In the heart of Beirut in a five-storey house owned by the Lebanese national telecommunication company, floors are about to be coated to make them anti-static, walls and ceilings will be insulated, and cabling systems installed so wires don’t become tangled. These and other details are set to be complete by mid-2020, when approximately 3000 processor cores, donated by CERN, will arrive.
The High-Performance Computing for Lebanon (HPC4L) project is part of efforts by Lebanese scientists to boost the nation’s research capabilities. Like many other countries that have been through conflict and seen their highly-skilled graduates leave to seek better opportunities, Lebanon is trying to stem its brain-drain. Though the new facility will not be the only HPC centre in the country, it is different because it involves both public and private institutions and has the full support of the government. “There are a few small-scale HPC facilities in different universities here, but they suffer from being isolated and hence are quickly outdated and underused,” says physicist Haitham Zaraket of Lebanese University in Beirut. “This HPC project puts together the main players in the realm of HPC in Lebanon.”
Having joined the LHC’s CMS experiment in 2016, Lebanese physicists want to develop the new facility into a CMS Tier-2 computing centre. High-speed internet will connect it to universities around the world and HPC4L has a mandate to ensure operation, maintenance, and user-interfacing for smooth and effective running of the facility. “We’ve been working with the government, private and public partners to prepare not just the infrastructure but also the team,” explains HPC4L project coordinator Martin Gastal of CERN. “CERN/CMS’s expertise and knowledge will help set up the facility and train users, but the team in Lebanon will run it themselves.” The Lebanese facility will also be used for computational biology, oil and gas discovery, financial forecasting, genome analysis and the social sciences.
Nepal is another country striving for greater digital storage and computing power. In 2017 Nepal signed a cooperation agreement with CERN. The following year, around 2500 cores from CERN enabled an HPC facility to be established at the government-run IT Park, with experts from Kathmandu University forming its core team. Rajendra Adhikari, project leader of Nepal’s HPC centre (pictured, second from right), also won an award from NVIDIA for the latest graphicscard worth USD 3000 and added it to the system. Nepal has never had computing on such a scale before, says Adhikari. “With this facility, we can train our students and conduct research that requires high-performance computing and data storage, from climate modelling, earthquake simulations to medical imaging and basic research.”
The Nepal facility is planning to store health data from hospitals, which is often deleted because of lack of storage space, and tests are being carried out to process drone images taken to map topography for hydropower feasibility studies. Even in the initial phases of the new centre, says Adhikari, computing tasks that used to take 45 days can now be processed in just 12 hours.
The SESAME light source in Jordan, which itself received 576 cores from CERN in 2017, is also using its experience to assist neighbouring regions in setting up and maintaining HPC facilities. “High-performance computing is a strong enabler of research capacity building in regions challenged by limited financial resources and talent exodus,” says Gastal. “By supporting the set up of efficient data processing and storage facilites, CERN, together with affiliated institutes, can assist fellow researchers in investing in the scientific potential of their own countries.”
An event held at CERN on 20–21 May revealed 170 projects that have been granted €100,000 of European Union (EU) funding to develop disruptive detection and imaging technologies. The successful projects, drawn from more than 1200 proposals from researchers in scientific and industrial organisations across the world, now have one year to prove the scientific merit and innovation potential of their ideas.
The 170 funded projects are part of the Horizon 2020 ATTRACT project funded by the EU and a consortium of nine partners, including CERN, the European Southern Observatory (ESO), European Synchrotron Radiation Facility (ESRF), European XFEL and Institut Laue-Langevin. The successful projects are grouped into four broad categories: data acquisition systems and computing; front-end and back-end electronics; sensors; and software and integration.
CERN researchers are involved in 19 of the projects, in areas from magnets and cryogenics to electronics and informatics. Several of the selected projects involve the design of sensors or signal-transmission systems that operate at very low temperatures or in the presence of radiation, and many target applications in medical imaging and treatment or in the aerospace sector. Others seek industrial applications, such as 3D printing of systems equipped with sensors, the inspection of operating cryostats or applications in environmental monitoring.
ESO’s astronomical technology and expertise will be applied to an imaging spectrograph suitable for clinical cancer studies and to single-photon visible-light imagers for adaptive optics systems and low-light-level spectroscopic and imaging applications. Among other projects connected with Europe’s major research infrastructures, four projects at the ESRF concern adaptive algebraic speckle tomography for clinical studies of osteoarticular diseases, a novel readout concept for 2D pixelated detectors, the transferral of indium-gallium-nitride epilayers onto substrates for full-spectrum LEDs, and artificial intelligence for the automatic segmentation of volumetric microtomography images.
“170 breakthrough ideas were selected based on a combination of scientific merit, innovation readiness and potential societal impact,” explained Sergio Bertolucci, chair of ATTRACT’s independent research, development and innovation committee. “The idea is to speed up the process of developing breakthrough technologies and applying them to address society’s key challenges.”
The outcomes of the ATTRACT seed-funding will be presented in Brussels in autumn 2020, and the most promising projects will receive further funding.
The open symposium of the European Strategy for Particle Physics (ESPP), which took place in Granada, Spain, from 13–16 May, revealed a vibrant field in flux as it grapples with how to attack the next big questions. Opening the event, chair of the ESPP strategy secretariat, Halina Abramowicz, remarked: “This is a very strange symposium. Normally we discuss results at conferences, but here we are discussing future results.” More than 10 different future-collider modes were under discussion, and the 130 or so talks and discussion sessions showed that elementary particle physics – in the wake of the discovery of the Higgs boson but so far no evidence of particles beyond the Standard Model (SM) – is transitioning into a new and less well-mapped realm of fundamental exploration.
Plain weird
Theorist Pilar Hernández of the University of Valencia described the SM as plain “weird”. The model’s success in describing elementary particles and their interactions is beyond doubt, but as an all-encompassing theory of nature it falls short. Why are the fermions arranged into three neat families? Why do neutrinos have an almost imperceptibly small mass? Why does the discovered Higgs boson fit the simplest “toy model” of itself? And what lies beneath the SM’s numerous free parameters? Similar puzzles persist about the universe at large: the mechanism of inflation; the matter–antimatter asymmetry; and the nature of dark energy and dark matter.
While initial results from the LHC severely constrain the most natural parameter spaces for new physics, said Hernández, the 10–100 TeV region is an interesting scale to explore. At the same time, she argued, there is a shift to more “bottom-up, rather than top-down”, approaches to beyond-SM (BSM) physics. The new quarries includes axion-like and long-lived particles, and searches for hidden, dark and feebly-interacting sectors – in addition to studying the Higgs boson, which has deep connections to many puzzles in the SM, with much greater precision. “Particle physics could be heading to crisis or revolution,” said Hernández.
Normally we discuss results at conferences, but here we are discussing future results
The accelerator, detector and computing technology needed for future fundamental exploration are varied and challenging. Reviewing Higgs-factory programmes, Vladimir Shiltsev, head of Fermilab’s Accelerator Physics Center, weighed up the pros and cons of linear versus circular machines. The former includes the International Linear Collider (ILC) and the Compact Linear Collider (CLIC); the latter a future circular electron–positron collider at CERN (FCCee) and the Circular Electron Positron Collider in China (CEPC). Linear colliders, said Shiltsev, are based on mature designs and organisation, are expandable to higher energies, and draw a wall-plug power similar to that of the LHC. On the other hand, they face challenges including their luminosity and number of interaction points. Circular Higgs factories offer a higher luminosity and more interaction points than linear options but require R&D into high-efficiency RF sources and superconducting cavities, said Shiltsev.
For hadron colliders, the three current options – CERN’s FCC-hh (100 TeV), China’s SppC (75 TeV) and a high-energy LHC (27 TeV) – demand next-generation superconducting dipole magnets. Akira Yamamoto of CERN/KEK said that while a lepton collider could begin construction in the next few years, the dipoles necessary for a hadron collider might take 10 to 15 years of R&D before construction could start.
The symposium also saw much discussion about muon colliders, which offer an energy-frontier lepton collider but for which it was widely acknowledged the technology is not yet ready. Concerning more futuristic acceleration technologies based on plasma wakefields, impressive results at facilities such as BELLA at Berkeley and AWAKE at CERN were on show.
Thinking ahead
From colliders to fixed-target to astrophysics experiments, said Francesco Forti of INFN and the University of Pisa, detectors face a huge variety of operating conditions and employ technologies deeply entwined with developments in industry. Another difficulty, he said, is how to handle non-standard physics signals, such as long-lived particles and monopoles. Like accelerators, detectors require long time scales – it was the very early 1990s when the first conceptual design reports for the LHC detectors were written.
In terms of data processing, the challenges ahead are immense, said Simone Campana of CERN and the HEP software foundation. The high-luminosity LHC (HL-LHC) presents a particular challenge, but DUNE, FAIR, BELLE II and other experiments will also create unprecedented data samples, plus there is the need to generate ever-more Monte Carlo samples. At the same time, noted Campana, the rate of advance in hardware performance has slowed in recent years, forcing the community to towards graphics processing units, high-performance computing and commercial cloud services. Forti and Campana both argued for better career opportunities and greater recognition for physicists who devote their time to detector and computing efforts.
The symposium also showed that the strategic importance of communications, education and outreach is becoming increasingly recognised.
Discussions in Granada revealed a community united in its desire for a post-LHC collider, but not in its choice of that collider’s form. Stimulating some heated exchanges, the ESPP saw proposals for future machines pitted against each other and against expectations from the HL-LHC in terms of their potential physics reach for key targets such as the Higgs boson.
Big questions
Gian Giudice, head of CERN’s Theory Department, said that the remaining BSM-physics space is “huge”, and pointed to four big questions for colliders: to what extent can we tell whether the Higgs is fundamental or composite? Are there new interactions or new particles around or above the electroweak scale? What cases of thermal relic WIMPs are still unprobed and can be fully covered by future collider searches? And to what extent can current or future accelerators probe feebly interacting sectors?
Though colliders dominated discussions, the enormous progress in neutrino physics since the previous ESPP was clear from numerous presentations. The open-symposium audience was reminded that neutrino masses, as established by neutrino oscillations, are the first particle-physics evidence for BSM phenomena. A vibrant programme is under way to fully measure the neutrino mixing matrix and in particular the neutrino mass ordering and CP violation phase, while other experiments are probing the neutrino’s absolute mass scale and testing whether they are of a Dirac or Majorana nature.
On 17 May in Granada, following the open symposium of the European Strategy for Particle Physics, the first meeting of a new international working group on the International Linear Collider (ILC) took place. The ILC is the most technologically mature of all current future-collider options, and was at the centre of discussions at the previous strategy update in 2013. Although its technology and costs have been revised since then, there is still no firm decision on the project’s location, governance or funding model. The new working group was set up by Japan’s KEK laboratory in response to a recent statement on the ILC from Japan’s Ministry of Education, Sports, Culture, Science and Technology (MEXT) that called for further discussions on these thorny issues. Comprising two members from Europe, two from North America and three from Asia (including Japan), the group will investigate and update several points, including: cost sharing for construction and operation; organisation and governance of the ILC; and the international sharing of the remaining technical preparations. The working group will submit a report to KEK by the end of September 2019 and the final report will be used by MEXT for discussions with other governments.
Around a fifth of the 160 input documents to the ESPP were linked to flavour physics, which is crucial for new-physics searches because it is potentially sensitive to effects at scales as high as 105 TeV, said Antonio Zoccoli of INFN. Summarising dark-matter and dark-sector physics, Shoji Asai of the University of Tokyo said that a shift was taking place from the old view, where dark-matter solutions arose as a byproduct of beyond-SM approaches such as supersymmetry, to a new paradigm where dark matter needs an explanation of its own. Asai called for more coordination and support between accelerator-based direct detection and indirect detection dark-sector searches, as exemplified by the new European Center for Astro-Particle Theory.
Jorgen D’Hondt of Vrije Universiteit Brussel listed the many dedicated experiments in the strong-physics arena and the open questions, including: how to reach an adequate precision of perturbative and non-perturbative QCD predictions at the highest energies? And how to probe the quark–gluon plasma equation of state and to establish whether there is a first-order phase transition at high baryon density?
Of all the scientific themes of the week, electroweak physics generated the liveliest discussions, especially concerning how well the Higgs boson’s couplings to fermions, gauge bosons and to itself can be probed at current and future colliders. Summary speaker Beate Heinemann of DESY cautioned that such quantitative estimates are extremely difficult to make, though a few things stand out. One is the impressive estimated performance from the HL-LHC in the next 15 years or so; another is that a long-term physics programme based on successive machines in a 100 km-circumference tunnel offers the largest overall physics reach on the Higgs boson and other key parameters. There is broad agreement, however, that the next major collider immediately after the LHC should collide electrons and positrons to fully explore the Higgs and make precision measurements of other electroweak parameters.
The big picture
The closer involvement of particle physics with astroparticle physics, in particular following the discovery of gravitational waves, was a running theme. It was argued that, in terms of technology, next-generation gravitational-wave detectors such as the Einstein Telescope are essentially “accelerators without beams” and that CERN’s expertise in vacuum and cryogenics would help to make such facilities a reality. Inputs from the astroparticle– and nuclear-physics communities, in addition to dedicated perspectives from Asia and the Americas, brought into sharp focus the global nature of modern high-energy physics and the need for greater coordination at all levels.
The open symposium of the ESPP update was a moment for physicists to take stock of the field’s status and future. The community rose to the occasion, aware that the decisions ahead will impact generations of physicists yet to be born. A week of high-quality presentations and focused discussions proved how far things have moved on since the previous strategy update concluded in 2013. Discussions illuminated both the immensity of efforts to evaluate the physics reach of the HL-LHC and future colliders, and the major task faced by the European Strategy Group (ESG) in plotting a path to the future. It is clear that new thinking, from basic theory to instrumentation, computing, analysis and global organisation, is required to sustain progress in the field.
No decisions were taken in Granada, stresses Abramowicz. “During the open symposium we mainly discussed the science. Now comes the time to assess the capacity of the community to realise the proposed scientific goals,” she says. “The Physics Preparatory Group is preparing the briefing book, which will summarise the scientific aspirations of the community, including the physics case for them.”
The briefing book is expected to be completed in September. The ESG drafting session will take place on 20–24 January 2020 in Bad Honnef, Germany, and the update of the ESPP is due to be completed and approved by CERN Council in May 2020.
Dieter Renker, who made some key contributions to the design and construction of the CMS experiment at the LHC, passed away on 16 March after a short illness. Dieter was born in Bavaria and studied physics in Munich and Berlin. He obtained his PhD from the Ludwig Maximilian University in Munich, based on experiments performed at SIN, now the Paul Scherrer Institute (PSI), in Villigen, Switzerland. In 1982 he joined SIN as a staff physicist, where he remained until his retirement at the end of 2009.
At SIN/PSI he participated in many experiments, providing excellent technical support, as well as designing new beamlines at the accelerator there. His technical aptitude in due course turned to detector development, which led to his greatest achievement. In the early days of CMS there were various ideas for the design of the electromagnetic calorimeter. Among these was the use of lead tungstate crystals, which although having many suitable properties for operation at the LHC, have a relatively small scintillation-light yield. Dieter contributed the key measurements which showed that avalanche photodiodes (APDs), with their key properties of internal gain and insensitivity to shower leakage, could be used to read out the crystals. This led to lead-tungstate crystals being adopted by CMS for the design of the calorimeter. Not only did they provide superb energy resolution for electrons and photons, enabling key discoveries such as the Higgs boson in 2012, but they also enabled a more compact detector with significantly reduced overall cost.
The development of the final APD was carried out over a period of many years by Hamamatsu Photonics (Japan), but under the close guidance of Dieter. Nearly 100 different APD prototypes were tested before the technology was deemed fit to be used in CMS. The size, capacitance, speed and, above all, radiation tolerance were the key parameters that needed to be improved, and the final choice was made very close to the deadline for commencing construction of the calorimeter. A complex multi- step screening process involving gamma irradiation and annealing also needed to be developed to ensure that the APDs installed met the demanding reliability requirements of CMS. Until now there has been no recorded failure of any of the 122,000 APDs installed in CMS.
Later, Dieter turned his attention to Geiger-mode APDs, which are now widely used in particle and astroparticle physics, as well as in PET scanners. Together with researchers at ETH Zurich, he started the development of the first camera based on these novel photo sensors for Cherenkov telescopes to measure very high-energy gamma rays from astrophysical sources. This camera was installed at the FACT telescope, located in La Palma, Spain, where the HEGRA experiment had also been operated with Dieter’s active participation. The FACT telescope has now been operating successfully for more than seven years, without any sensor-related problems.
After his retirement Dieter returned to his spiritual home, Munich, where he continued his work at the Technical University.
Dieter was a curious physicist with an exceptional talent for novel detector concepts. He pursued new ideas with a strong focus on achieving his goals. He had a very open mind, and was willing to advise and assist colleagues with great patience and good humour. In his free time his interests included classical music and cooking as well as searching the woods for unusual edible mushrooms. Many colleagues and visitors have fond memories of invitations to his home, embellished with fine cooking.
His sudden illness was a shock to many. Dieter leaves behind his partner, Ulrike.
Nikhef particle physicist and prominent member of the ATLAS experiment at CERN, Olga Igonkina, passed away on 19 May in Amsterdam at the age of 45.
Olya, as she was known to most of us, was born in 1973 in Moscow. Her father was an engineer, her mother a biological scientist. At age 14 she went to a special school for children talented in mathematics and in 1991 started her studies in physics at the Moscow Institute for Physics and Technology. Two years later Olya moved to the ITEP institute to specialise in particle physics, working at the ARGUS experiment and later the HERA-B experiment at DESY.
Olya wrote her dissertation about J/ψ production in HERA-B, with Mikhail Danilov as her supervisor. In 2002 she moved to BaBar at SLAC as a postdoc with the University of Oregon in the group of Jim Brau, where she worked on searches for lepton-flavour-violating tau decays and became convener of the BaBar tau working group. In 2006 she moved to CERN to spearhead Oregon’s new ATLAS group. Her work in ATLAS concentrated on the trigger, where she contributed to many activities with great ideas and enthusiasm, in particular as the trigger-menu coordinator during the startup of the LHC, and later on physics with tau leptons. She began her appointment at Nikhef in 2008 and in 2015 became a professor at Radboud University in Nijmegen.
For her efforts on the ATLAS trigger, Olya was given an ATLAS outstanding achievement award in 2018. Physics-wise, her passion was lepton flavour violation, in particular in tau decays. Intrigued by the hints of lepton-flavour violation in B decays reported by the LHCb experiment and B factories, and always on the lookout for a niche in a large collaboration, in 2018 Olya moved some of her efforts from tau to B physics. She took responsibility for the B-hadron triggers with the aim of collecting an even larger sample of B decays in ATLAS for the final year of Run 2. She was working on preparations for an RK measurement until her very last days.
Besides being a talented scientist, Olya was a dedicated teacher. She supervised an impressive number of PhD students and was very successful in obtaining research grants. She was also very active in outreach activities, with masterclasses and open days at Nikhef, and in community building at ATLAS. Recently she organised the 15th International Workshop on Tau Lepton Physics conference in Amsterdam.
Olya was a passionate physicist who was bursting with ideas. Among several tributes from her colleagues, Olya was described as a future experiment leader. She had a memorably strong work ethos, and until the very last moment refused to let her illness affect her work. She was always cheerful and always positive. Her attitude to work and life will remain a source of inspiration to many of us.
Olya leaves behind her husband, Wouter Hulsbergen of Nikhef, and two children.
Vacuum technology for particle accelerators has been pioneered by CERN since its early days. The Intersecting Storage Rings (ISR) brought the most important breakthroughs. Half a century ago, this technological marvel – the world’s first hadron collider – required proton beams of unprecedented intensity and extremely low vacuum pressures in the interaction areas (below 10–11 mbar). Addressing the former challenge led to innovative surface treatments such as glow-discharge cleaning, while the low-vacuum requirement drove the development of materials and their treatments. It also led to novel high-performance cryogenic pumps and vacuum gauges that are still in use today, and CERN’s record for the lowest ever achieved pressure at room temperature (2 × 10–14 mbar) still stands.
The Large Electron Positron (LEP) collider opened a new chapter in CERN’s vacuum story. Even though LEP’s residual gas density and current intensities were less demanding than those of the ISR, its exceptional length and intense synchrotron-light power triggered the need for unconventional solutions at reasonable cost. Responding to this challenge, the LEP vacuum team developed extruded aluminium vacuum chambers and introduced, for the first time, linear pumping by non-evaporable getter (NEG) strips. In parallel, LEP project leader Emilio Picasso launched another fruitful development that led to the production of the first superconducting radio-frequency cavities based on niobium thin-film coating on copper substrates. It was a great success, and the present accelerating RF cavities of the LHC and HIE-ISOLDE are essentially based on the expertise assimilated for LEP.
The coexistence at CERN of both NEG and thin-film expertise was the seed for another breakthrough in vacuum technology: NEG thin-film coatings, driven by the requirements of the LHC and its project leader Lyn Evans. The NEG material, a micron-thick coating made of a mixture of titanium, zirconium and vanadium, is deposited onto the inner wall of vacuum chambers and, after activation by heating in the accelerator, provides pumping for most of the gas species present in accelerators. The Low Energy Ion Ring was the first CERN accelerator to implement extensive NEG coating in around 2006. For the LHC, one of the technology’s key benefits is its low secondary-electron emission, which suppresses the growth of electron clouds in the room-temperature part of the machine.
New concepts
Synchrotron radiation-induced desorption and electron clouds at temperatures of around 4.3 K had to be studied in depth for the LHC, leading CERN’s vacuum experts to develop new concepts for vacuum systems at cryogenic temperatures, in particular the beam screen. The more intense beams of the high-luminosity LHC (HL-LHC) upgrade, presently under way, will amplify the effect of electron clouds on both the beam stability and the thermal load to the cryogenic systems. Since NEG coatings are limited for room-temperature beam pipes, an alternative strategy had to be found for the parts of the accelerators that cannot be heated, such as those in the HL-LHC’s inner triplet magnets.
Following an idea that originated at CERN in 2006, thin-film coatings made from carbon offer a solution and the material has already been deposited on tens of SPS vacuum chambers within the LHC Injectors Upgrade project. Another idea to fight electron clouds for the HL-LHC involves laser-treating surfaces to make them more rough, so that secondary electrons are intercepted by the surrounding surfaces. In collaboration with UK researchers and GE Inspection Robotics, CERN’s vacuum team has recently developed a miniature robot that can direct the laser onto the LHC beam screen (see image above). The possibility of in situ surface treatments by lasers opens new perspectives for vacuum technology in the next decades, including studies for future circular colliders. Another benefit of this study is the development of small robots for the in situ inspection of long ultra-high vacuum beam pipes, such as those of the LHC’s arcs.
The Compact Linear Collider (CLIC) project, which envisages a high-energy linear electron–positron collider at CERN, demands quadrupole magnets with a very small-diameter beam pipe (about 8 mm) supporting pressures in the ultra-high vacuum range. This can be obtained by NEG-coating the vacuum vessel, but the coating process in such a high aspect-
ratio geometry is not easy due to the very small space available for the material source and the plasma needed for its sputtering. This troublesome issue has been solved by a complete change of the production process in which the NEG material is no longer directly coated on the wall of the tiny pipe, but instead is coated on the external wall of a sacrificial mandrel made of high-purity aluminium.
Next-generation synchrotron-light sources share CLIC’s need for very compact magnets with magnetic poles as close as possible to the beam, so as to reduce costs and improve beam performance. CERN’s vacuum group collaborates closely with vacuum experts of light sources, MAX IV in Sweden and PSI in Switzerland being prominent examples, to develop the required very-small-diameter vacuum pipes. Further technology transfer has come from the sophisticated simulations necessary for the HL-LHC and the Future Circular Collider study, which have also found applications beyond the accelerator field, from the coating of electronic devices to space simulation.
Relations with industry are key to the operation of CERN’s accelerators, especially those in the LHC chain. The vacuum industry across CERN’s member countries provides us with state-of-art components, valves, pumps, gauges and control equipment that have contributed to the high reliability of the lab’s vacuum systems. In return, the LHC gives high visibility to industrial products. Indeed, the variety of projects and activities performed at CERN provide us with a continuous stimulation to improve and extend our competences in vacuum technology. In addition to future colliders are: antimatter physics, which requires very low gas density; radioactive-beam physics, which imposes severe controls on contamination and gas exhausting; and gravity-wave physics for which the tradeoff between cost and performance of vacuum systems is essential for the approval of next-generation observatories.
An orthogonal driver of innovation in vacuum technology is the reduction of costs and operational downtime of CERN’s accelerators. Achieving ultra-high vacuum in a matter of a few hours at a reduced cost would also have an impact well beyond the high-energy physics community. This and other challenges involved in fundamental exploration are guaranteed to drive further advances in vacuum technology.
The open symposium of the European Strategy for Particle Physics (ESPP) update, which drew to a close last week in Granada, Spain, was a moment for physicists to take stock of their field’s status and future. A week of high-quality presentations and focused discussions proved how far things have moved on since the previous strategy update concluded in 2013. In the past few years the LHC has proved the existence of the Higgs boson and so far suggested that there are no new particles beyond the SM at the electroweak scale. Spectacular progress has been made with neutrinos, dark-matter searches, flavour and electroweak physics, and gravitational-wave astronomy is beginning to take off. The deepest puzzles of the standard models of particle physics and cosmology remain at large, however, and large colliders are one of the best tools to address them.
Recommendations from the ESPP are due early next year. Dominating discussions at the open symposium last week was which project should succeed the LHC after its operations cease in the 2030s. The decision has significant consequences for the next generation of particle physicists, not just in Europe but internationally. Perspectives from Asia and the Americas, in addition to national views and inputs from the astroparticle– and nuclear-physics communities, brought into sharp focus the global nature of modern high-energy physics and the need for greater coordination at all levels.
The 130 or so talks and discussion sessions in Granada revealed a community united in its desire for a post-LHC collider, but less so in its choice of that collider’s form. Enormous efforts have gone into weighing up the physics reach of the various projects under study, a task complicated by the complexity of future accelerator technologies, detectors and analyses. Stimulating some heated exchanges, the ESPP saw the International Linear Collider (ILC) in Japan, a Compact Linear Collider (CLIC) or future circular electron–positron collider (FCC-ee) at CERN and a Circular Electron Positron Collider in China (CEPC) pitted against each other and against expectations from the high-luminosity LHC in terms of their potential in key areas such as Higgs physics.
Summary sessions
Summing up the situation for beyond-SM (BSM) physics, Gian Giudice of CERN said that the remaining BSM-physics space is “huge”, and pointed to four big questions for colliders: to what extent can we tell whether the Higgs is fundamental or composite? Are there new interactions or new particles around or above the electroweak scale? What cases of thermal relic WIMPs are still unprobed and can be fully covered by future collider searches? And to what extent can current or future accelerators probe feebly interacting sectors?
Neutrinos, the least well known of all the SM particles, were the subject of numerous presentations. The ESPP audience was reminded that neutrino masses, as established by neutrino oscillations, are the first particle-physics evidence for BSM phenomena. A vibrant programme is under way to fully measure the neutrino mixing matrix and in particular the neutrino mass ordering and CP violation phase. Other experiments are probing the neutrino’s absolute mass scale and testing whether they are of a Dirac or Majorana nature. Along with gravitational waves, neutrinos play a powerful role in multimesseneger astronomy.
Flavour physics is crucial for BSM searches since it is potentially sensitive to effects at scales as high as 105 TeV, said Antonio Zoccoli of INFN in his summary. There is also much complementarity between low-energy physics, the high-energy frontier and searches for feebly interacting particles, he said. Oddities in b-decays seen by the LHCb collaboration are of particular interest. “Flavour is a major legacy of LHC,” Zoccoli concluded. “Charged hadron particle-ID should be mandatory for a full physics programme at future colliders.”
Summarising ESPP sessions on dark-matter and dark-sector physics, Shoji Asai of the University of Tokyo drew attention to a shift in sociology that is taking place. In the old view, dark-matter solutions arose as a byproduct of “top-down” approaches (such as supersymmetry) to solve the SM’s problems. The “new sociology” holds that dark matter needs an explanation of its own, and it’s to be considered a bonus if such a solution also elucidates important issues such as the strong-CP problem or baryogenesis. Among the “big questions” identified in this sector at the ESPP update were: What are the main differences between light hidden-sector dark matter and WIMPs? How broad is the parameter space for the QCD axion? How do we compare the results of different experiments in a more model-independent way? And how will direct and indirect dark-matter detection experiments inform/guide accelerator searches and vice versa? Asai said that consensus has emerged on the need for more coordination and support between accelerator-based direct detection and indirect detection dark-sector searches, as exemplified by the new European Center for AstroParticle Theory.
In summarising interests in the strong sector, Jorgen D’Hondt of Vrije Universiteit Brussel listed the many dedicated experiments in this area and the open questions identified at the ESPP symposium: “What are the experimental and theoretical prerequisites to reach an adequate precision of perturbative and non-perturbative QCD predictions at the highest energies? What can be learned from beams-on-target experiments at current and potential future accelerators? How to probe the quark–gluon plasma equation of state and to establish whether there is a first-order phase transition at high baryon density? What is known about the make-up of the proton (mass, radius, spin, etc) and how to extract it? And what is the role of strong interactions at very low and very high (up to astrophysical) energies?”
Electroweak sparks
Of all the scientific themes of the week, electroweak physics generated the most lively discussions, especially concerning how well the Higgs boson’s couplings to fermions, gauge bosons and to itself can be probed at current and future colliders. Summary speaker Beate Heinemann of DESY cautioned that such quantitative estimates should be treated with a degree of flexibility at this time, though a few things stand out: one is the impressive estimated performance from the HL-LHC in the next 15 or so years; another is that a long-term physics programme based on successive machines in a 100 km-circumference tunnel offers the largest overall physics reach on the Higgs boson and other key parameters. The long timescales required to master the technology for the next hadron collider were well noted. There is broad agreement that the next major collider after the LHC should collide electrons and positrons to fully explore the Higgs boson and make precision measurements of other electroweak parameters that are sensitive to phenomena at higher energy scales. Whether that machine is circular or linear, and built in Asia or Europe, are the billion-dollar questions facing the community now.
The closer involvement of particle physics with astroparticle physics, in particular following the discovery of gravitational waves, was the running theme of the open symposium. It was argued that, in terms of technology, next-generation gravitational-wave detectors such as the Einstein Telescope are essentially “accelerators without beams” and that CERN’s expertise in vacuum and cryogenic technologies (a result of the lab’s continual pursuit and execution of big-collider projects) would help to make such facilities a reality.
The closing discussion of the symposium offered a final hour for physicists to air their views, many of which were met with applause. Proponents of circular machines highlighted the high flexibility and exploratory potential of projects such as FCC-ee, pointing out that it would serve as an electroweak as well as a Higgs factory. Linear-minded participants cited factors such as the extendable nature of linacs, and the independence of their tunnels from a subsequent hadron collider. For others, the priority for CERN should be to enter negotiations as soon as possible for a 100 km tunnel in the Geneva region, buying time to decide which physics option should be installed. Warm applause followed a remark that CERN decides for itself what its next project should be, without relying on other labs. But there were reminders from others that high-energy physics is an international field and that, in times of scarce resources, all options should be considered.
The high-energy physics community has risen to the occasion of the ESPP update. New thinking, from basic theory to instrumentation, computing, analysis and global organisation, is clearly required to sustain the recent rate of progress. Now that the open symposium is over, the European Strategy Group (ESG) will start to prepare a briefing book. Further input can be submitted to the strategy secretariat during the next months, and at a special session organised by the European Committee for Future Accelerators on 14 July 2019 during the European Physical Society Conference on High Energy Physics in Ghent, Belgium. An ESG drafting session will take place on 20–24 January 2020 in Bad Honnef, Germany, and the update of the ESPP is due to be completed and approved by the CERN Council in May 2020.
Swiss physicist Hans-Jürg Gerber passed away on 28 August last year. Born in Langnau/Kanton Bern, he studied and did his PhD from 1949 to 1959 at ETH Zurich on “Scattering and polarization effects of 3.27 MeV neutrons on deuterons”. He then worked at the University of Illinois in the US, before joining CERN from 1962 to 1968. There, he carried out experiments at the 28 GeV Proton Synchrotron (PS). He studied high-energy neutrino interactions using a spark chamber, and performed measurements of lepton universality. He also tested time-reversal invariance in the charged decay mode of the Λ hyperon. He was also PS coordinator from 1965 to 1966.
In 1968, Gerber became head of the research department at the Swiss Institute for Nuclear Physics (SIN). He was elected by the Swiss Federal Council to become associate professor of experimental physics in 1970, and in 1977 promoted to full professor. Gerber initiated basic research at SIN and later at the Paul Scherrer Institute (PSI) with his precision experiments on the decay of charged muons – experiments that continue to this day at PSI (see p45). His flair for the fundamental led to the most general determination of the leptonic four-fermion interaction for the normal and inverse muon decay using experimental data, which brought him international recognition.
In the 1980s and 1990s, Hans-Jürg returned to CERN to help set up and operate experiment PS195 (CPLEAR) for studying CP violation using a tagged neutral-kaon beam. The concept of the experiment, which involved tagging the flavour of the neutral kaon at the point of production, was opposite to already operational kaon experiments based on K-short and K-long beams. As a skilled experimenter, he contributed significantly to the success of CPLEAR with unconventional ideas. For example, during a crisis when the liquid-scintillator started to develop air bubbles due to the heat from nearby electronics, he invented a system to remove the air dissolved in the liquid using ultrasound. CPLEAR’s measurements on the violation of time-reversal invariance (T-invariance) and tests of quantum mechanics were the starting point for significant theoretical work he undertook on T, CP and CPT invariance.
While he retired in the spring of 1997 after a long and extremely successful career, he still continued working on particle physics with various publications on the interpretation of the CPLEAR results regarding testing of quantum mechanics, T- and CPT-violation.He was also a contributor to the review of particle physics in the Particle Data Group.
Experiment, theory and teaching formed a unity for Hans-Jürg. This was particularly evident in his lectures, in which he enthusiastically conveyed the joy of physics to his students. We also remember dinners with Hans-Jürg after long working days setting up experiments, where we talked about all possible physics questions.
He is survived by his wife Hildegard, his three children and grandchildren.
The eminent mathematician Michael Atiyah died in Edinburgh on 11 January, aged 89. He was one of the giants of mathematics whose work influenced an enormous range of subjects, including theoretical high-energy physics.
Atiyah’s most notable achievement, with Isadore Singer, is the “index theorem”, which occupied him for more than 20 years and generated results in topology, geometry and number theory using the analysis of elliptic differential operators. In mid-life, he learned that theoretical physicists also made use of the theorem and this opened the door to an interaction between the two disciplines, which he pursued energetically until the end of his life. It led him not only to mathematical results on Yang–Mills equations, but also to encouraging the importation of concepts from quantum field theory into pure mathematics.
Early years
Born of a Lebanese father and a Scottish mother, his early years were spent in English schools in the Middle East. He then followed the natural course for a budding mathematician in that environment by attending the University of Cambridge, where he ended up writing his thesis under William Hodge and becoming a fellow at Trinity College. As a student he had little interest in physics, but went to hear Dirac lecture largely because of his fame. The opportunity then arose to spend a year at the Institute for Advanced Study in Princeton in the US, where he met his future collaborators and close friends Raoul Bott, Fritz Hirzebruch and Singer.
A visit by Singer to the University of Oxford (where Atiyah had recently moved) in 1962 began the actual work on the index theorem, where the Dirac operator would play a fundamental role. This ultimately led to Atiyah being awarded a Fields Medal in 1966 and, with Singer, the Abel Prize in 2004. Over the years, proofs and refinements of the index theorem evolved. Although topology was at the forefront of the first approaches, in the early 1970s techniques using “heat kernels” became more analytic and closer to the calculations that theoretical physicists were performing, especially in the context of anomalies in quantum field theory. In the 1980s, in a proof by Luis Álvarez-Gaumé (who subsequently became a member of the CERN theoretical physics unit for 30 years), Hirzebruch’s polynomials in the Pontryagin classes – which form the topological expression for the index – emerged as a natural consequence of supersymmetry.
Singer visited Oxford again in 1977, this time bringing mathematical questions concerning Yang-Mills theory. Using quite sophisticated algebraic geometry and the novel work of Roger Penrose, this yielded a precise answer to physicists’ questions about instantons, specifically the so-called ADHM (Atiyah, Drinfeld, Hitchin, Manin) construction. That mathematicians and physicists had common ground in a completely new context made a huge impression on Michael, and he was energetic in facilitating this cooperation thereafter. He frequently engaged in correspondence and discussions with Edward Witten, out of which emerged the current fashion in mathematics of topological quantum field theories – beginning with a formalism that described new invariants of knots. Despite the quantum language of this domain, Michael’s mathematical work with a physical interface was more concerned with classical solutions, and the soliton-like behaviour of monopoles and skyrmions.
Founding father
During his life he took on many administrative tasks, including the presidency of the Royal Society and mastership of Trinity College. He was also the founding director of the Isaac Newton Institute for Mathematical Sciences in Cambridge.
With his naturally effervescent personality he possessed, in Singer’s words, “speed, depth, power and energy”. Collaborations were all-important, bouncing ideas around with both mathematicians and physicists. Beauty in mathematics was also a feature he took seriously, as was a respect for the mathematicians and physicists of the past. He even campaigned successfully for a statue of James Maxwell to be erected in Edinburgh, his home city, in later years.
As for the index theorem itself, it is notable that one of the more subtle versions – the “mod 2 index” – played an important role in Kane and Mele’s theoretical prediction of topological insulators. As they wrote in their 2005 paper: “it distinguishes the quantum spin-Hall phase from the simple insulator.” A fitting tribute to an outstanding pure mathematician, whose intuition and technical power revealed so much in so many domains.
Muon colliders have been the topic of much discussion this week during the update of the European Strategy for Particle Physics (ESPP) in Granada. Proposals for such a machine are much less advanced than those for other projects under consideration for a post-LHC collider. A muon collider is therefore not being considered on the same footing as circular and linear electron—positron projects such as FCC-ee, CEPC, ILC and CLIC. Nevertheless, its potential to produce very high-energy collisions that can transfer all energy into the production of new particles in a relatively small facility is proving difficult for physicists to resist.
In one of 160 written inputs to the ESPP update, the Muon Collider Working Group points out that a 14 TeV muon collider provides an effective energy reach similar to that of a 100 TeV FCC proton—proton machine. In addition to its energy reach, the report states, a dedicated muon collider is able to precisely measure the Higgs boson’s mass and width, along with other observables: “A muon collider is therefore ideal to search for and/or study new physics and for resolving narrow resonances both as a precision facility and/or as an exploratory machine”. A 14 TeV muon machine would fit neatly into the tunnel that is currently occupied by the LHC, and in principle the technology could go to much higher energies.
Being some 200 times more massive than electrons, muons suffer significantly less from synchrotron-radiation losses and therefore can be accelerated efficiently in a circular machine. But reaching high luminosities is extremely tough owing to short muon lifetime at rest (2.2 μs) and the difficulty of producing large numbers of muons in suitably shaped bunches.
One way to conquer this problem is to “cool” an initial low-energy muon beam, which has large transverse and longitudinal emittances, by several orders of magnitude in the 6D phase-space and then rapidly accelerate it. Last year, the UK-based Muon Ionisation Cooling Experiment (MICE) demonstrated the principle of ionisation cooling, by observing 4D cooling in a low-flux muon beam. “The results point the way to an exciting programme in which muon beams of high-brightness are exploited to seek new insights into the properties of neutrinos and to explore the energy frontier with multi-TeV lepton-antilepton collisions,” says MICE spokesperson Ken Long of Imperial College, London, who was present at the ESPP symposium. Fermilab in the US also pursued such technologies in its dedicated Muon Accelerator Program (MAP), while an experiment at J-PARC in Japan last year accelerated muons by a radio-frequency accelerator for the first time.
Recently, physicists in Italy and France proposed an alternative concept for a muon collider called the Low Emittance Muon Accelerator (LEMMA), which offers a naturally cooled muon beam with a long lifetime in the laboratory frame by capturing muon–antimuon pairs created in electron–positron annihilation.
The Muon Collider Working Group, which was established by CERN in 2017, has performed a first, high-level review of these two muon collider schemes. “The focus has been on the positron-based scheme, which it was really promising but it has been found to require consolidation,” said Daniel Schulte of CERN, reporting the group’s findings in Granada this week. The group recommends that an international collaboration “promote muon colliders and organise the effort on the development of both accelerators and detectors and to define the road-map towards a CDR by the next ESPP update”. The production of muon neutrinos with a well-known flux and energy spectrum would also serve as the source for a neutrino factory.
Given the broad spectrum of views expressed in Granada this week, both during the sessions and in the sidelines, the path to a muon collider could be bumpy. “At the Higgs pole, it’s not competitive with any of the other machines, although the perspective for Higgs physics at a very high-energy muon collider is actually very good,” pointed out one participant during a discussion session about Higgs and electroweak physics. “It’s total fantasy!” said another. But the view from the expert working group is more positive. Can muon colliders at this moment be considered for the next project? “Not yet,” said Schulte, but enormous progress has been made and it is clearly worthwhile to continue R&D. “A muon collider may be the best option for very high lepton collider energies beyond 3 TeV, and has strong synergies with other projects such as magnet and RF development,” concluded Schulte. “We should not miss this opportunity.”
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