive scientist, but also the historical, scientific and political context in which the work was done.

Many people are happy to take life a little easier when they retire. Not so Herwig Schopper. For him, science is centre stage, and given his vast knowledge in science and science policy, his advice and help are still in high demand. A chapter on science for peace illustrates the rocky path he and dedicated colleagues took to create a science project in the Middle East, SESAME, with the goal to bring scientists from hostile countries to work together on excellent scientific projects.

While the main part of this very readable text describes the life and work of Schopper in the words of his co-author Gillies, each chapter ends with a section in Schopper’s own words in which he shares some very personal memories with the reader, speaking about his family and friends, and his deep love for music. The book also ends with an epilogue and some reflections by the man himself. Building on 100 years of experience, these words provide much food for thought.

Many of Herwig Schopper’s colleagues and friends have encouraged him to write about his life, as he has seen and done so many things. The result of those requests, this biography is a captivating documentation of an exemplary life in particle physics. But more than the story of a fulfilled life, this biography is a textbook about how scientific progress depends on individuals, scientific excellence, inspiration, perseverance and the fortune needed to bring it all off, as the French say.

Photonuclear summit takes place in Paphos

The 15th edition of Electromagnetic Interactions with Nucleons and Nuclei (EINN) attracted 100 delegates to Paphos in Cyprus from 31 October to 4 November 2023. EINN covers theoretical and experimental developments in hadron physics, including the partonic structure of nucleons and hadron spectroscopy, the muon magnetic moment, dark-matter searches, the electroweak structure of light nuclei, new experimental facilities and physics searches, lattice QCD, the integration of machine-learning methodologies in QCD and the potential of quantum computing in QCD.

A highlight of the conference was the evening plenary poster session. Luis Alberto Rodriguez Chacon (The Cyprus Institute), Cornelis Mommers (Mainz University) and Sotiris Pitelis (Mainz) were recognised with the prestigious EPS poster prize, and presented their work on the calculation of the gluon momentum fraction in mesons through lattice QCD simulations, exotic atoms, and the X17 discovery potential from γD → e⁺e^–pn with neutron tagging. This edition of EINN also hosted topical workshops on the QCD analysis of nucleon structure and experimental opportunities at the Electron-Ion Collider. Preceding the conference, a two-day meeting on careers in photonuclear physics was tailored to be a platform for PhD students and postdoctoral researchers to establish professional networks.

With QCD taking a central role in contemporary physics research worldwide, the EINN conference is poised to maintain its crucial role as an international forum for the field.

CERN teams up with ET on civil engineering

The Einstein Telescope (ET), a proposed third-generation gravitational-wave observatory in Europe with a much higher sensitivity than existing facilities, requires a new underground infrastructure in the form of a triangle with 10 km-long arms. At each corner a large cavern will host complex mirror assemblies that detect relative displacements as small as 10^–22 m caused by momentary stretches and contractions of space–time. Access to the underground structure, which needs to be at a depth of between 200 and 300 m to mitigate environmental and seismic noise, will be provided by either vertical shafts or inclined tunnels. Currently there are two candidate sites for the ET: the Meuse–Rhine Euroregion and the Sardinia region in Italy, each with their own geology and environment.

CERN is already sharing its expertise in vacuum, materials, manufacturing and surface treatments with the gravitational-wave community. Beginning in 2022, a collaboration between CERN, Nikhef and INFN is exploring practical solutions for the ET vacuum tubes which, with a diameter of 1 to 1.2 m, would represent the largest ultrahigh vacuum systems ever built (CERN Courier September/October 2023 p45).

In September 2023, the ET study entered a further agreement with CERN to support the preparation of a site-independent technical design report. With civil-engineering costs representing a significant proportion of the overall implementation budget, detailed studies are needed to ensure a cost-efficient design and construction methodology. Supported financially by INFN, Nikhef and IFAE, CERN will provide technical assistance on how to optimise the tunnel placement, for example via software tools to generate geological profiles. Construction methodology and management of excavated materials, carbon footprint, environmental impact, and project cost and schedule, are other key aspects. CERN will also provide recommendations during the technical review of the associate documents that feed into the site selection.

“We are advising the ET study on how we managed similar design studies for colliders such as CLIC, ILC, the FCC and the HL-LHC upgrade,” explains John Osborne of CERN’s site and civil-engineering department. “CERN is acting as an impartial third party in the site-selection process.”

A decision on the most suitable ET site is expected in 2027, with construction beginning a few years later. “The collaboration with CERN represents an element of extreme value in the preparation phase of the ET project,” says ET civil-engineering team leader Maria Marsella. “CERN’s involvement will help to design the best infrastructure at any selected sites and to train the future generation of engineers who will have to face the construction of such a large underground research facility.”

US and CERN sign joint statement of intent

In April, CERN and the US government released a joint statement of intent concerning future planning for large research infrastructures, advanced scientific computing and open science. The statement was signed in Washington, DC by CERN Director-General Fabiola Gianotti and principal deputy US chief technology officer Deirdre Mulligan of the White House Office of Science and Technology.

Acknowledging their longstanding partnership in nuclear and particle physics, CERN and the US intend to enhance collaboration in planning activities for large-scale, resource-intensive facilities. Concerning the proposed Future Circular Collider, FCC-ee, the text states: “Should the CERN Member States determine that the FCC-ee is likely to be CERN’s next world-leading research facility following the high-luminosity Large Hadron Collider, the US intends to collaborate on its construction and physics exploitation, subject to appropriate domestic approvals.” A technical and financial feasibility study for the proposed FCC is due to be completed in March 2025.

CERN and the US also intend to discuss potential collaboration on pilot projects to incorporate new analytics techniques and tools such as AI into particle-physics research at scale, and affirm their collective mission “to take swift strategic action that leads to accelerating widespread adoption of equitable open research, science and scholarship throughout the world”.

How skills pursue diversity and inclusion

Students from under-represented populations, including those at institutions serving minorities, have traditionally faced barriers to participating in high-energy physics (HEP). These include a lack of research infrastructure and opportunities, insufficient mentoring, lack of support networks, and financial hardship, among many others.

To help overcome these barriers, in 2022 the US CMS collaboration designed a pilot programme called PURSUE – the Program for Undergraduate Research Summer Experience. Due to the COVID pandemic, the collaboration initially worked virtually with 16 students, before an in-person pilot was launched in 2023. The programme has changed the career paths of several students, and a third edition with 20 undergraduates is now underway.

The power of collaboration

Two thirds of the HEP workforce go on to develop careers outside the field. The skills developed in HEP can lead to careers in many sectors, from software and electronics to health and finance. With skills-based labour markets currently a hot topic in business, a more guided and organised approach towards skills has the potential to reinforce the workforce pipeline for both HEP and industry, and benefit the many young researchers who look for jobs outside of academia.

The LHC experiments are a perfect seedbed for this. Comprising some 1200 physicists, graduate students, engineers, technicians and computer scientists from 55 universities and institutes, the US CMS collaboration each year trains about 200 students, 100 postdocs and produces 45 PhDs. It is therefore in a strong position to provide pathways to involve many young researchers in every aspect of the experiment and to prepare hundreds of next-generation scientists for careers in physics and industry alike.

The PURSUE undergraduate internship offers opportunities in state-of-art detector design and upgrades, operations, novel techniques in data taking and analysis, scientific presentations and international partnerships. It doesn’t matter if you are a US citizen or not. The basic requirement is that you are a student inside the US. This year’s cohort comprises students from Africa, South and Central America, and Asia.

This one-of-its-kind programme relies on a large team of dedicated collaborators

At the start of each year, invitations are sent out to all US CMS institutes asking them to propose projects and mentors. This year almost 30 applications were received, which were then matched as closely as possible to the individual interests of the students. Being a diverse and sprawling collaboration – rather than a single institution – is an attractive part of the programme.

At the beginning of the internship, all students meet at the LHC Physics Center at Fermilab for two weeks of software training, during which they gain skills in Unix, Python, machine learning and other areas that will equip them in any research area and throughout industry. This part of PURSUE was developed within the framework of the IRIS-HEP project, which is funded by the US National Science Foundation to address the computing challenges of the High-Luminosity LHC, and the CERN-based HEP Software Foundation. These skills are also key requirements for industry, with 42% of companies identifying AI and big data as a strategic priority for the next five years, according to the World Economic Forum’s Future of Jobs Report 2023.

During the remaining eight weeks of their internship, students travel to the US institution where their mentor is located. The students stay connected throughout this period via meetings and Zoom talks on physics and careers topics, and at the end of the programme they come together to produce a final presentation and poster. Some continue their research during the following semester, enabling a deeper dive into the field.

Success story

This one-of-its-kind programme relies on a large team of dedicated collaborators who take precious time out of their routines to battle the lack of diversity in HEP. And PURSUE’s interns are already succeeding. For example, from the 2022 cohort, Sneha Dixit has been admitted to graduate school at the University of Nebraska–Lincoln to pursue doctoral research on the CMS experiment, and Gabriel Soto has taken up a PhD in accelerator physics at the University of California Davis.

PURSUE also provides a way to engage new institutes with HEP. The initial funding for the programme was provided by a US Department of Energy grant awarded to Tougaloo College in Mississippi along with Brown University, the University of Puerto Rico and the University of Wisconsin. Tougaloo College had no previous connection to particle physics, but it is now hoped that it will become a member of the US CMS collaboration.

The driving force behind PURSUE was Meenakshi Narain of Brown University, an inspirational leader and champion of diversity in CMS and beyond, who passed away in January last year. We hope that the programme inspires similar initiatives in other experiments, fields and regions.

Philip John Bryant 1942–2024

Accelerator physicist Phil Bryant, who made significant contributions to machines at CERN and beyond, passed away on 15 April 2024.

Just married, and fresh from his PhD from University College London, Phil was recruited by CERN in November 1968 to work in the magnet group of the Intersecting Storage Rings (ISR) division, where his first task was to oversee the manufacture of the skew quadrupoles. The group, later renamed the beam optics and magnets group, was strongly involved in the commissioning and development of the collider. Phil set up and tested a low-beta scheme, built from recuperated iron-core magnets, to validate this technology for the ISR, paving the way for the first superconducting low-beta insertion in a working accelerator. Later, he led the design and construction of the beamline from the PS that enabled pp collisions at the ISR, a development with which he became deeply involved. His name is also associated with coupling compensation, and generally with the smooth operation of the collider until it closed in 1983.

A skilled communicator, Phil moved on to assist Kjell Johnsen with setting up the CERN Accelerator School (CAS). He served as director of the school from 1985 to 1991, delivering many lectures himself, and laying the foundations for it to become the valued institution that it is today. He then participated in a study of a B-meson factory for CERN before turning his attention to medical accelerators. Under his leadership this culminated in the Proton–Ion Medical Machine Study (PIMMS) of a synchrotron and its beamlines, which became the basis of the now operating medical centres for cancer treatment in Italy (CNAO) and Austria (MedAustron).

In addition to his managerial competence, Phil brought a contagious enthusiasm to the table

In the early 2000s, Phil joined the LHC effort, serving as chair of the specification committee and taking responsibility for the contract office. Having to navigate deadlines, he shuttled between physicists, engineers, procurement officers and CERN’s legal team. In addition to his managerial competence, Phil brought a contagious enthusiasm to the table, and would apply diplomatic skill in the conclusion of protocols with funding agencies and institutes. On his official retirement from CERN in 2007, Phil moved to Austria to be available for the medical facility under construction there. As this activity wound down, he increased his collaboration with the Vienna-based company Cividec, developing diamond radiation detectors, as well as continuing to improve the WINAGILE program that he had developed for accelerator design, and lecturing – notably for CAS and JUAS, the Joint Universities Accelerator School.

Phil enjoyed scientific work, developing new ideas and writing. The author of numerous papers, his 2012 report on the advancement of colliders due to work done at the ISR is exemplary. A prodigious worker, he was nevertheless modest and always anxious to acknowledge the contribution of collaborators. Besides being a talented physicist and engineer, Phil was also good at drawing, his cartoons being especially appreciated. An inveterate “bricoleur”, when not busy advancing accelerator technology he was active with his hands at home. Phil will be sorely missed.

Yves Baconnier 1934–2024

Yves Baconnier, who made important technical and managerial contributions to a surprising number of CERN accelerators, passed away on 21 January 2024.

Born in 1934 in the Ardèche in the South of France, Yves completed his studies at the Institut Polytechnique de Grenoble. He joined CERN in 1963 as engineer-in-charge of the Proton Synchrotron (PS) and quickly took a strong interest in analysing and improving the slow-ejection procedure. He became leader of the machine study team before moving to the Super Proton Synchrotron (SPS) project in the early 1970s, where his experience with beam extraction was very welcome.

Once the SPS extraction was operational at the end of the 1970s, Yves moved on to the Large Electron Positron (LEP) collider and, in particular, its injection system, which at the beginning was imagined as a new system without a link to the existing accelerators. His decisive idea to use the deadtime between the proton cycles of the SPS – dictated by limited cooling power – to insert the low-dissipation e⁺e^– acceleration cycles from 3.5 to 20 GeV was the key element for accepting the existing accelerator chain PS and SPS with all its infrastructure as the LEP injector. This cut short all discussions on other possible LEP sites in Europe.

After this memorable success with LEP, Yves moved back to his first love, the PS, and took responsibility for the PS ring proper to define and oversee an upgrade programme enabling the elderly machine to accelerate electrons and positrons from 0.6 to 3.5 GeV. The complete vacuum system had to be modified to withstand the synchrotron radiation emitted from the lepton beams, and the campaign reached its climax during a very long shutdown in 1987, during which a stainless-steel vacuum chamber was installed around the ring. Since the PS magnets had combined-focussing, i.e. a quadrupole magnetic field on top of the dipole field, the synchrotron radiation would not allow for stable operation. To counter this, two Robinson-type wiggler magnets had to be inserted. Yves and his team designed this unique magnet, tested the prototype in the PS and at the DCI ring at LAL in Orsay and, finally, introduced it successfully in the PS.

In the early 1990s, Yves went on to join the teams designing a beauty factory to be housed in the tunnel of the former Intersecting Storage Rings, and took the lead in the design of a tau–charm factory to be built on a green-field site in Spain. However, since these projects did not then materialise, he continued his work at the CLIC test facility to which the linear accelerator of LEP had to be converted. In 1984, in parallel with his other widespread activities, Yves took an active interest in the LHC design study, leading to the project’s official approval in 1994. He moved into project management in the mid-1990s and was entrusted with chairing the influential LHC parameter committee until his retirement in 1999.

Yves will be remembered for his thorough and well-thought approach to his work, always seeking to understand ab initio, and for his meticulous insistence on checking hardware through prototyping and extended testing. Unassuming but sharp and exacting, he was a well-respected colleague, an appreciated lecturer and a leader with wide-ranging interests.

Stefano Catani 1958–2024

Stefano Catani, a theoretical particle physicist in the Florence section of Istituto Nazionale di Fisica Nucleare (INFN), passed away on 16 January 2024. Stefano was one of the world’s leading experts in quantum chromodynamics (QCD) and its phenomenological application to high-energy collider physics, leaving an irreplaceable void among his colleagues, friends and family.

Stefano studied physics at the University of Florence and obtained his PhD in 1987 under the supervision of Marcello Ciafaloni, who passed away in September 2023. He was a postdoctoral fellow at the University of Cambridge from 1989 to 1991, and a member of CERN’s theory division from 1991 to 1993. After 1993 he developed his scientific career at INFN Florence, with a period as a CERN staff member between 1997 and 2002.

Discussing physics with Stefano was a fantastic experience. His depth and vision were simply unique. He was one of the great pioneers in the development of QCD as a precision science, thanks to his extraordinary ability to embrace the entire field without interruption, from the physics of “soft” gluons and their resummation to the perturbative regime. His research achievements are internationally recognised as being fundamental to the success of the high-energy collider physics programme, in particular for precision studies of the Higgs boson and the top quark.

His work is recognised as fundamental to the success of the high-energy collider physics programme

Among his most important contributions are the formulation of jet clustering algorithms at lepton and hadron colliders (a key component of most experimental analyses), a general expression for the determination of the infrared singularities of scattering amplitudes (the so-called Catani formula), the design of general algorithms for the perturbative calculation of cross sections and differential observables, which have become a standard in the community (the well-known Catani–Seymour dipole subtraction and the q_T subtraction schemes), and the innovative Catani–Krauss–Kuhn–Webber algorithm for Monte Carlo simulations of many-jet processes.

Stefano’s work was especially motivated by the application of QCD to collider data. He was convinced that our understanding of QCD singularities could be formulated in a way that any user could make a next-to-leading-order calculation of any suitable observable, not just dedicated calculations by experts. He also studied factorisation properties and coherence effects in the high-energy limit (the Catani–Ciafaloni–Fiorani–Marchesini equation) and proposed a generalisation of collinear factorisation that accounts for potential factorisation breaking effects at very high perturbative orders. The countless messages received from collaborators and colleagues all over the world, affected by the premature loss of a dear friend and extraordinary colleague, highlight Stefano’s great qualities of generosity, human warmth and scientific rigour that will be sorely missed by all.

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