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.
