Originally set to take place in Prague, the International Conference of High Energy Physics (ICHEP) took place virtually from 28 July to 6 August. Running a major biennial meeting virtually was always going to be extremely difficult, but the local organisers rose to the challenge by embracing technologies such as Zoom and YouTube. To allow global participation, the conference was spread over eight days rather than the usual six, with presentations compressed into five-hour slots that were streamed twice: first as a live “premiere” and later as recorded “replay” sessions, for the benefit of participants in different time zones.
This was the first ICHEP meeting since the publication of the update of the European strategy for particle physics, which presented an ambitious vision for the future of CERN. Though VIP-guest Peter Gabriel – rock star and human rights advocate – may not have been aware of this when delivering his opening remarks, his urging that delegates speak up for science and engage with politicians resonated with the physicists virtually present.
Many scientific highlights were covered at ICHEP and it is only possible to scratch the surface here. The results from all four major LHC experiments were particularly impressive and the collective progress in understanding the properties of neutrinos shows no sign of slowing down.
ATLAS and CMS presented the first evidence for the decay of the Higgs boson into a pair of muons. Combined, the results provide strong evidence for the coupling of the Higgs boson to the muon, with the strength of the coupling consistent with that predicted in the Standard Model. Prior to these new results, the Higgs had only been observed to couple to the much heavier third-generation fermions and the W and Z gauge bosons. The measurements also provide further evidence for the linearity of the Higgs coupling, now over four orders of magnitude (from the muon to top quark), indicating the universality of the Standard-Model Higgs boson as the mechanism through which all Standard Model particles acquire mass. These are highly non-trivial statements.
ATLAS also presented a combined measurement of the Higgs signal strength, which describes a common scaling of the expected Higgs-boson yields in all processes, of 1.06 ± 0.07. In this measurement, the experimental and theoretical uncertainties are now roughly equal, emphasising the ever-increasing importance of theoretical developments in keeping up with the experimental progress; a feature that will ultimately determine the precision that will be reached by the LHC and high-luminosity LHC (HL-LHC) Higgs physics programmes.
The range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive
More generally, the precision we are seeing from the ATLAS and CMS Run 2 proton–proton data is truly impressive, and an exciting indication of what is to come as the integrated luminosity accumulated by the experiments ramps up, and then ramps up again in the HL-LHC era. One interesting new example was the first observation of WW production from photon–photon collisions, where the photons are radiated from the incoming proton beams. This is a neat measurement that demonstrates the breadth of physics accessible at the LHC.
Overall, the range of Standard Model measurements presented at ICHEP 2020 by ATLAS and CMS was truly impressive and we should not forget that it is still relatively early in the LHC programme. In parallel, direct searches for new phenomena, such as supersymmetry and the “unexpected”, continues apace. Results from direct searches at the energy frontier were covered in numerous parallel session presentations. The current status was summarised succinctly by Paris Sphicas (Athens) in his conference summary talk: “Looked for a lot of possible new things. Nothing has turned up yet. Still looking intensively.”
Over the last few years, a number of deviations from theoretical predictions have been observed in B-meson decays to final states with leptons. Discrepancies have been observed in ratios of decays to different lepton species, and in the angular distribution of decay products. Taken alone, each of these discrepancies are not particularly significant, but collectively they may be telling us something new about nature. At ICHEP 2020, the LHCb experiment presented their recently published results on the angular analysis in B0 → K*0 μ+μ–. The overall picture remains unchanged. The full analysis of the LHCb Run-2 data set, including updated measurements of the relative rates of the muon and electron decay modes (RK and RK*), is eagerly awaited.
The search for rare kaon decays continues to attract interest
The search for rare kaon decays continues to attract interest. One of the most impressive results presented at ICHEP was the recent observation by NA62 of the extremely rare kaon decay, K+ → π+νν̄. Occurring only once in every 10 billion decays, this is an incredibly challenging measurement and the new NA62 result is the first statistically significant observation of this decay, based on just 17 events. Whilst the observed rate is consistent with the Standard Model expectation, its observation opens up a new future avenue for exploring the possible effects of new physics.
Neutrino physics continues to be one of the most active areas of research in particle physics, so it was not surprising that the neutrino parallel sessions were the best attended of the conference. This is a particularly interesting time, with long-baseline neutrino oscillation experiments becoming sensitive to the neutrino mass ordering, and beginning to provide constraints on CP violation. Updates were presented by the NOvA experiment in the USA and the T2K experiment in Japan. Both experiments favour the normal-ordering hypothesis, although not definitively, and there is currently a slight tension between the CP violation results from the two experiments. It is worth noting that the combined interpretation of the two experiments is quite complex. The NOvA and T2K collaborations are working on a combined analysis to clarify the situation.
There were also a number of presentations on the next generation of long-baseline neutrino oscillation experiments, DUNE in the US and Hyper-Kamiokande in Japan, which aim to make the definitive discovery of CP violation in the neutrino sector. In the context of DUNE, the progress with liquid-argon time-projection- chamber (LArTPC) detector technology is impressive. It was particularly pleasing to see a number of physics results from MicroBooNE at Fermilab, and the single-phase DUNE detector prototype at CERN (ProtoDUNE-SP), that are based on the automatic reconstruction of LArTPC images – a longstanding challenge.
A vast range of high-qualify scientific research was covered in the 800 parallel session presentations and summarised in the 44 plenary talks at ICHEP 2020. The quality of the presentations was high, and speakers coped well with the challenge of pre-recording talks. The “replay” sessions worked extremely well too – an innovation that is likely to persist in the post-COVID world. About 3000 people registered for the meeting, which is more than double the previous two events. It was particularly pleasing to learn that almost 2500 connected to the parallel sessions.
Despite the many successes, we all missed the opportunity to meet colleagues in person; it is often the informal discussions over coffee or in restaurants and bars that generate new ideas and, importantly, lead to new collaborations. Whilst virtual conferences are likely to remain a feature in the post- COVID world, they will not replace in-person events.