Moriond’s electroweak delights

5 July 2024
Moriond 2024

Packed sessions, more than 100 talks and lively discussions at Rencontres de Moriond electroweak, held from 24 to 31 March in La Thuile, Italy, captured the latest thinking in the field. The Standard Model (SM) emerged intact, while new paths of enquiry were illuminated.

Twelve years after the discovery of the Higgs boson, H, a wide variety of analyses by ATLAS and CMS are bringing the new scalar into sharper focus. This includes its mass, for which CMS has reported the most precise single measurement using the H  ZZ → 4ℓ channel: 125.04 ± 0.11 (stat) ± 0.05 (syst) GeV. A Run 2 legacy mass measurement combining ATLAS and CMS results is under way, while projections for the HL-LHC indicate that an uncertainty at the 10–20 MeV level is attainable. For the H width, which is potentially highly sensitive to new physics but notoriously difficult to measure at a hadron collider, the experiments constrain its value to be less than three times the SM width at 95% confidence level using an indirect method with reasonable assumptions. A precision of about 20% is expected from the full HL-LHC dataset.

New generation

The measured H cross sections in all channels continue to support the simplest incarnation of the SM H sector, with a new result from CMS testing the bbH production mode in the ττ and WW channels. Now that the H couplings to the most massive particles are well established, the focus is moving to the second-generation fermions. Directly probing the shape of the Brout–Englert–Higgs potential, and sensitive to new-physics contributions, the H self-coupling is another key target. HH production has yet to be observed at the LHC due to its very low cross section (the combined ATLAS and CMS limit is currently 2.5–3 times the SM value), but an extensive measurement programme utilising multiple channels is under way and Moriond saw new results presented based on HH → bbbb and HH → γγττ decays (see “Homing in on the Higgs self-interaction“).

Searches for exotic H decays, or for additional low-mass scalar bosons as predicted by two-Higgs-doublet extensions to the SM, were a Moriond highlight. A wide scope of new H-boson (a, A) searches have been released by ATLAS and CMS, including a new search for H → aa → muons by CMS in the mass range 0.2–60 GeV and, on the higher mass side, new limits on H/A → tt by ATLAS and A → ZH → ℓℓ tt by CMS. Although none show significant deviations from the SM, most of the searches are statistically limited and there remains a large amount of phase space available for extended H sectors. Generating much conversation in the corridors was a new-physics interpretation of ATLAS and CMS data in terms of a Higgs-triplet model, based on results  in the HH → γγ channel and top-quark differential distributions.

The LHC experiments are making stunning progress in precision electroweak measurements, as exemplified by a new measurement by CMS of the effective leptonic electroweak mixing angle sin2θeff= 0.23157 ± 0.00031, the first LHC measurement of the W-boson width by ATLAS, and precise measurements of the W and Z cross sections at 13.6 TeV. ATLAS announced at Moriond the most precise single-experiment test of lepton-flavour universality in comparisons between W-boson decays to muons and electrons. A wide-ranging presentation of electroweak results based on two-photon collisions at the LHC described recent attempts by CMS to extract the anomalous magnetic moment of the tau lepton. And LHCb showcased its capabilities in providing an independent measurement of the W-boson mass and the Z-boson cross section. Participants heard about the increasing relevance of lattice QCD in precision electroweak measurements, for example in determining the running of alpha and the weak mixing angle. A tension between the predictions from lattice QCD and from more traditional dispersive approaches exists, with a similar origin to that for the anomalous magnetic moment of the muon.

Following the recent observation of entanglement in top-quark pairs by ATLAS and CMS, a presentation addressing the intriguing ability of colliders to carry out fundamental tests of quantum mechanics generated much discussion. Offering full access to spin information, collider experiments can study quantum correlations, wavefunction collapse and decoherence at unprecedented energies, possibly enabling a Bell measurement at the HL-LHC and the first observation of toponium.

Seeking signals from beyond

Searches for long-lived particles by ATLAS, CMS and LHCb – including the first at LHC Run 3 by CMS – were high on the Moriond agenda. Heavy gauge and scalar bosons, left–right gauge boson masses and heavy neutral leptons are among other new-physics scenarios being constrained. Casting the net as wide as possible, the LHC experiments are developing AI anomaly-detection algorithms, while the power of effective field theory (EFT) in parameterising the effect of heavy new particles on LHC measurements continues to grow via a diverse range of analyses. Even at O(6) in the SMEFT, no fewer than 59 Wilson coefficients, each related to different underlying physics processes, need to be to measured.

Neutrinoless double-beta decay, which would be an unambiguous sign of new physics, continues to be hunted by a host of experiments

Tensions between theory and experiment remain in some processes involving b → s or b → c quark transitions. Moriond saw much discussion on such processes, including new results from Belle II on the branching ratio of the highly suppressed decay B → Kνν. Participants heard about the need for theory progress, as has been the case recently with impressive calculations of b → sγ. Predictions for b → sμμ – which show a tension with experiment and that are independent of the R(K) parameters clocking the relative rates of B → +μ and B → Ke+e – are excellent ways to probe new physics. Concerning b → c transitions, updates on R(D*) from Belle II and on R(D*) and R(D) from LHCb based on the muonic decay of the tau lepton take the world-average tension to 3.17σ. The stability of the SM prediction of R(D*) was also questioned.

New flavours

The flavour sector is awash with new results. LHCb presented fresh analyses exploring mixing and CP violation in the charm sector – a unique gateway to the flavour structure of up-type quarks – while CMS presented a new measurement of CP violation in Bs→ J/ψ K+K decays. In ultra-rare kaon decays, KOTO presented a new upper limit on the branching ratio of K0L→ πνν (< 2 × 10–9 at 90% confidence level) and projects a sensitivity < 10–13 with the proposed KOTO II upgrade. NA62 presented a preliminary measurement of the branching ratio of the very rare decay π0→ e+e (5.86 ± 0.37 × 10–8), in agreement with the SM, and results for K+→ πγγ, the latter offering the first evidence that second-order terms must be included in chiral perturbation theory. Belle and Belle II showed new radiative and electroweak penguin results concerning processes such as B0→ γγ, and BESIII presented a precise measurement of the CKM matrix element Vcs. A sweeping theory perspective on the mysterious flavour structure of the SM introduced participants to “flavour modular symmetries” – a promising new game in town for a potential theory of flavour based on modular forms, which are well known in mathematics and were used in the proof of Fermat’s last theorem.

The final sessions of Moriond electro­weak turned to neutrinos, dark matter and astroparticle physics. KATRIN is soon to release an update on the neutrino mass limit based on six times more data, with an expected uncertainty of mν < 0.5 eV, and is undertaking R&D towards a proposed upgrade (KATRIN++) that would use new technology to push the mass limit down further. The collaboration is also stepping up its search for new physics via high-precision spectroscopy and is working towards an upgrade called TRISTAN that will soon zone in on the sterile neutrino hypothesis.

Rencontre at Moriond

In Japan, the T2K facility has undergone an extensive renewal period including its first operation with the near-detector ND280 upgrade in August 2023, which increased the acceptance. Designed to explore neutrino mass ordering and leptonic CP violation, T2K data so far show a slight preference for the “normal” mass ordering while admitting a CP-conserving phase at the level of 2σ. However, a joint analysis between T2K and NOvA, a neutrino oscillation experiment in the US with a longer baseline and complementary sensitivity, prefers a more degenerate parameter space where either CP conservation or the inverted ordering are acceptable solutions. The combined data place a strong constraint on Δm32.

Neutrinoless double-beta decay (NDBD), which would reveal the neutrino to be a Majorana particle and be an unambiguous sign of new physics, continues to be hunted by a host of experiments. LEGEND-200’s first physics data was shown, setting up an ultimate goal of placing a lower limit on the NDBD half-life of 1028 years for 76Ge. Also located at Gran Sasso, CUORE, which has been collecting data since 2019, will operate for one more year before an upgrade is planned. In parallel, designs for a next-generation tonne-scale upgrade, CUPID, are being finalised. Neutrino aficionados were also treated to scotogenic three-loop models, in which neutrinos gain a Dirac mass term from radiative corrections, and to the latest results from FASER at the LHC, including the first emulsion-detector measurements of the νe and νμ cross sections at TeV energies, and a search for axion-like particles.

IceCube, which studies resonant disappearance of antineutrinos due to matter effects, showed intriguing results that delve into new-physics territory. Adding sterile neutrinos improves global fits by 7σ, participants heard, but brings inconsistencies too. Generating much interest, the global p-value for the null hypothesis of the sterile neutrino in the muon disappearance channel is 3.1%, in tension with MINOS. The Deep Core IceCube upgrade will increase the number of strings in the observatory, while the more significant Gen-2 upgrade will expand its overall area. A theory overview of the status of sterile neutrinos, taking into account recent results from MiniBooNE, MicroBooNE, PROSPECT, STEREO, GALEX, SAGE, BEST and others, concluded that experimental evidence for such a fourth neutrino state is fading but not excluded. The so-called reactor anomaly is probably explained by smaller uranium contribution than previously accounted for, while the upgraded Neutrino-4 experiment will shed light on tensions with PROSPECT and STEREO.

Cosmological constraints

The status of dark photons was also reviewed. Constraints are being placed from many sources, including colliders, astrophysical and cosmological bounds, haloscopes, and most recently radio telescopes, the James Webb Space Telescope and beam-dump experiments. PandaX-4T, which seeks to constrain WIMP dark matter and NDBD, is about to restart data-taking. LZ, another large liquid-xenon detector, has placed record limits on dark matter based on its first 60 days of data-taking. Results from the first observing run of a novel kind of laser-interferometric detector, LIDA, to observe axion-like particles in the galactic halo are promising.

No particle-physics conference would be complete without the anomalous magnetic moment of the muon

The latest supersymmetry and dark-matter searches at ATLAS and CMS were also presented, including a new result on R-parity violating supersymmetry and fresh limits on the chargino mass. BESIII reported on exotic searches for massive dark photons, muon-philic particles, glueballs and the QCD axion. Searches for axion-like particles are multiplying in many shapes and forms. In terms of flavour probes of axions, the strongest bounds come from NA62. Less conventionally, probing ultralight dark matter by searching for oscillatory behaviour in gravitational waves is gaining traction. Recent NanoGrav data show no signs of such a signal.

All eyes on the muon

No contemporary particle-physics conference would be complete without the anomalous magnetic moment of the muon – a powerful quantity that takes into account all known and unknown particles, for which the measured value is in significant tension with the SM prediction. As the Fermilab Muon g-2 experiment continues to improve the experimental precision (currently 0.2 ppm), all eyes are on how the SM calculation is performed – specifically the systematic uncertainty associated with a process called hadronic vacuum polarisation. A huge amount of work is going into understanding this quantity, both in terms of the calculational machinery and underlying data used. When computed using lattice QCD, the tension between experiment and theory is significantly reduced. However, the calculations are so complex that few groups have been able to execute them. That is set to change this year, Moriond participants heard, as new lattice calculations are unblinded ahead of the Lattice 2024 meeting in August, followed by a decision on whether to include such results in the official SM prediction at the seventh plenary workshop of the Muon g-2 Theory Initiative at KEK in September.

Experimentally and theoretically, all tools are being thrown at the SM in an attempt to find an explanation for dark matter, the cosmological baryon asymmetry, neutrino masses and other outstanding mysteries. The many high-quality talks at this year’s Moriond electroweak session, including an impressive batch of flash talks in dedicated young-researcher sessions, covered all aspects of the adventure and set the standard for future analyses. An incredible interplay between astrophysical, cosmological, collider and other experimental measurements is rapidly eating into the available parameter space for new physics. Ten years ago, the Moriond theory-summary speaker remarked “new physics must be around the corner, but we see no corner”. While the same could be said today, physicists have a much clearer view of the road ahead.

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