LHC physicists spill the beans in Boston

5 July 2024
The 12th LHCP conference in Boston
Boston collisions The 12th LHCP conference took place at Northeastern University in Boston. Credit: T Orimoto

Dedicated solely to LHC physics, the LHCP conference is a vital gathering for experts in the field. The 12th edition was no exception, attracting 450 physicists to Northeastern University in Boston from 3 to 7 June. Participants discussed recent results, data taking at a significantly increased instantaneous luminosity in Run 3, and progress on detector upgrades planned for the high-luminosity LHC (HL-LHC).

The study of the Higgs boson remains central to the LHC programme. ATLAS reported a new result on Standard Model (SM) Higgs-boson production with decays to tau leptons, achieving the most precise single-channel measurement of the vector-boson-fusion production mode to date. Determining the production modes of the Higgs boson precisely may shed light on the existence of new physics that would be observed as deviations from the SM predictions.

Beyond single Higgs production, the di-Higgs production (HH) search is one of the most exciting and fundamental topics for LHC physics in the coming years as it directly probes the Higgs potential (see “Homing in on the Higgs self-interaction“). ATLAS has combined results for HH production in multiple final states, providing the best-expected sensitivity to the HH production cross-section and Higgs-boson self-coupling, allowing κλ (the Higgs self-coupling with respect to the SM value) to be within the range –1.2 < κλ< 7.2.

The search for beyond-the-SM (BSM) physics to explain the many unresolved questions about our universe is being conducted with innovative ideas and methods. CMS has presented new searches involving signatures with two tau leptons, examining the hypotheses of an excited tau lepton and a heavy neutral spin-1 gauge boson (Z) produced via Drell-Yan and, for the first time, via vector boson fusion. These results set stringent constraints on BSM models with enhanced couplings to third-generation fermions.

Other new-physics theoretical models propose additional BSM Higgs bosons. ATLAS presented a search for such particles being produced in association with top quarks, setting limits on their cross-section that significantly improve upon previous ATLAS  results. Additional BSM Higgs bosons could explain puzzles such as dark matter, neutrino oscillations and the observed matter–antimatter asymmetry in the universe.

The dark side

Some BSM models imply that dark-matter particles could arise as composite mesons or baryons of a new strongly-coupled theory that is an extension of the SM. ATLAS investigated this dark sector through searches for high-multiplicity hadronic final states, providing the first direct collider constraints on this model to complement direct dark-matter-detection experimental results.

CMS have used low-pileup inelastic proton–proton collisions to measure event-shape variables related to the overall distribution of charged particles. These measurements showed the particle distribution to be more isotropic than predicted by theoretical models.

LHCP conference talk

The LHC experiments also presented multiple analyses of proton–lead (p–Pb) and pp collisions, exploring the potential production of quark–gluon plasma (QGP) – a hot and dense phase of deconfined quarks and gluons found in the early universe that is frequently studied in heavy-ion Pb–Pb collisions, among others, at the LHC. Whether it can be created in smaller collision systems is still inconclusive.

ALICE reported a high-precision measurement of the elliptic flow of anti-helium-3 in QGP using the first Run-3 Pb–Pb run. The much larger data sample compared to the previous Run 2 measurement allowed ALICE to distinguish production models for these rarely produced particles for the first time. ALICE also reported the first measurement of an impact-parameter-dependent angular anisotropy in the decay of coherently photo-produced ρ0 mesons in ultra-peripheral Pb–Pb collisions. In these collisions, quantum interference effects cause a decay asymmetry that is inversely proportional to the impact parameter.

CMS reported its first measurement of the complete set of optimised CP-averaged observables from the process B0 K*0μ+μ. These measurements are significant because they could reveal indirect signs of new physics or subtle effects induced by low-energy strong interactions. By matching the current best experimental precision, CMS contributes to the ongoing investigation of this process.

LHCb presented measurements of the local and non-local contributions across the full invariant-mass spectrum of B0* K*0μ+μ, tests of lepton flavour universality in semileptonic b decays, and mixing and CP violation in D  Kπ decays.

The future of the field was discussed in a well-attended panel session, which emphasised exploring the full potential of the HL-LHC and engaging younger generations

From a theoretical perspective, progress in precision calculations has exceeded expectations. Many processes are now known to next-to-next-to-leading order or even next-to-next-to-next-to-leading order (N3LO) accuracy. The first parton distribution functions approximating N3LO accuracy have been released and reported at LHCP, and modern parton showers have set new standards in perturbative accuracy.

In addition to these advances, several new ideas and observables are being proposed. Jet substructure, for instance, is becoming a precision science and valuable tool due to its excellent theoretical properties. Effective field theory (EFT) methods are continuously refined and automated, serving as crucial bridges to new theories as many ultraviolet theories share the same EFT operators. Synergies between flavour physics, electroweak effects and high-transverse-momentum processes at colliders are particularly evident within this framework. The use of the LHC as a photon collider showcases the extraordinary versatility of LHC experiments and their synergy with theoretical advancements.

Discovery machine

The HL-LHC upgrade was thoroughly discussed, with several speakers highlighting the importance and uniqueness of its physics programme. This includes fundamental insights into the Higgs potential, vector-boson scattering, and precise measurements of the Higgs boson and other SM parameters. Thanks to the endless efforts by the four collaborations to improve their performances, the LHC already rivals historic lepton colliders for electroweak precision in many channels, despite the cleaner signatures of lepton collisions. The HL-LHC will be capable of providing extraordinarily precise measurements while also serving as a discovery machine for many years to come.

The future of the field was discussed in a well-attended panel session, which emphasised exploring the full potential of the HL-LHC and engaging younger generations. Preserving the unique expertise and knowledge cultivated within the CERN community is imperative. Next year’s LHCP conference will be held at National Taiwan University in Taipei from 5 to 10 June.

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