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.”
A new record for the highest luminosity at a particle collider has been set by SuperKEKB at the KEK laboratory in Tsukuba, Japan. On 15 June, electron–positron collisions at the 3 km-circumference double-ring collider reached an instantaneous luminosity of 2.22×1034 cm-2 s-1 — surpassing the LHC’s record of 2.14×1034 cm-2s-1 set with proton–proton collisions in 2018. A few days later, SuperKEKB pushed the luminosity record to 2.4×1034 cm-2s-1. This milestone follows more than two years of commissioning of the new machine, which delivers asymmetric electron–positron collisions to the Belle II detector at energies corresponding to the Υ(4S) resonance (10.57 GeV) to produce copious amounts of B and D mesons and τ leptons.
We can spare no words in thanking KEK for their pioneering work in achieving results that push forward both the accelerator frontier and the related physics frontier
Pantaleo Raimondi
SuperKEKB is an upgrade of the KEKB b-factory, which operated from 1998 until June 2010 and held the luminosity record of 2.11×1034 cm−2s−1 for almost ten years until the LHC edged past it. SuperKEKB’s new record was achieved with a product of beam currents less than 25% of that at KEKB thanks to a novel “nano-beam” scheme originally proposed by accelerator physicist Pantaleo Raimondi of the ESRF, Grenoble. The scheme, which works by focusing the very low-emittance beams using powerful magnets at the interaction point, squeezes the vertical height of the beams at the collision point to about 220 nm. This is expected to decrease to approximately 50 nm by the time SuperKEKB reaches its design performance.
“We, as the accelerator community, have been working together with the KEK team since a very very long time and we can spare no words in thanking KEK for their pioneering work in achieving results that push forward both the accelerator frontier and the related physics frontier,” says Raimondi.
The first collider to employ the nano-beam scheme and to achieve a β*y focusing parameter of 1 mm, SuperKEKB required significant upgrades to KEKB including a new low-energy ring beam pipe, a new and complex system of superconducting final-focusing magnets, a positron damping ring, and an advanced injector. The most recent improvement, completed in April, was the introduction of crab-waist technology, which stabilises beam-beam blowup using carefully tuned sextupole magnets located symmetrically on either side of the interaction point (IP). It was first used at DAΦNE, which had much less demanding tolerances than SuperKEKB, and differs from the “crab-crossing” technology based on special radio-frequency cavities which was used to boost the luminosity at KEKB and is now being implemented at CERN for the high-luminosity LHC.
This luminosity milestone marks the start of the super B-factory era
Yukiyoshi Ohnishi
“The vertical beta at the IP is 1 mm which is the smallest value for colliders in the world. Now we are testing 0.8 mm,” says Yukiyoshi Ohnishi, commissioning leader for SuperKEKB. “The difference between DAΦNE and SuperKEKB is the size of the Piwinski angle, which is much larger than 1 as found in ordinary head-on or small crossing-angle colliders.”
In the coming years, the luminosity of SuperKEKB is to be increased by a factor of around 40 to reach its design target of 8×1035 cm−2s−1. This will deliver to Belle II, which produced its first physics result in April, around 50 times more data than its predecessor, Belle, at KEKB over the next ten years. The large expected dataset, containing about 50 billion B-meson pairs and similar numbers of charm mesons and tau leptons, will enable Belle II to study rare decays and test the Standard Model with unprecedented precision, allowing deeper investigations of the flavour anomalies reported by LHCb and sensitive searches for very weakly interacting dark-sector particles.
“This luminosity milestone, which was the result of extraordinary efforts of the SuperKEKB and Belle II teams, marks the start of the super B-factory era. It was a special thrill for us, coming in the midst of a global pandemic that was difficult in so many ways for work and daily life,” says Ohnishi. “In the coming years, we will significantly increase the beam currents and focus the beams even harder, reducing the β*y parameter far below 1 mm. However, there will be many more difficult technical challenges on the long road ahead to design luminosity, which is expected towards the end of the decade.”
The crab-waist scheme is also envisaged for a possible Super Tau Charm factory and for the proposed Future Circular Collider (FCC-ee) at CERN, says Raimondi. “For both these projects there is a solid design based on this concept and in general all circular lepton colliders are apt to take benefit from it.”
Accelerator physicist John Flanagan, who made important contributions to beam instrumentation for the KEKB and SuperKEKB projects in Japan, passed away on 13 March.
John Flanagan was born in 1964 and grew up in The Valley of Vermont, attending Philips Academy and graduating from Harvard University in 1987 with a degree in physics, astronomy and astrophysics. After working for a few years at software companies and at the Space Sciences Laboratory at Berkeley, he attended graduate school in physics at the University of Hawai’i at Manoa in 1992. Emeritus professor Steve Olsen recalls: “John was one of our best ever graduate students at the UH. Although he was initially attracted to Hawaii because of his love for scuba diving, his deepest dive as a grad student was to the bottom of the Super-Kamkiokande water tank.”
John joined the Super-Kamkiokande experiment at an early stage. He was a beloved member of the construction team and quite a favourite of the miners at Kamioka, who presented him with the snake’s beating heart at the mine-tunnel dedication. John took the first data-taking shift on the experiment on 1 April 1996 and the following year married Mika Masuzawa, who was at that time a postdoc from Boston University working on the Super-K construction. This was around the time John completed his thesis on the first observation of atmospheric neutrino oscillations at Super-Kamkiokande, supervised by John Learned. After completing his PhD, he moved back to Japan and was a research fellow in the KEK accelerator division. His talent was quickly recognised. He was appointed as an assistant professor in 1999, an associate professor in 2008 and promoted to full professor in 2016.
He was also known for his activities on gender-equality issues
A world-leading accelerator physicist, John was well known for his immense contributions to the KEKB and SuperKEKB projects. His work on the photoelectron instability, monitoring of the beam size via synchrotron radiation light and X-rays, and feedback systems played a key role in KEKB’s achievement of the world highest luminosity at an electron-positron accelerator. Flanagan-san (sometimes nicknamed “furigana-san”) participated in nearly every aspect of the construction, monitoring and operation of KEKB. He is most celebrated for his outstanding work on the synchrotron radiation (SR) light monitor using interferometery, which allows real-time measurement of micron-level beam sizes. For SuperKEKB, he greatly improved the SR monitor by using a diamond mirror; this eliminated the systematics from thermal expansion of the mirror that had plagued the SR monitoring system in KEKB.
John also led work on the remediation of the electron-cloud effect, in particular concerning the onset of the electron-cloud blowup and its relation to the head-tail instability, which has been quite visible in the global accelerator community. In addition to being one of the key accelerator problems for KEKB and SuperKEKB, a solution to the electron-cloud problem is also needed for successful operation of the damping rings for the future International Linear Collider. Finally, he developed an innovative X-ray beam profile monitoring technique by adapting techniques from X-ray astronomy and using innovative high-speed electronics. John carried out early tests of the system in collaboration with colleagues at CESR-TA (Cornell Electron Storage Ring Test Accelerator) in Cornell and at the ATF2 (Accelerator Test Facility) at KEK. He also developed a collaboration with SLAC and the University of Hawai’i within the framework of the US-Japan Cooperation Program in High Energy Physics. In the near future, an upgraded version of this X-ray monitor will be used to realise John’s dream of bunch-by-bunch measurements of small vertical beam sizes.
In addition to his fluent command of the Japanese language and understanding of Japanese manners, John was a modest and kind person who was beloved by his colleagues in the KEK accelerator division and by those on the Belle and Belle II experiments. He was also known for his activities on gender-equality issues including participation in the Japanese Physical Society taskforces and committees as well as serving as an instructor at the Rikejo science camp for high-school girls.
John is survived by his wife, a professor at KEK, and by his daughter Mariko. We will all remember him with the greatest respect as a splendid person, as innovative scientist, and someone who we are very proud to have had the opportunity to work with.
The discovery of the Higgs boson by the ATLAS and CMS collaborations at the LHC in 2012 marked a turning point in particle physics. Not only was it the last of the Standard Model particles to be found, but it is completely different to any particle seen before: a fundamental scalar, with profound connections to the structure of the vacuum. Extensive measurements so far suggest that the particle is the simplest possible version that nature permits. But the study of the Higgs boson is still in its infancy and its properties present enigmas, including why it is so light, which the Standard Model cannot explain. Particle physics is entering a new era of exploration to address these and other outstanding questions, including unknowns in the universe at large, such as the nature of dark matter.
The 2020 update of the European strategy for particle physics (ESPPU), which was released today during the 199th session of the CERN Council, sets out an ambitious programme to carry the field deep into the 21st century. Following two years of discussion and consultation with particle physicists in Europe and beyond, the ESPPU has identified an electron–positron Higgs factory as the highest priority collider after the LHC. The ultraclean collision environment of such a machine (which could start operation at CERN within a timescale of less than 10 years after the full exploitation of the high-luminosity LHC in the late 2030s) will enable dramatic progress in mapping the diverse interactions of the Higgs boson with other particles, and form an essential part of a research programme that includes exploration of the flavour puzzle and the neutrino sector.
We have started to concretely shape CERN’s future after the LHC
Ursula Bassler
To prepare for the longer term, the ESPPU prioritises that Europe, together with its international partners, explore the technical and financial feasibility of a future proton–proton collider at CERN with a centre-of-mass energy of at least 100 TeV. In addition to allowing searches for new phenomena at unprecedented scales, this machine would enable the detailed study of how the Higgs boson interacts with itself – offering a deeper understanding of the electroweak phase transition in the early universe after which the vacuum gained a non-zero expectation value and particles were enabled to acquire mass.
“The strategy is above all driven by science and presents the scientific priorities for the field,” said Ursula Bassler, president of the CERN Council. “We have started to concretely shape CERN’s future after the LHC, which is a difficult task because of the different paths available.”
Setting the stage The strategy update is the second since the process was launched in 2005. It aims to ensure the optimal use of global resources, serving as a guideline to CERN and enabling a coherent science policy in Europe. Building on the previous strategy update, which concluded in 2013, the 2020 update states that the successful completion of the high-luminosity LHC should remain the focal point of European particle physics, together with continued innovation in experimental techniques. Europe, via the CERN neutrino platform, should also continue to support the Long Baseline Neutrino Facility in the US and neutrino projects in Japan. Diverse projects that are complementary to collider projects are an essential pillar of the ESPPU recommendations, which urge European laboratories to support experiments enabling, for example, precise investigations of flavour physics and electric or magnetic dipole moments, and searches for axions, dark-sector candidates and feebly interacting particles.
The continuing ability of CERN, European laboratories and the particle-physics community to realise compelling scientific projects is essential for scientific progress, states the report. Cooperative programmes between CERN and research centres and national institutes in Europe should be strengthened and expanded, in addition to building strong collaborations with the astroparticle and nuclear physics communities.
Exploring the next frontier The 2013 ESPPU recommended that options for CERN’s next machine after the LHC be explored. Today, there are four possible options for a Higgs factory in different regions of the world: an International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) at CERN, a Future Circular Collider (FCC-ee) at CERN, and a Circular Electron Positron Collider (CEPC) in China. As Higgs factories, the ESPPU finds all four to have comparable reach, albeit with different time schedules and with differing potentials for the study of physics topics at other energies. While not specifying which facility should be built, the ESPPU states that the large circular tunnel necessary for a future hadron collider at CERN would also provide the infrastructure needed for FCC-ee as a possible first step. In addition to serving as a Higgs factory, FCC-ee is able to provide huge numbers of weak vector bosons and their decay products that would enable precision tests of electroweak physics and the investigation of the flavour puzzle.
Considering colliders at the energy frontier, a 3 TeV CLIC and a 100 TeV circular hadron collider (FCC-hh) were considered in depth. While the proposed 380 GeV CLIC also offers a Higgs factory as a first stage, the dramatic increase in energy possible with a future hadron collider compared to the 13 TeV of the LHC has led the ESPPU to consider this technology as the most promising for a future energy-frontier facility. Europe together with international partners will therefore begin a feasibility study into building such a machine at CERN with the FCC-ee Higgs and electroweak factory as a possible first stage, to be established as a global endeavour and completed on the timescale of the next strategy update later this decade. It is also expected that Europe invests further in R&D for the high-field superconducting magnets for FCC-hh while retaining a programme in the advanced accelerator technology developed for CLIC, which also has significant potential applications in accelerator-based science beyond high-energy physics.
Europe should keep the door open to participate in other headline projects
Halina Abramowicz
The report also notes that the timely realisation of the ILC in Japan would be compatible with this strategy and, in that case, the European particle physics community would wish to collaborate. “The natural next step is to explore the feasibility of the highest priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains Halina Abramowicz, chair of the European Strategy Group, which was charged with organizing the 2020 update. “Europe should keep the door open to participate in other headline projects which will serve the field as a whole.”
Ramping up accelerator R&D To achieve the ambitious ESPPU goals, particle physicists are urged to undertake vigorous R&D on advanced accelerator technologies, in particular concerning high-field superconducting magnets including those based on high-temperature superconductors. Europe should develop a technology roadmap, taking into account synergies with international partners and other communities such as photon and neutron science, fusion energy and industry, urges the ESPPU report, which also stresses the proven ability of innovative accelerator technology to drive many other fields of science, industry and society. In addition to high-field magnets, the roadmap should include R&D for plasma-acceleration schemes, an international design study for a muon collider, and R&D on high-intensity, multi-turn energy-recovery linacs.
It is an historic day for CERN and for particle physics in Europe and beyond
Fabiola Gianotti
The ESPPU recommendations strongly emphasise the need to continue with efforts to minimise the environmental impact of accelerator facilities and maximise the energy efficiency of future projects. Europe should also continue to vigorously support theoretical research covering the full spectrum of particle physics, pursuing new research directions and links with cosmology, astroparticle physics and nuclear physics. The development of software and computing infrastructures that exploit recent advances in information technology and data science are also to be pursued in collaboration with other fields of science and industry, while particle physicists should forge stronger relations with the European Commission and continue their leadership in promoting knowledge-sharing through open science.
“It is an historic day for CERN and for particle physics in Europe and beyond. We are all very excited and we are ready to work on the implementation of this very ambitious but cautious plan,” said CERN Director-General Fabiola Gianotti following the unanimous adoption of the resolution to update the strategy by the CERN Council’s national representatives. “We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s member states and beyond. These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”
This year’s Guido Altarelli awards, which recognise exceptional achievement by young scientists in the field of deep inelastic scattering (DIS), and related topics, have been presented to Pier Francesco Monni of CERN and Philip Ilten of the University of Birmingham. Monni was recognised for his pioneering contributions to the theory and phenomenology of multi-scale QCD resummation, and Ilten, a member of the LHCb collaboration, for his exceptional contributions to bridging the gap between experiment and phenomenology in QCD and proton structure.
The prizes, now in their fifth iteration, and sponsored this year by European Physical Journal C, World Scientific and Centro Fermi, are awarded each year to a theorist and an experimentalist with a maximum of eight years of research experience following their PhD. The ceremony took place last week during the LHCb collaboration meeting, as its traditional venue, the annual DIS conference, had to be cancelled due to the ongoing coronavirus pandemic.
“Guido Altarelli was one of the founders of QCD and one of the fathers of the DIS conferences,” explains chair of the selection committee Elisabetta Gallo. “His legacy and his mentorship of young scientists inspired the leaders of the DIS conference series to honour his legacy through this prize.”
First isolated in 2004 by physicists at the University of Manchester using pieces of sticky tape and a graphite block, the one-atom-thick carbon allotrope graphene has been touted as a wonder material on account of its exceptional electrical, thermal and physical properties. Turning these properties into scalable commercial devices has proved challenging, however, which makes a recently agreed collaboration between CERN and UK firm Paragraf on graphene-based Hall-probe sensors especially novel.
There is probably no other facility in the world to be able to confirm this, so the project has been a big win on both sides
Ellie Galanis
With particle accelerators requiring large numbers of normal and superconducting magnets, high-precision and reliable magnetic measurements are essential. While the workhorse for these measurements is the rotating-coil magnetometer with a resolution limit of the order of 10–8 Vs, the most important tool for local field mapping is the Hall probe, which passes electrical current proportional to the field strength when the sensor is perpendicular to a magnetic field. However, measurement uncertainties in the 10–4 range required for determining field multipoles are difficult to obtain, even with the state-of-the-art devices. False signals caused by non-perpendicular field components in the three-dimensional sensing region of existing Hall probes can increase the measurement uncertainty, requiring complex and time-consuming calibration and processing to separate true signals from systematic errors. With an active sensing component made of atomically thin graphene, which is effectively two-dimensional, a graphene-based Hall probe in principle suffers negligible planar Hall effects and therefore could enable higher precision mapping of local magnetic fields.
Inspiration strikes
Stephan Russenschuck, head of the magnetic measurement section at CERN, spotted the potential of graphene-based Hall probes when he heard about a talk given by Paragraf – a recent spin-out from the department of materials science at the University of Cambridge – at a magnetic measurement conference in December 2018. This led to a collaboration, formalised between CERN and Paragraf in April, which has seen several graphene sensors installed and tested at CERN during the past year. The firm sought to develop and test the device ahead of a full product launch by the end of this year, and the results so far, based on well-calibrated field measurements in CERN’s reference magnets, have been very promising. “The collaboration has proved that the sensor has no planar effect,” says Paragraf’s Ellie Galanis. “This was a learning step. There is probably no other facility in the world to be able to confirm this, so the project has been a big win on both sides.”
The graphene Hall sensor also operates over a wide temperature range, down to liquid-helium temperatures at which superconducting magnets in the LHC operate. “How these sensors behave at cryogenic temperatures is very interesting,” says Russenschuck. “Usually the operation of Hall sensors at cryogenic temperatures requires careful calibration and in situ cross-calibration with fluxmetric methods. Moreover, we are now exploring the sensors on a rotating shaft, which could be a breakthrough for extracting local, transversal field harmonics. Graphene sensors could get rid of the spurious modes that come from nonlinearities and planar effects.”
CERN and Paragraf, which has patented a scalable process for depositing two-dimensional materials directly onto semiconductor-compatible substrates, plan to release a joint white paper communicating the results so far and detailing the sensor’s performance across a range of magnetic fields.
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional
Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
