The first direct image of a black hole, obtained by the Event Horizon Telescope (EHT) collaboration earlier this year, has been recognized by the 2020 Breakthrough Prize in Fundamental Physics. The $3 million prize will be shared equally between 347 researchers who were co-authors of the six papers published by the EHT collaboration on 10 April.
The EHT is a network of eight radio dishes in Antarctica, Chile, Mexico, Hawaii, Arizona and Spain that creates an Earth-sized interferometer. Its ultra-high angular resolution images of radio emission from a supermassive black hole at the heart of galaxy M87* opened a new window on black holes and other phenomena. Recently, a team at Brookhaven National Laboratory used the EHT image to disfavour “fuzzy” models of ultra-light boson dark matter.
Also announced were six New Horizons Prizes worth $100,000 each, which recognize early-career achievements in physics and mathematics. In physics, Jo Dunkley (Princeton); Samaya Nissanke (University of Amsterdam) and Kendrick Smith (Perimeter Institute) were awarded for the development of novel techniques to extract fundamental physics from astronomical data. Simon Caron-Huot (McGill University) and Pedro Vieira (Perimeter Institute) were recognized for their “profound contributions to the understanding of quantum field theory”.
The Breakthrough Prize was founded in 2012 by former physicist and entrepreneur Yuri Milner, with sponsors including Google’s Sergey Brin and Facebook’s Mark Zuckerberg. In August, a Special Breakthrough Prize in Fundamental physics was awarded to Sergio Ferrara, Daniel Freedman and Peter van Nieuwenhuizen for the discovery of supergravity.
All prize recipients, along winners in mathematics and biology, will receive their awards at a ceremony in California on 3 November.
Hooman Davoudiasl and Peter Denton of Brookhaven National Laboratory have used the recent Event Horizon Telescope image of supermassive black hole M87* to disfavour “fuzzy” models of ultra-light boson dark matter with masses of the order of a few 10-21 eV (Phys. Rev. Lett. 123 021102). The inferred mass, spin and age of the black hole are incompatible with the existence of such fuzzy dark matter given the principle of superradiance, whereby quantum fluctuations deplete the angular momentum of a rotating black hole by populating a cloud of bosons around it. The effect depends only on the bosons’ mass, and does not presuppose any non-gravitational interactions. Future measurements of M87* and other spinning supermassive black holes have the potential to exclude the entire parameter space for fuzzy dark matter.
An intriguing alternative to cold dark matter, fuzzy dark matter could address the “core-cusp problem”, wherein observations of an approximately constant dark matter density in the inner parts of galaxies conflict with the steep power-law-like behaviour of cosmological simulations. The particles’ long de Broglie wavelengths, of the order of a kiloparsec, would suppress structure at this scale.
On 10 July, CERN and the Astroparticle Physics European Consortium (APPEC) founded a new research centre for astroparticle physics theory called EuCAPT. Led by an international steering committee comprising 12 theorists from institutes in France, Portugal, Spain, Sweden, Germany, the Netherlands, Italy, Switzerland and the UK, and from CERN, EuCAPT aims to coordinate and promote theoretical physics in the fields of astroparticle physics and cosmology in Europe.
Astroparticle physics is undergoing a phase of profound transformation, explains inaugural EuCAPT director Gianfranco Bertone, who is spokesperson of the Centre for Gravitation and Astroparticle Physics at the University of Amsterdam. “We have recently obtained extraordinary results such as the discovery of high-energy cosmic neutrinos with IceCube, the direct detection of gravitational waves with LIGO and Virgo, and we have witnessed the birth of multi-messenger astrophysics. Yet we have formidable challenges ahead of us: understanding the nature of dark matter and dark energy, elucidating the origin of cosmic rays, understanding the matter-antimatter asymmetry problem, and so on. These are highly interdisciplinary problems that have ramifications in cosmology, particle, and astroparticle physics, and that are best addressed by a strong and diverse community of scientists.”
The construction of experimental astroparticle facilities is coordinated by APPEC, but until now there was no Europe-wide coordination of theoretical activities, says Bertone. “We want to be open and inclusive, and we hope that all interested scientists will feel welcome to join this new initiative.” On a practical level, EuCAPT aims to coordinate scientific and training activities, help researchers attract adequate resources for their projects, and promote a stimulating and open environment in which young scientists can thrive. CERN will act as the central hub of the consortium for the first five years.
It is not a coincidence that CERN has been chosen as the central hub of EuCAPT, says Gian Giudice, head of CERN’s theory department. “The research that we are doing at CERN-TH is an exploration of the possible links between physics at the smallest and largest scales. Creating a collaborative network among European research centres in astroparticle physics and cosmology will boost activities in these fields and foster dialogue with particle physics,” he says. “Dark matter, dark energy, inflation and the origin of large-scale structures are big questions regarding the universe. But there are good hints that suggest that their explanation has to be looked for in the domain of particle physics.”
Strengthening connections between particle physics and related disciplines, CERN signed a collaboration agreement with the European Space Agency (ESA) on 11 July to address the challenges of operating equipment in harsh radiation environments. Such environments are found in both particle-physics facilities and outer space, and the agreement identifies several high-priority projects, including: high-energy electron tests; high-penetration heavy-ion tests; assessment of commercial components and modules; radiation-hard and radiation-tolerant components and modules; radiation detectors, monitors and dosimeters; and simulation tools for radiation effects. Important preliminary results have already been achieved in some areas, including high-energy electron tests of electronics for the Jupiter Icy Moons Explorer (JUICE) mission performed at CERN’s CLEAR/VESPER facility.
The Bs meson is a bound state of a strange quark and a beauty antiquark – as such it possesses both beauty and strangeness. For many years the search for its extremely rare decay to a μ+μ– pair was a holy grail of particle physics, because of its sensitivity to theories that extend the Standard Model (SM). The SM predicts the decay rate for Bs→μ+μ– to be only about 3.6 parts per billion (ppb). Its lighter cousin, the B0, which is made from a down quark and a beauty antiquark, has an even lower predicted branching fraction for decays to a μ+μ– pair of 0.1 ppb. If beyond-the-SM particles exist, however, the predictions could be modified by their presence, giving the decays sensitivity to new physics that rivals and might even exceed that of direct searches.
It took more than a quarter of a century of extensive effort to establish Bs→μ+μ–, and the first observation was presented in 2013, in a joint publication by the CMS and LHCb collaborations based on LHC Run 1 data. The same paper reported evidence for B0→μ+μ– with a significance of three standard deviations, however, this signal has not subsequently been confirmed by CMS, LHCb or ATLAS analyses. A new CMS Run 2 analysis now looks set to bolster interest in these intriguing decays.
The CMS collaboration has updated its 2013 analysis with higher centre-of-mass-energy Run 2 data from 2016, permitting an observation of Bs→μ+μ– with a significance of 5.6 standard deviations (figure 1). The results are consistent with the latest results from ATLAS and LHCb, and while no significant deviation from the SM is observed by any of the experiments, all three decay rates are found to lie slightly below the SM prediction. The slight deficit is not significant, but the trend is intriguing because it could be related to so-called flavour anomalies recently observed by the LHCb experiment in other rare decays of B mesons (CERN Courier May/June p9). This makes the new CMS measurement even more exciting. The new analysis showed no sign of B0→μ+μ–, and a stringent 95% confidence limit of less than 0.36 ppb was set on its rate.
CMS also managed to measure the effective lifetime of the Bs meson using the several dozen Bs→μ+μ– decay events that were observed. The interest in measuring this lifetime is that, just as for the branching fraction, new physics might alter its value from the SM expectation. This measurement yielded a lifetime of about 1.7 ps, consistent with the SM. The measured CMS value is also consistent with the only other such lifetime measurement, performed by LHCb.
With three times more Run 2 data yet to be analysed by CMS, the next update – based on the full Run 1 and Run 2 datasets – may shed more light on this fascinating corner of physics, and move us closer to the ultimate goal, which is the observation of the B0→μ+μ– decays.
The LHC completed its Run 2 operations in December 2018, delivering a large dataset of proton–proton collisions at a centre-of-mass energy of 13 TeV. The ATLAS detector maintained a high level of readiness and performance throughout Run 2, resulting in 139 fb–1 of data for physics analyses.
An increasingly consistent picture of the properties of the Higgs boson is being drawn in light of the Run 2 data. This is thanks to a wide range of measurements, and particularly through the establishment of its couplings with third-generation quarks following the observation of the H → bb decay and associated ttH production.
The H → γγ and H → ZZ* → 4ℓ final states, where 4ℓ denotes 4e, 2e2μ or 4μ, provide clean experimental signatures that played a leading role in the discovery of the Higgs boson, and are ideal for precision measurements that could reveal subtle effects from new physics. ATLAS presented updated results for these two channels using the full Run 2 dataset at the 2019 summer conferences.
Using improved identification and energy calibration of leptons, photons and jets, and new analysis techniques, a sample of about 210 H → ZZ* → 4ℓ signal events (figure 1) and 6550 H → γγ signal events were selected to perform a series of measurements. The properties of the Higgs boson are investigated by measuring inclusive, differential and per-production-mode cross sections that are sensitive to different modelling aspects.
In the 4ℓ channel, differential cross-section measurements are performed as a function of the transverse momentum of the Higgs boson and the number of jets produced in association with it. The different production mechanisms of the Higgs boson are measured inclusively and in various regions of kinematic phase space, which are cleanly separated by neural networks.
In the high-statistics γγ channel, differential cross sections are measured for a set of variables related to the Higgs boson kinematics, as well as the kinematics and multiplicity of jets produced in association with the Higgs boson. The measured distributions are used to constrain modified interactions of the Higgs boson with SM particles.
The measurements in both channels are found to be well described by the SM predictions. Their combination yields a total Higgs-production cross section of 55.4 ± 4.3 pb, in agreement with the SM prediction of 55.6 ± 2.5 pb. The combined measurement of the transverse-momentum differential cross section (figure 2) has significantly improved in precision compared to earlier results. It is sensitive to the virtual processes governing the dominant Higgs-boson production through gluon fusion and to direct contributions from new physics.
Achieving 8% precision on the Higgs cross section is a significant step towards studying the electroweak symmetry breaking mechanism. Numerous additional measurements are being pursued by ATLAS in the Higgs-boson sector with the full Run 2 dataset to perform detailed tests of SM predictions and hunt for new phenomena.
The Humboldt Kolleg conference Discoveries and Open Puzzles in Particle Physics and Gravitation took place at Kitzbühel in the Austrian Alps from 24 to 28 June, bringing Humboldt prize winners, professors and research-fellow alumni together with prospective future fellows. The meeting was sponsored by the Humboldt Foundation, based in Bonn, whose mission is to promote cooperation between scientists in Germany and elsewhere. The programme focused on connections between particle physics and the large-scale cosmological structure of the universe.
The most recent LHC experimental results were presented by Karl Jakobs (Freiburg and ATLAS spokesperson), confirming the status of the Standard Model (SM). A key discussion topic raised by Fred Jegerlehner (DESY-Zeuthen) is whether the SM’s symmetries might be “emergent” at the relatively low energies of current experiments: in contrast to unification models that exhibit maximal symmetry at the highest energies, the gauge symmetries could emerge in the infrared, but “dissolve” in the extreme ultraviolet. Consider the analogy of a carpet: it looks flat and invariant under translations when viewed from a distance, but this smoothness dissolves when we look at it close up, e.g. as perceived by an ant crawling on it. A critical system close to the Planck scale – the scale where quantum-gravity effects should be important – could behave similarly: the only modes that can exist as long-range correlations, e.g. light-mass particles, self-organise into multiplets with a small number of particles, just as they do in the SM. The vector modes become the gauge bosons of U(1), SU(2) and SU(3); low-energy symmetries such as baryon- and lepton-number conservation would all be violated close to the Planck scale.
Ideas connecting particle physics and quantum computing were also discussed by Peter Zoller (Innsbruck) and Erez Zohar (MPQ, Munich). Here, one takes a lattice field theory that is theoretically difficult to solve and maps it onto a fully controllable quantum system such as an optical lattice that can be programmed in experiments to do calculations – a quantum simulator. First promising results with up to 20 qubits have been obtained for the Schwinger model (QED in 1+1 dimensions). This model exhibits dynamical mass generation and is a first prototype before looking at more complicated theories like QCD.
The cosmological constant is related to the vacuum energy density, which is in turn connected to possible phase transitions in the early universe.
A key puzzle concerns the hierarchies of scales: the small ratio of the Higgs-boson mass to the Planck scale plus the very small cosmological constant that drives the accelerating expansion of the universe. Might these be related? The cosmological constant is related to the vacuum energy density, which is in turn connected to possible phase transitions in the early universe. Future gravitational-wave experiments with LISA were discussed by Stefano Vitale (Trento) and are expected to be sensitive to the effects of these phase transitions.
A main purpose of Humboldt Kolleg is the promotion of young scientists from the central European region. Student poster prizes sponsored by the Kitzbühel mayor Klaus Winkler were awarded to Janina Krzysiak (IFJ PAN, Krakow) and Jui-Lin Kuo (HEPHY, Vienna).
Almost 750 high-energy physicists met from 10–17 July in Ghent, Belgium, for the 2019 edition of EPS-HEP. The full scope of the field was put under a microscope by more than 500 parallel and plenary talks and a vibrant poster session. The ongoing update of the European Strategy for Particle Physics (ESPP) was a strong focus, and the conference began with a session jointly organised by the European Committee for Future Accelerators to seek further input from the community ahead of the publication of the ESPP briefing book in September.
The accepted view, explained ESPP secretary Halina Abramowicz, is that an electron–positron collider should succeed the Large Hadron Collider (LHC). The question is whether to build a linear collider that is extendable to higher energies, or a circular collider whose infrastructure could later be reused for a hadron collider. DESY’s Christophe Grojean weighed up the merits of a Large Electron Positron collider (LEP)-style Z-pole run at a high-luminosity circular machine – a “tera-Z factory” – against the advantages of the polarised beams proposed at linear facilities, and questioned the value of polarisation to measurements of the Higgs boson at energies above 250 GeV. Furthermore, he said, sensitivities should be evaluated in light of the expected performance of the high-luminosity LHC (HL-LHC).
Blue skies required
Presentations on accelerator and detector challenges emphasised the importance of sharing development between competing projects: while detector technology for an electron–positron machine could begin production within about five years, proposed hadron colliders require a technological leap in both radiation hardness and readout speed. CERN’s Ariella Cattai expressed concern for excessive utilitarianism in detector development, with only 5% of R&D being blue-sky despite the historical success of this approach in developing TPC, RICH and silicon strip detectors, among others. She also pointed out that although 80% of R&D specialists believe their work has potential social outcomes, less than a third feel adequately supported to engage in technology transfer. Delegates agreed on the need for more recognition for those who undertake this crucial work. CERN’s Graeme Stewart highlighted the similar plight of theorists developing event generators, whose work is often not adequately rewarded or supported. The field also needs to keep pace with computing developments outside the field, he said, by designing data models and code that are optimised for graphics-processing units rather than CPUs (central-processing units).
The accepted view is that an electron–positron collider should succeed the LHC
The beginning of the main EPS conference was dominated by impressive new results from ATLAS and CMS, as they begin to probe Higgs couplings to second-generation fermions, and as the experiments continue to search for new phenomena and rare processes. Several speakers noted that the LHC even has the potential to exceed LEP in precision electroweak physics: although the hadronic environment increases systematic uncertainties, deviations arising from beyond-Standard Model (SM) phenomena are expected to scale with the centre-of-mass energy squared. Giulia Zanderighi of the Max Planck Institute and Claude Duhr of CERN also highlighted the need to improve the precision of theoretical calculations if they are to match experimental precision by the end of the HL-LHC’s run, showcasing work to extend next-to-next-to-leading order (NNLO) calculations to two-to-three processes, and the latest moves to N3LO calculations.
The flavour-physics scene was updated with new SM-consistent constraints from Belle on the ratios R(D) and R(D*), somewhat lessening the suggestion of lepton-universality violation in B-meson decays. With the advent of Belle II, and the impending analysis of LHCb’s full Run 2 dataset, the flavour anomalies will surely soon be confirmed or resolved. LHCb also presented new measurements of the gamma angle of the unitarity triangle, which show a mild 2σ tension between the values obtained from B+ and Bs0 decays. Meanwhile, long-baseline neutrino-oscillation experiments provided tantalising information on leptonic CP violation, with T2K data excluding CP conservation at 2σ irrespective of the neutrino mass hierarchy, and NOVA disfavouring an inverted hierarchy of neutrino mass eigenstates at 1.9σ.
Background checks
A refrain common to both collider and non-collider searches for dark-matter candidates was the need to eliminate backgrounds. A succession of talks scaled the 90 orders of magnitude in mass that dark-matter candidates might occupy. CERN’s Kfir Blum explained that: “The problem with gravity is that it doesn’t matter if you’re a neutrino or a rhinoceros – if you sit on a geodesic you’re going to move in the same way,” making it difficult to infer the nature of dark matter with cosmological arguments. Nevertheless, he reported work on the recent black-hole image from the Event Horizon Telescope, which excludes some models of ultra-light dark matter. Above this, helioscopes such as CAST continue to encroach on the parameter space of QCD axions, while more novel haloscopes cut thin swathes down to low couplings in the 20 orders of magnitude of mass explored by searches for axion-like particles. Meanwhile, searches for WIMPs are sensitive to masses just beyond this, from 1 to 1000 GeV/c2. Carlos de los Heros of Uppsala University explained that experiments such as XENON1t are pushing close to the so-called neutrino floor, and advocated for the development of directional detection methods that can distinguish solar neutrinos from WIMPs, and plunge into what is rather a neutrino “swamp”.
An exciting synergy between heavy-ion physics and gravitational waves was in evidence, with the two disparate approaches both now able to probe the equation of state of nuclear matter. Particular emphasis was placed on the need to marry the successful hydrodynamical and statistical description of ion–ion collisions with that used to describe proton–proton collisions, especially in the tricky proton-ion regime. These efforts are already bearing fruit in jet modelling. On the cosmological side, speakers reflected on the enduring success of the ΛCDM model to describe the universe in just six parameters, with François Bouchet of the Institut d’Astrophysique de Paris declaring that “the magic of the cosmic microwave background is not dead”, and explaining that Planck data have ruled out several models of inflation. Interdisciplinarity was also on display in reports on multi-messenger astronomy, with particular excitement reserved for the proposed European-led Einstein Telescope gravitational-wave observatory, which Marek Kowalski of DESY reported will most likely be built in either Italy or the Netherlands, and that will boast 10-times better sensitivity than current instruments.
This year’s EPS prize ceremony rewarded the CDF and D0 collaborations for the discovery of the top quark, and the WMAP and Planck collaborations for their outstanding contributions to astroparticle physics and cosmology. Today’s challenges are arguably even greater, and the spirit of EPS-HEP 2019 was to reject a false equivalence between physics being “new” and being beyond the SM. Participants’ hunger for the technological innovation required to answer the many remaining open questions was matched by an openness to reconsider theoretical thinking on fine tuning and naturalness, and how these principles inform the further exploration of the field.
EPS-HEP 2021 will take place in Hamburg from 21–28 July.
More than 400 researchers convened in Brussels from 24 to 28 June for the annual meeting of the Future Circular Collider (FCC) study. In addition to innovations in superconductivity, high-field magnets, superconducting radio-frequency systems and civil-engineering studies, discussions sought to clarify issues surrounding the physics research topics that FCC can address.
The meeting also marked the final event of the Horizon 2020 EuroCirCol project – a European Union project to produce a conceptual design study for a post-LHC research infrastructure based on an energy-frontier 100 TeV circular hadron collider. Since June 2015 the project has produced a wealth of results in high-tech domains via the collaborative efforts of partners in Europe and other countries such as the US, Japan, Korea and Russia. These include impressive progress toward 16 T magnets and in the performance of superconducting wires. Breakthroughs in both fields, such as a first accelerator-type magnet exceeding 14 T (see Advanced dipole sets high-field record) and an increase in the critical current density of Nb3Sn wire, promise to significantly reduce the costs of exploring the high-energy frontier and could find practical applications outside particle physics.
The four-volume FCC conceptual design report was also presented. Authored by 1350 people from 150 institutes, the report “underlines the global attractiveness of the FCC and documents the far-reaching benefits that the project can have for Europe and future generations,” said Frédérick Bordry, CERN director for accelerators and technologies.
A wide range of talks focused on a future circular lepton collider (FCC-ee) as the first step of the FCC programme, followed by an energy-frontier proton collider (FCC-hh). Results testify to the technological readiness of the FCC-ee, which could be operational by the end of the 2030s and therefore allow time to develop the novel technologies required for a 100 TeV proton–proton collider.
In his keynote talk, Nima Arkani- Hamed of the Institute for Advanced Study highlighted the importance of scrutinising the Higgs boson at a post-LHC machine. Speakers also stressed the complementarity between the different FCC options in searching for dark-matter candidate particles and other new physics. Finally, the potential for studying the strong interaction with heavy-ion collisions, and detailing parton distribution functions with a proton–electron interaction point, were demonstrated.
The sustainability of research infrastructures and the assessment of their societal impact were other highlights of FCC week 2019, as discussed at a special “Economics of Science” workshop. Experts from the field of economics shared lessons learned with representatives from CERN and other research organisations, including SKA, ESA and ESS, demonstrating the many benefits beyond physics that major international projects bring.
The XVIII International Conference on Strangeness in Quark Matter (SQM 2019) was held from 10 to 15 June in Bari, Italy. With 270 delegates from 32 countries, the largest participation ever for the SQM series, the conference focused on the role of strange and heavy-flavour quarks in heavy-ion collisions and astrophysics. The scientific programme consisted of 50 invited plenary talks, 76 contributed parallel talks and a rich poster session with more than 60 contributions.
A state-of-the-art session opened the conference, also including a tribute to the late Roy Glauber entitled “The Glauber model in high-energy nucleus–nucleus collisions”. Subsequent sessions were dedicated to highlights from theory and experiment, and included reports on results from low- and high-energy collisions, as well as on hyperon interactions in lattice QCD and thermal models. Representatives from all major collaborations at CERN’s LHC and SPS, Brookhaven’s RHIC, the Heavy Ion Synchrotron SIS at the GSI Darmstadt and the NICA project at the JINR Dubna made special efforts to release new results at SQM 2019.
Among the highlights were reports that particle-yield measurements are close to determining where phenomena such as strangeness enhancement are localised in phase space. Collective behaviour in small systems was also a much-discussed topic, with new results from the PHENIX experiment showing that p-Au, d-Au and 3He-Au collisions exhibit elliptic flow coefficients consistent with expectations regarding their initial collision geometry. Results from ALICE, CMS and STAR consistently corroborate the presence of elliptic flow in small systems.
There is also increasing interest in transverse-momentum differential baryon-to-meson ratios in the heavy-flavour sector. Recent results from pp and Pb-Pb collisions from both ALICE and CMS suggest that the same dynamics observed in the ratio Λ/K0S may be present in Λc/D, despite the fact that strange and charm quarks are thought to be created in different stages of the system’s evolution. Further studies and future measurements may be needed.
A promising new perspective for the LHC data is to use high-energy pp and p-Pb collisions as factories of identified hadrons created by a source of finite radius and then to measure the ensuing interactions between these hadrons using femtoscopy. This technique has allowed the ALICE collaboration to study interactions that were so far not measured at all and probe, for instance, the p-Ξ and p-Ω interaction potentials. These results provide fundamental constraints to the QCD community and are significant in the context of the astrophysics.
New results on the onset of deconfinement were shown by the NA61/SHINE collaboration. First results on strangeness production at low energy from HADES and BM@N also enriched the discussion at SQM 2019.
Presentations at the final session showed good prospects for future measurements at FAIR (GSI Darmstadt), NICA (JINR Dubna), the Heavy-Ion Project (J-PARC), and at CERN, givenongoing detector upgrades, the high-luminosity programme, and possible next-generation colliders. Perspectives for QCD measurements at future electron–ion colliders were also presented. On the theory side, new developments and strong research efforts are bringing a better understanding of strangeness production and open heavy-flavour dynamics in heavy-ion collisions.
Young scientist prizes sponsored by the Nuclear Physics European Collaboration Committee were awarded to Bong-Hwi Lim of Pusan National University, Korea, and to Olga Soloveva of Goethe University, Frankfurt for their poster contributions. The inaugural Andre Mischke Award (established at SQM2019) for the young scientist with the best experimental parallel talk was given to Erin Frances Gauger of the University of Texas, Austin.
The next edition of SQM will take place in Busan, Korea, in May 2021.
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