Breaking records at EPS-HEP

5 October 2021

In this year’s unusual Olympic summer, high-energy physicists pushed back the frontiers of knowledge and broke many records. The first one is surely the number of registrants to the EPS-HEP conference, hosted online from 26 to 30 July by the University of Hamburg and DESY: nearly 2000 participants scrutinised more than 600 talks and 280 posters. After 18 months of the COVID pandemic, the community showed a strong desire to meet and discuss physics with international colleagues. 

200 trillion b-quarks, 40 billion electroweak bosons, 300 million top quarks and 10 million Higgs bosons

The conference offered the opportunity to hear about analyses using the full LHC Run-2 data set, which is the richest hadron-collision data sample ever recorded. The results are breathtaking. As my CERN colleague Michelangelo Mangano explained recently to summer students, “The LHC works and is more powerful than expected, the experiments work and are more precise than expected, and the Standard Model works beautifully and is more reliable than expected.” About 3000 papers have been published by the LHC collaborations in the past decade. They have established the LHC as a truly multi-messenger endeavour, not so much because of the multitude of elementary particles produced – 200 trillion b-quarks, 40 billion electroweak bosons, 300 million top quarks and 10 million Higgs bosons – but because of the diversity of scientifically independent experiments that historically would have required different detectors and facilities, built and operated by different communities. “Data first” should always remain the leitmotif of the natural sciences. 

Paula Alvarez Cartelle (Cambridge) reminded us that the LHC has revealed new states of matter, with LHCb confirming that four or even five quarks can assemble themselves into new long-lived bound states, stabilised by the presence of two charm quarks. For theorists, these new quark-molecules provide valuable input data to tune their lattice simulations and to refine their understanding of the non-perturbative dynamics of strong interactions.

Theoretical tours de force

While Run 1 was a time for inclusive measurements, a multitude of differential measurements were performed during Run 2. Paolo Azzurri (INFN Pisa) reviewed the transverse momentum distribution of the jets produced in association with electroweak gauge bosons. These offer a way to test quantum chromodynamics and electroweak predictions at the highest achievable precision through higher-order computations, resummation and matching to parton showers. The work is fuelled by remarkable theoretical tours de force reported by Jonas Lindert (Sussex) and Lorenzo Tancredi (Oxford), which build on advanced mathematical techniques, including inspiring new mathematical developments in algebraic geometry and finite-field arithmetic. We experienced a historic moment: the LHC definitively became a precision machine, achieving measurements reaching and even surpassing LEP’s precision. This new situation also induced a shift more towards precision measurements, model-independent interpretations and Standard Model (SM) compatibility checks, and away from model-dependent searches for new physics. Effective-field-theory analyses are therefore gaining popularity, explained Veronica Sanz (Valencia and Sussex).

We know for certain that the SM is not the ultimate theory of nature. How and when the first cracks will be revealed is the big question that motivates future collider design studies. The enduring and compelling “B anomalies” reported by LHCb could well be the revolutionary surprise that challenges our current understanding of the structure of matter. The ratios of the decay widths of B mesons, either through charged or neutral currents, b→cℓν and b→sℓ+, could finally reveal that the electron, muon and tau lepton differ by more than just their masses.

The statistical significance of the lepton flavour anomalies is growing, reported Franz Muheim (Edinburgh and CERN), creating “cautious” excitement and stimulating the creativity of theorists like Ana Teixeira (Clermont-Ferrand), who builds new physics models with leptoquarks and heavy vectors with different couplings to the three families of leptons, to accommodate the apparent lepton-flavour-universality violations. Belle II should soon bring new additional input to the debate, said Carsten Niebuhr (DESY).

Long-awaited results

The other excitement of the year came from the long-awaited results from the muon g-2 experiment at Fermilab, presented by Alex Keshavarzi (Manchester). The spin precession frequency of a sample of 10 billion muons was measured with a precision of a few hundred parts per million, confirming the deviation from the SM prediction observed nearly 20 years ago by the E821 experiment at Brookhaven. With the current statistics, the deviation now amounts to 4.2σ. With an increase by a factor 20 of the dataset foreseen in the next run, the measurement will soon become systematics limited. Gilberto Colangelo (Bern) also discussed new and improved lattice computations of the hadronic vacuum polarisation, significantly reducing the discrepancy between the theoretical prediction and the experimental measurement. The jury is still out – and the final word might come from the g-2/EDM experiment at J-PARC.

Accelerator-based experiments might not be the place to prove the SM wrong. Astrophysical and cosmological observations have already taught us that SM matter only constitutes around 5% of the stuff that the universe is made of. The traditional idea that the gap in the energy budget of the universe is filled by new TeV-scale particles that stabilise the electroweak scale under radiative corrections is fading away. And a huge range of possible dark-matter scales opens up a rich and reinvigorated experimental programme that can profit from original techniques exploiting electron and nuclear recoils caused by the scattering of dark-matter particles. A front-runner in the new dark-matter landscape is the QCD axion originally introduced to explain why strong interactions do not distinguish matter from antimatter. Babette Döbrich (CERN) discussed the challenges inherent in capturing an axion, and described the many new experiments around the globe designed to overcome them.

Progress could also come directly from theory

Progress could also come directly from theory. Juan Maldacena (IAS Princeton) recalled the remarkable breakthroughs on the black-hole information problem. The Higgs discovery in 2012 established the non-trivial vacuum structure of space–time. We are now on our way to understanding the quantum mechanics of this space–time.

Like at the Olympics, where breaking records requires a lot of work and effort by the athletes, their teams and society, the quest to understand nature relies on the enthusiasm and the determination of physicists and their funding agencies. What we have learnt so far has allowed us to formulate precise and profound questions. We now need to create opportunities to answer them and to move ahead.

One cannot underestimate how quickly the landscape of physics can change, whether the B-anomalies will be confirmed or whether a dark-matter particle will be discovered. Let’s see what will be awaiting us at the next EPS-HEP conference in 2023 in Hamburg – in person this time!

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