New Perspectives on Einstein’s E = mc2 mixes historical notes with theoretical aspects of the Lorentz group that impact relativity and quantum mechanics. The title is a little perplexing, however, as one can hardly expect nowadays to discover new perspectives on an equation such as E = mc2. The book’s true aim is to convey to a broader audience the formal work done by the authors on group theory. Therefore, a better-suited title may have been “Group theoretical perspectives on relativity”, or even, more poetically, “When Wigner met Einstein”.
The first third of the book is an essay on Einstein’s life, with historical notes on topics discussed in the subsequent chapters, which are more mathematical and draw heavily on publications by the authors – a well-established writing team who have co-authored many papers relating to group theory. The initial part is easy to read and includes entertaining stories, such as Einstein’s mistakes when filing his US tax declaration. Einstein, according to this story, was calculating his taxes erroneously, but the US taxpayer agency was kind enough not to raise the issue. The reader has to be warned, however, that the authors, professors at the University of Maryland and New York University, have a tendency to make questionable statements about certain aspects of the development of physics that may not be backed up by the relevant literature, and may even contradict known facts. They have a repeated tendency to interpret the development of physical theories in terms of a Hegelian synthesis of a thesis and an antithesis, without any cited sources in support, which seems, in most cases, to be a somewhat arbitrary a posteriori assessment.
There is a sharp distinction in the style of the second part of the book, which requires training in physics or maths at advanced undergraduate level. These chapters begin with a discussion of the Lorentz group. The interest then quickly shifts to Wigner’s “little groups”, which are subgroups of the Lorentz group with the property of leaving the momentum of a system invariant. Armed with this mathematical machinery, the authors proceed to Dirac spinors and give a Lorentz-invariant formulation of the harmonic oscillator that is eventually applied to the parton model. The last chapter is devoted to a short discussion on optical applications of the concepts advanced previously. Unfortunately, the book finishes abruptly at this point, without a much-needed final chapter to summarise the material and discuss future work, which, the previous chapters imply, should be plentiful.
Young Suh Kim and Marilyn Noz’s book may struggle to find its audience. The contrast between the lay and expert parts of this short book, and the very specialised topics it explores, do not make it suitable for a university course, though sections could be incorporated as additional material. It may well serve, however, as an interesting pastime for mathematically inclined audiences who will certainly appreciate the formalism and clarity of the presentation of the mathematics.
A career as a surveyor offers the best of two worlds, thinks Dominique Missiaen, a senior member of CERN’s survey, mechatronics and measurements (SMM) group: “I wanted to be a surveyor because I felt I would like to be inside part of the time and outside the other, though being at CERN is the opposite because the field is in the tunnels!” After qualifying as a surveyor and spending time doing metrology for a cement plant in Burma and for the Sorbonne in Paris, Missiaen arrived at CERN as a stagier in 1986. He never left, starting in a staff position working on the alignment of the pre-injector for LEP, then of LEP itself, and then leading the internal metrology of the magnets for the LHC. From 2009–2018 he was in charge of the whole survey section, and since last year has a new role as a coordinator for special projects, such as the development of a train to remotely survey the magnets in the arcs of the LHC.
“Being a surveyor at CERN is completely different to other surveying jobs,” explains Missiaen. “We are asked to align components within a couple of tenths of a millimetre, whereas in the normal world they tend to work with an accuracy of 1–2 cm, so we have to develop new and special techniques.”
A history of precision
When building the Proton Synchrotron in the 1950s, engineers needed an instrument to align components to 50 microns in the horizontal plane. A device to measure such distances did not exist on the market, so the early CERN team invented the “distinvar” – an instrument to ensure the nominal tension of an invar wire while measuring the small length to be added to obtain the distance between two points. It was still used as recently as 10 years ago, says Missiaen. Another “stretched wire” technique developed for the ISR in the 1960s and still in use today replaces small-angle measurements by a short-distance measurement: instead of measuring the angle between two directions, AB and AC, using a theodolite, it measures the distance between the point B and the line AC. The AC line is realised by a nylon wire, while the distance is measured using a device invented at CERN called the “ecartometer”.
Invention and innovation haven’t stopped. The SMM group recently adapted a metrology technique called frequency sweeping interferometry for use in a cryogenic environment to align components inside the sealed cryostats of the future High-Luminosity LHC (HL-LHC), which contract by up to 12 mm when cooled to operational temperatures. Another recent innovation, in collaboration with the Institute of Plasma Physics in Prague that came about while developing the challenging alignment system for HIE-ISOLDE, is a non-diffractive laser beam with a central axis that diverges by just a few mm over distances of several hundred metres and which can “reconstruct” itself after meeting an obstacle.
The specialised nature of surveying at CERN means the team has to spend a lot of time finding the right people and training newcomers. “It’s hard to measure at this level and to maintain the accuracy over long distances, so when we recruit we look for people who have a feeling for this level of precision,” says Missiaen, adding that a constant feed of students is important. “Every year I go back to my engineering school and give a talk about metrology, geodesy and topometry at CERN so that the students understand there is something special they can do in their career. Some are not interested at all, while others are very interested – I never find students in between!”
We see the physics results as a success that we share in too
CERN’s SMM group has more than 120 people, with around 35 staff members. Contractors push the numbers up further during periods such as the current long-shutdown two (LS2), during which the group is tasked with measuring all the components of the LHC in the radial and vertical direction. “It takes two years,” says Jean-Frederic Fuchs, who is section leader for accelerators, survey and geodesy. “During a technical stop, we are in charge of the 3D-position determination of the components in the tunnels and their alignment at the level of a few tenths of a millimetre. There is a huge number of various accelerator elements along the 63 km of beam lines at CERN.”
Fuchs did his master’s thesis at CERN in the domain of photogrammetry and then left to work in Portugal, where he was in charge of guiding a tunnel-boring machine for a railway project. He returned to CERN in the early 2000s as a fellow, followed by a position as a project associate working on the assembly and alignment of the CMS experiment. He then left to join EDF where he worked on metrology inside nuclear power plants, finally returning to CERN as a staff member in 2011 working on accelerator alignment. “I too sought a career in which I didn’t have to spend too much time in the office. I also liked the balance between measurements and calculations. Using theodolites and other equipment to get the data is just one aspect of a surveyor’s job – post-treatment of the data and planning for measurement campaigns is also a big part of what we do.”
With experience in both experiment and accelerator alignment, Fuchs knows all too well the importance of surveying at CERN. Some areas of the LHC tunnel are moving by about 1 mm per year due to underground movement inside the rock. The tunnel is rising at point 5 (where CMS is located) and falling between P7 and P8, near ATLAS, while the huge mass of the LHC experiments largely keeps them at the same vertical position, therefore requiring significant realignment of the LHC magnets. During LS2, the SMM group plans to lower the LHC at point 5 by 3 mm to better match the CMS interaction point by adjusting jacks that allow the LHC to be raised or lowered by around 20 mm in each direction. For newer installations, the movement can be much greater. For example, LINAC4 has moved up by 5 mm in the source area, leading to a slope that must be corrected. The new beam-dump tunnels in the LHC and the freshly excavated HL-LHC tunnels in points 1 and 5 are also moving slightlycompared to the main LHC tunnel. “Today we almost know all the places where it moves,” says Fuchs. “For sure, if you want to run the LHC for another 18 years there will be a lot of measurement and realignment work to be done.” His team also works closely with machine physicists to compare its measurements to those performed with the beams themselves.
It is clear that CERN’s accelerator infrastructure could not function at the level it does without the field and office work of surveyors. “We see the physics results as a success that we share in too,” says Missiaen. “When the LHC turned on you couldn’t know if a mistake had been made somewhere, so in seeing the beam go from one point to another, we take pride that we have made that possible.”
Pierre Lazeyras, who played leading roles in the ALEPH experiment, neutrino beams and silicon detectors during a 35-year-long career at CERN, passed away on 4 April aged 88.
Pierre graduated from the École supérieure de physique et chimie industrielle (ESPCI) in Paris in 1954 and, after working in Anatole Abragam’s group at CEA Saclay, he joined CERN as a staff member in October 1961. He was one of the early collaborators in the Track Chamber (TC) division, which built the two-metre bubble chamber and the Big European Bubble Chamber (BEBC). In parallel, he headed the team that developed one of the first superconducting bending magnets for BEBC’s “beam s3”.
Pierre directed the TC SPS neutrino beam group from 1972, which included the construction of the horns, the 185 m-long iron muon shielding and the beam monitoring, for which silicon-diode particle detectors were employed. After some initial teething troubles, the SPS neutrino beams operated for nearly 20 years without major problems. The silicon monitors were found to be more precise than the early gas-filled ion chambers, and this was the beginning of the era of silicon micro-strip detectors. Pierre encouraged the microelectronics developments for this new technology and its integrated readout circuits. These advances also came just in time for the UA2 experiment at the SPS and for wider applications in the LEP experiments.
Pierre was instrumental in the formation and success of ALEPH. From the conception of the experiment in 1982 right through to the LEP2 phase in 1996, he was ALEPH technical coordinator – a role that was quite new to those of us coming from smaller experiments. Pierre made sure we were realistic in our ambitions and our estimates of the difficulties and planning constraints, and we owe it mainly to him that the various parts of ALEPH were assembled without major problems. He was always available for advice even if, in his careful and reserved style, he did not try to direct or micro-manage everything.
In addition to being responsible for general safety in the experiment (which had no major incidents during its 11 years of operation), Pierre ensured that the construction of ALEPH was completed within budget. He also played an essential role at a crucial moment for the experiment in the early 1990s: the problem with the superconducting magnet cryostat. Under Pierre’s supervision, a vacuum leak was located, close to the edge of the magnet, and the cryostat then underwent “surgery” using a milling machine suspended from a crane. It was a wonderful exercise in imagination and, to the relief of all, a complete success. Pierre had always insisted that such a huge superconducting magnet and cryostat inherently constituted a fragile device, and had objected to the idea of warming up the magnet during annual shutdowns, citing the mechanical stress resulting from this procedure. He was absolutely right.
Pierre was also involved in the design of the large stabilised superconductors for the LHC-experiment magnets and served as a member of the magnet advisory group of the LHC into his retirement, his wisdom being highly appreciated. He was also an active member of the CERN Staff Association. Following his retirement in 1996, he joined the Groupement des Anciens and was a representative on the CERN health insurance supervisory committee, where his advice and opinions were always wise and measured.
Pierre was not only highly talented and used his experience most effectively, he was also a warm person, someone on whom one could always rely. He would always tell you straight how things were and then suggest how any problems could be tackled. A typical remark by Pierre would be: “Ask me to approve or reject your ideas, do not ask me what work I have for you.” We will remember him as a very dear friend and colleague.
Aldo Michelini, who led OPAL and other important experiments at CERN, passed away at Easter at the age of 89. He was known as much for his kindness and care for his colleagues, particularly those embarking on their careers, as for the physics at which he excelled.
Aldo first came to CERN in 1960, bringing experience from several tracking-chamber experiments, including a stint with Jack Steinberger at Columbia University, and he lost no time in making an impact. One of his earliest contributions was to equip CERN’s Wilson chamber magnet with spark chambers, which he then used as part of a CERN/ETH/Imperial College/Saclay collaboration to measure properties of the K02 meson and pp and K–p charge-exchange interactions using a polarised target.
As the 1960s advanced, Aldo formed a partnership and life-long friendship with his compatriot, Mario Morpurgo, who was an early pioneer of superconducting magnet technology. The two were part of the small team spearheading the development of the Omega spectrometer, a general-purpose device built around a large superconducting magnet that could be arranged and configured according to the physics to be studied. Omega was initially equipped with spark chambers and installed on a PS beamline, receiving its first beam in 1972, and moved to the SPS in 1976 where it became the backbone of the fixed-target programme there for 20 years.
In 1973, Aldo headed a similar project to build a general-purpose spectrometer for the North Area. This became NA3, which was the first experiment to receive beam in the new SPS hadron hall, EHN1, in May 1978. NA3 embarked on a programme of high-mass dimuon production with π+, π–, K+, K–, p and p beams, enabling the first observation of upsilon production by pions. It also probed the structure of the incoming particles via the Drell–Yan process. The spectrometer carried out a string of valuable experiments under Aldo’s guidance until 1981, when he became spokesperson of the OPAL experiment being planned for LEP. Aldo remained at the helm of OPAL right up to his retirement in 1995.
OPAL was built around tried and tested technology, including a paradoxical novelty for Morpurgo: a warm magnet. Huge for its time, with a collaboration of some 300 people, OPAL was nevertheless the smallest of the four LEP experiments. It was a scale that lent itself well to Aldo’s unique style of management – leading through example and consensus. Colleagues remember him smiling and looking very worried, or more often than not, the other way around. This was strangely motivational, with team members striving to make him smile more and worry less. His personality shaped the unique OPAL team spirit. Despite his gentle nature, Aldo was more than capable of making tough choices, and winning over those who might initially have disagreed with him.
When OPAL detected the first Z boson at LEP on 13 August 1989, Aldo was heard to remark that the young people had taken over. The average age of those in the control room that day was well under 30, and that youthfulness was no accident. Aldo actively supported the young members of the collaboration, making sure that they were visible at collaboration meetings and conferences. He also imbued them and the whole collaboration with a culture of never publishing even preliminary results before being absolutely certain of them. As a result, OPAL’s scientists built a strong reputation, with many conference conversations including the words, “let’s wait and see what OPAL has to say”. Aldo’s faith in the younger generation was rewarded by some 300 successful PhD theses from OPAL, while more than 100 CERN fellows passed through the collaboration over its lifetime.
Aldo was a great leader, commanding respect and affection in equal measure. That the collaboration was still able to gather more than 100 members in 2019 to celebrate the 30th anniversary of that first Z decay is testimony to the kind of person Aldo was, and to the spirit that he engendered. Although he was unable to attend that gathering, he sent a message, and was loudly cheered. He will be sorely missed.
Distinguished CERN physicist Adolf Minten passed away on 21 March at the age of 88.
After graduating from the University of Bonn, where he worked in the team of Wolfgang Paul on the 500 MeV electron synchrotron, Adolf joined the CERN Track Chamber division in 1962. Working under Charles Peyrou, he set up beamlines for the two-metre bubble chamber and actively participated in its broad physics programme. Another important milestone of his career was his time as a visiting scientist at SLAC from 1966 to 1967, where he took part in the early experiments on hadron electro-production and electron scattering at the new two-mile accelerator.
Adolf returned to CERN at a time of decisive developments in accelerator and detector technologies. In parallel to his continued participation in bubble-chamber experiments, he became interested in the physics programme of the Intersecting Storage Rings, the world’s first proton–proton collider, which started operation in 1971. To cope with the high interaction rates expected at this new machine, the development of track detectors focused on the multi-wire proportional chamber (MWPC) developed by Georges Charpak. One of the designs was a large multi-purpose spectrometer called the split-field magnet (SFM). At that time, a large-scale application of the revolutionary MWPC technology, hitherto available only in single-wire devices or small-surface detectors, presented a formidable challenge. In 1969, Adolf became responsible for the construction of the SFM facility, which covered the full solid angle with an unprecedented 300 m2 detector surface, and 70,000 wires and electronics channels. Major detector, electronics and software developments were needed to bring this project into operation in 1974.
In 1975, to prepare for the next generation of experiments at the new SPS machine, the CERN management proposed the creation of a new Experimental Facilities (EF) division. Adolf was elected to lead the new EF division, a position that required a combination of strong scientific and technical authority, and in which he commanded the unreserved respect of his collaborators. Following support provided to the major facilities for the SPS fixed-target programme, such as BEBC, the Omega spectrometer and the neutrino, muon and other experiments, his new division soon became involved in the successful experiments at the SPS proton–antiproton collider.
In 1984 Adolf stepped down from his position as EF division leader and joined the ALEPH experiment at LEP. The LEP experiments were a quantum leap in size and complexity when compared to previous experiments, and demanded new organisational structures. As head of the ALEPH steering committee, Adolf was instrumental in setting up an organisation whose role he compared to an “orchestra, where it is not sufficient that all the instruments be properly tuned, they must also harmonise”. However, his true role of an “elder statesman” went far beyond organisational responsibilities; equally important were his human qualities, which were remarkable indeed and for which he was respected by both young and old.
Adolf maintained a constant interest in DESY, where he was highly appreciated. In 1981 Bjorn Wiik’s study group had finished the HERA design report, and DESY set up an international evaluation committee to analyse it in detail. Adolf was invited to chair this committee. Its positive recommendation was a significant step towards the approval of the HERA project. He chaired the DESY scientific council from 1987 until 1990, during the main construction phase of the storage rings and the H1 and ZEUS multi-purpose detectors.
Adolf retired from CERN in 1996. We remember him as a supremely well-organised scientist of deep and incisive intelligence, unafraid to challenge and question preconceived ideas, and always inspiring others to do the same. At the same time, he was a modest person who cared profoundly for all the people around him, and their families.
Antonino Pullia, who passed away in April aged 84, was a student of Giuseppe Occhialini at the University of Milan and obtained his laurea in 1959. For the next 60 years he devoted himself to teaching, administration and the rich physics research programmes at the INFN and the universities of Milan and Milano-Bicocca, playing a major role in establishing the new physics department at the latter. He had a great passion for teaching undergraduates, continuing well into retirement.
Pullia’s research ranged over many topics including neutrino physics, proton decay, double-beta decay, DELPHI at LEP, CMS at LHC and dark-matter searches. He also played a prominent role in the discovery of neutral currents at CERN using the Gargamelle bubble chamber.
In March 1972 he presented the vertex distribution of possible neutral-current events that had no lepton candidate but one or more pions. The distribution was seen to be uniform, just like the events with muon candidates, leading immediately to the formation of working groups concentrating on neutral-current searches in both hadronic and purely leptonic modes. After a remarkable scanning and measurement effort many candidates for neutral currents had been found, but the burning issue was the size of the background due to neutron interactions. Pullia recognised the importance of a special class of events, namely genuine neutrino events with a detected final-state muon and a neutron emitted at the interaction vertex and detected downstream in the visible part of the bubble chamber. Such events were rare, but very valuable, since in this case the downstream event was surely induced by a neutron. It was clear that the major source of background neutrons was coming from neutrino events in the material surrounding Gargamelle. With this knowledge, it turned out that the predicted background was far too small to explain the observed number of neutral-current candidates and thus, at the end of July 1973, the collaboration was able to announce the great discovery of neutral currents. The Italian Physical Society awarded the 2011 Fermi prize to Pullia in recognition of his important contribution.
At the beginning of the 1980s Tonino, as he was known, joined the DELPHI collaboration at LEP where he worked with his group on the construction of the electromagnetic calorimeter, along with the reconstruction and analysis software. The Milan group, under his constant support, was extremely active in DELPHI, proposing many original analyses, as well as many PhD and master theses, contributing to the exceptionally rich LEP physics results.
In 2012 Tonino became interested in the detection of dark matter, deciding to resurrect a special type of bubble chamber developed 50 years ago – called “the Geyser” – which is remarkable in its simplicity. With no moving parts, and the ability to reset itself a few seconds after a bubble is formed, the device was ideal for underground experiments. He also formed the MOSCAB collaboration, which successfully produced a small detector with the required superheat needed for dark-matter searches.
Each of us who had the privilege to work with, or simply to talk to, Tonino has been enlightened in some way in our efforts to have a deeper understanding of fundamental physics. He was always extremely kind and open to alternative views. We will sadly miss him for his human qualities, and as a physicist.
Teresa Rodrigo Anoro, professor of atomic and nuclear physics at the University of Cantabria, passed away at her home on 20 April after a long illness. She was a leading figure within the particle-physics community and played a key role in shaping Spanish particle-physics policy, with an emphasis on promoting the participation of women in science.
After her bachelor’s degree in physics from the University of Zaragoza, Teresa joined the high-energy physics group of La Junta de Energía Nuclear in Madrid (currently
CIEMAT), earning a PhD in 1985 with a thesis on the production of strange particles at the NA23 experiment at CERN. She then moved to CERN to participate in the development of the Uranium–TMP calorimeter for the upgrade of the UA1 experiment, where she started her personal journey towards finding the top quark. This eventually brought her to the CDF experiment at Fermilab, where she carried out the detailed modelling of the W+jet background, a crucial input to the top’s discovery. In 1994 she took up a faculty position at the Instituto de Física de Cantabria (IFCA) in Santander, incorporating the IFCA group into both the CDF experiment and the newly formed CMS collaboration at CERN. Under her direction, the group continued her study of the properties of the top quark and opened up a new line of research towards the discovery of the Higgs boson.
More recently, moving away from hadron beams for the first time, Teresa promoted new approaches to the search for light dark-matter at the DAMIC experiment. She was well aware of the importance of technology development and detector building in high-energy physics and orchestrated her group’s contribution to the construction of the CMS muon spectrometer, in particular its muon alignment system, and to the building of CDF’s time-of-flight detector.
Teresa’s scientific insight and strong commitment to whatever endeavour she was engaged in were recognised by the international community: she was elected chair of the CMS collaboration board (2011–2012) and served as a member of several scientific policy committees, including the European Physical Society HEPP board (2006–2013) and the CERN scientific policy committee (2012–2017). Outside academia, she was a member of several Spanish ministerial scientific panels and of the technical and research panel of the Princesa de Asturias awards. She also held an honorary doctorate from the Menéndez Pelayo International University, received the silver medal of the University of Cantabria and the first Julio Peláez award for female pioneers in science, among other recognitions.
Teresa’s influence on the Santander HEP group and the IFCA institute that she directed until a few months before her death remains very visible. During her tenure, the group grew considerably and greatly expanded its activities. The institute was awarded the greatest distinction of excellence of the Spanish science system, the Maria de Maeztu grant, and the gender-equality prize awarded by the Spanish National Research Council.
Those of us who were fortunate enough to know Teresa and to share some of her scientific passions, are aware of how kind, approachable, righteous and sympathetic
she was, though with a strong character that came from her deep honesty. Teresa’s legacy stands as a testament to her leadership, her vision and her ability to mentor rising colleagues. She will be sorely missed.
Danila Tlisov, a member of the CMS collaboration at CERN, passed away on 14 April in Russia due to complications associated with COVID-19. He was just 36 years old.
Danila joined the INR Moscow group in 2010 as a young researcher after graduating with honours from Moscow State University and defending his dissertation. Following his contributions to early heavy-neutrino searches, he started to work on the CMS hadron calorimeter (HCAL) subsystem in 2012. Danila served as the hub of the multinational CMS HCAL upgrade effort, leading the CERN-based team that received individual components from India, Russia, Turkey and the US, and assembling them into a working detector. Danila recently brought his unique mix of strengths to the CMS HCAL management team as deputy project manager and a member of the CMS management.
In the physics analysis realm, Danila worked with the University of Rochester group on a measurement of the electroweak mixing angle using the forward–backward asymmetry in Drell–Yan events, where he focused on critical improvements to the calibration of the electron-energy measurements in challenging regions of Drell–Yan kinematic phase space.
CMS friends and colleagues remember fondly the warm smile and incredibly effective leadership of Danila. His practical know-how and excellent judgement were critical as we worked together through the tough challenges of a major detector upgrade.
Danila was an accomplished backcountry touring skier. Because of his great physical strength and focus on climbing, it was often said that he may have been faster going uphill than downhill, and that is saying a lot.
Among his many colleagues, Danila will be remembered for his pleasant, cheerful disposition, even during times of intense pressure. He challenged us with his brilliant ideas, guided students with patience and grace, and inspired us all. He will be sorely missed.
Experimental physicist Ronald Fortune, who joined CERN’s first nuclear research group in January 1956, passed away on 16 June 2019 at the age of 90.
Ron graduated with a degree in physics and mathematics from the University of Aberdeen, UK, before joining electrical engineering firm AEI in Manchester, where he acquired a valuable practical training in several departments and research experience in high-voltage techniques and electron-microscope design. This training was put to immediate use in his first post as scientific officer in the British Royal Naval Scientific Service, where he developed automated instrumentation for the study of atomic-weapon explosions at the Woomera test range in Australia.
Ron’s main career was as a senior scientist at CERN, where he spent 17 years engaged in a wide variety of projects. This included six years in high-energy physics research studying K-mesons, relativistic ionisation effects and hunting for quarks, during which Ron pioneered methods for identifying high-energy particles by measurement of their momentum and ionising power, and developed high-precision optical equipment for the photography of high-energy particles. For his work on relativistic ionisation, he was awarded a doctorate by the University of Geneva. The next eight years were spent in CERN’s applied-physics divisions, where he was a member of the team that developed the world’s first radio-frequency particle separator. Ron also coordinated a large CERN–Berkeley–Rutherford team in the extensive study of accelerator shielding problems. The final phase of his career at CERN was spent in organising the large-scale production of particle detectors (wire chambers) for the nuclear-physics divisions.
In 1973 Ron resigned his staff position at CERN to direct an independent consultancy in physics, engineering-physics and project management. In 1976 the firm signed a contract with the Dutch government, where he was charged with the construction of a five-metre superconducting solenoid for the muon channel of the National Institute for Nuclear Physics Research in Amsterdam, which was successfully brought into operation in 1981.
In later years Ron actively collaborated in neuroscience research carried out at the Geneva University Hospital, co-authoring several peer-reviewed articles in specialised journals.
Ron was a most charming person, always very cheerful and positive with an extraordinary sense of humour.
An intriguing low-energy excess of background events recorded by the world’s most sensitive WIMP dark-matter experiment has sparked a series of preprints speculating on its underlying cause. On 17 June, the XENON collaboration, which searches for excess nuclear recoils in the XENON1T detector, a one-tonne liquid-xenon time-projection chamber (TPC) located underground at Gran Sasso National Laboratory in Italy, reported an unexpected excess in electronic recoils at energies of a few keV, just above its detection threshold. Though acknowledging that the excess could be due to a difficult-to-constrain tritium background, the collaboration says solar axions and solar neutrinos with a Majorana nature, both of which would signal physics beyond the Standard Model, are credible explanations for the approximately 3σ effect.
Who needs the WIMP if we can have the axion?
Elena Aprile
“Thanks to our unprecedented low event rate in electronic recoils background, and thanks to our large exposure, both in detector mass and time, we could afford to look for signatures of rare and new phenomena expected at the lowest energies where one usually finds lots of background,” says XENON spokesperson Elena Aprile, of Columbia University in New York. “I am especially intrigued by the possibility to detect axions produced in the Sun,” she says. “Who needs the WIMP if we can have the axion?”
The XENON collaboration has been in pursuit of WIMPs, a leading bosonic cold-dark-matter candidate, since 2005 with a programme of 10 kg, 100 kg and now 1 tonne liquid-xenon TPCs. Particles scattering in the liquid xenon create both scintillation light and ionisation electrons; the latter drift upwards in an electric field towards a gaseous phase where electroluminescence amplifies the charge signal into a light signal. Photomultiplier tubes record both the initial scintillation light and the later electroluminescence, to reveal 3D particle tracks, and the relative magnitudes of the two signals allows nuclear and electronic recoils to be differentiated. XENON1T derives its world-leading limit on WIMPs – the strictest 90% confidence limit being a cross-section of 4.1×10−47 cm2 for WIMPs with a mass of 30 GeV – from the very low rate of nuclear recoils observed by XENON1T from February 2017 to February 2018.
A surprise was in store, however, in the same data set, which also revealed 285 electronic recoils at the lower end of XENON1T’s energy acceptance, from 1 to 7 keV, over the expected background of 232±15. The sole background-modelling explanation for the excess that the collaboration has not been able to rule out is a minute concentration of tritium in the liquid xenon. With a half-life of 12.3 years and a relatively low amount of energy liberated in the decay of 18.6 keV, an unexpected contribution of tritium decays is favoured over XENON1T’s baseline background model at approximately 3σ. “We can measure extremely tiny amounts of various potential background sources, but unfortunately, we are not sensitive to a handful of tritium atoms per kilogram,” explains deputy XENON1T spokesperson Manfred Lindner, of the Max Planck Institute for Nuclear Physics in Heidelberg. Cryogenic distillation plus running the liquid xenon through a getter is expected to remove any tritium below the level that would be relevant, he says, but this needs to be cross-checked. The question is whether a minute amount of tritium could somehow remain in liquid xenon or if some makes it from the detector materials into the liquified xenon in the detector. “I personally think that the observed excess could equally well be a new background or new physics. About 3σ implies of course a certain statistical chance for a fluctuation, but I find it intriguing to have this excess not at some random place, but towards the lower end of the spectrum. This is interesting since many new-physics scenarios generically lead to a 1/E or 1/E2 enhancement which would be cut off by our detection threshold.”
Solar axions
One solution proposed by the collaboration is solar axions. Axions are a consequence of a new U(1) symmetry proposed in 1977 to explain the immeasurably small degree of CP violation in quantum chromodynamics – the so-called strong CP problem — and are also a dark-matter candidate. Though XENON1T is not expected to be sensitive to dark-matter axions, should they exist they would be produced by the sun at energies consistent with the XENON1T excess. According to this hypothesis, the axions would be detected via the “axioelectric” effect, an axion analogue of the photoelectric effect. Though a good fit phenomenologically, and like tritium favoured over the background-only hypothesis at approximately 3σ, the solar-axion explanation is disfavoured by astrophysical constraints. For example, it would lead to a significant extra energy loss in stars.
Axion helioscopes such as the CERN Axion Solar Telescope (CAST) experiment, which directs a prototype LHC dipole magnet at the Sun and could convert solar axions into X-ray photons, will help in testing the hypothesis. “It is not impossible to have an axion model that shows up in XENON but not in CAST,” says deputy spokesperson Igor Garcia Irastorza of University of Zaragoza, “but CAST already constraints part of the axion interpretation of the XENON signal.” Its successor, the International Axion Observatory (IAXO), which is set to begin data taking in 2024, will have improved sensitivity. “If the XENON1T signal is indeed an axion, IAXO will find it within the first hours of running,” says Garcia Irastorza.
A second new-physics explanation cited for XENON1T’s low-energy excess is an enhanced rate of solar neutrinos interacting in the detector. In the Standard Model, neutrinos have a negligibly small magnetic moment, however, should they be Majorana rather than Dirac fermions, and identical to their antiparticles, their magnetic moment should be larger, and proportional to their mass, though still not detectable. New physics beyond the Standard Model could, however, enhance the magnetic moment further. This leads to a larger interaction cross section at low energies and an excess of low-energy electron recoils. XENON1T fits indicate that solar Majorana neutrinos with an enhanced magnetic moment are also favoured over the background-only hypothesis at the level of 3σ.
The absorption of dark photons could explain the observed excess.
Joachim Kopp
The community has quickly chimed in with additional ideas, with around 40 papers appearing on the arXiv preprint server since the result was released. One possibility is a heavy dark-matter particle that annihilates or decays to a second, much lighter, “boosted dark-matter” particle which could scatter on electrons via some new interaction, notes CERN theorist Joachim Kopp. Another class of dark-matter model that has been proposed, he says, is “inelastic dark matter”, where dark-matter particles down-scatter in the detector into another dark-matter state just a few keV below the original one, with the liberated energy then seen in the detector. “An explanation I like a lot is in terms of dark photons,” he says. “The Standard Model would be augmented by a new U(1) gauge symmetry whose corresponding gauge boson, the dark photon, would mix with the Standard-Model photon. Dark photons could be abundant in the Universe, possibly even making up all the dark matter. Their absorption in the XENON1T detector could explain the observed excess.”
“The strongest asset we have is our new detector, XENONnT,” says Aprile. Despite COVID-19, the collaboration is on track to take first data before the end of 2020, she says. XENONnT will boast three times the fiducial volume of XENON1T and a factor six reduction in backgrounds, and should be able to verify or refute the signal within a few months of data taking. “An important question is if the signal has an annual modulation of about 7% correlated to the distance of the sun,” notes Lindner. “This would be a strong hint that it could be connected to new physics with solar neutrinos or solar axions.”
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