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, 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, 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.

Jacques Haissinski, who played an important role in major particle-physics experiments, passed away on 25 March 2024 at the age of 89. His father Moïse worked with Marie Curie and had been a long-time collaborator of her daughter Irène Joliot-Curie.

Jacques entered Ecole Normale Supérieure in 1954 and later went to Stanford, where he worked under Burton Richter on the pioneering Colliding Beam machine to collide electrons in flight using two storage rings. After his military service, Jacques joined the Laboratoire de l’Accélérateur Linéaire in Orsay, to undertake a doctorate on the AdA (Anello di Accumulazione) ring. Built in Frascati from an idea of Bruno Touschek to collide in-flight electrons and positrons stored in the same vacuum chamber, AdA had been brought to Orsay by Pierre Marin to take advantage of the high intensity of the linac beams. Jacques mastered all aspects of the ground-breaking experiment and succeeded in detecting the very first time-in-flight collisions in 1963.

In accelerator physics, following a discovery on the ACO ring at Orsay, Jacques published, in 1967, a basic paper on the longitudinal equilibrium of particles in a storage ring that contained the now widely used “Haïssinski equation”. He also collaborated with Stanford on the commissioning of SPEAR and later SLC, the very first and so-far only linear collider. In phenomenology, following Touschek, he led a programme on radiative corrections and later gave lectures on this subject in preparation for LEP at Ecole de Gif in 1989.

But the main scientific activity of Jacques Haïssinski was experimental particle physics. He took part in many experiments, directed theses in Orsay on ACO, and was spokesperson of the CELLO experiment at DESY. During the construction of LEP, Jacques served as chairperson of the LEP committee at CERN.

After LEP, Jacques turned his interests to astroparticle physics and cosmology, notably giving courses on the subject and collaborating on the EROS experiment and the Planck mission. During that time, he also took responsibilities in the management of Paris-Sud University (at Orsay), and later as a leader in IN2P3 and in the Saclay Laboratory DAPNIA (now IRFU). His leadership was greatly appreciated by the French high-energy physics community.

An outstanding teacher, Jacques also campaigned for the dissemination of knowledge to the public. He was a great humanist who was deeply concerned with social injustice and criminal wars. He presented his views publicly and believed that other physicists should do so. Generous with his precious time, he was always available to pass on his knowledge and vast scientific culture. He marked and inspired several generations of particle and accelerator physicists.

Mats Lindroos, who made major contributions to accelerator technology, passed away on 2 May 2024 aged just 62.

Mats received his PhD in subatomic physics from Chalmers University of Technology in Gothenburg, Sweden in 1993 under the supervision of Björn Jonson. As a PhD student he studied decay properties and hyperfine interactions from oriented nuclei, making use of the low-temperature nuclear orientation facilities at ISOLDE, Daresbury and Studsvik. He joined CERN as a research fellow in 1993 and became a staff member in 1995.

While at CERN, Mats filled a number of diverse roles including being responsible for PS Booster operation and the technical coordination of the ISOLDE facility. He was one of the driving forces behind the HIE-ISOLDE project that commenced construction in 2009 and is now one of the major accelerated radioactive beam facilities worldwide. While at CERN he also played leading roles in several European Union-supported design studies for future conceptual accelerator facilities: the nuclear-physics radioactive beam facility EURISOL and the beta-beam neutrino factory. 

In 2009, when Sweden and Denmark were selected to be the host countries for the European Spallation Source (ESS), Mats returned to his roots in Sweden on secondment from CERN, formally joining the ESS in 2015. As one of the earliest members of the ESS organisation, he was responsible for establishing the nascent accelerator organisation as well as the accelerator collaboration, set up as a CERN-like collaboration, between major European accelerator laboratories across 10 countries to undertake the technical design of this important part of the facility. Mats led the technical design for the 5 MW proton linac of the ESS, and from 2013 as head of the 100-strong accelerator division he led the linac project that is now in the late stages of construction and installation. Even after stepping down from his leadership roles because of illness, he enthusiastically accepted a new one to advise the ESS management. He was fully involved in the process, and undoubtedly would have been instrumental in guiding the future evolution of the facility.

He set up a CERN-like collaboration between major European accelerator laboratories across 10 countries

As a globally recognised expert on accelerator technology, Mats served on many committees in an advisory role, such as the IJC Lab strategic advisory board (France), IN2P3 scientific committee (France), J-PARC technical advisory committee (Japan), PIP-II Fermilab technical advisory committee (US) and CERN’s scientific policy committee. As an adjunct professor at Lund University he enjoyed teaching and supervising students in addition to his numerous research, management and committee roles. Despite all these work activities, Mats found time to oversee, together with his partner Anette, the construction of a house on the south Swedish coast, where they enjoyed walking, gardening and being active in the local community.

Mats has touched all our lives with his energy and passion for research, his creativity for new ideas, his worldly knowledge, his sense of humour, and most importantly, his humanity and kindness. He will be greatly missed by all of us who had the privilege to count him as a friend and colleague.