Standard Model stands strong at Moriond

8 May 2019

The 66th Rencontres de Moriond, held in La Thuile, Italy, took place from 16 to 30 March, with the first week devoted to electroweak interactions and unified theories, and a second week to QCD and high-energy interactions. More than 200 physicists took part, presenting new results from precision Standard Model (SM) measurements to new exotic quark states, flavour physics and the dark sector.

A major theme of the electroweak session was flavour physics, and the star of the show was LHCb’s observation of CP violation in charm decays (see LHCb observes CP violation in charm decays). The collaboration showed several other new results concerning charm- and B-meson decays. One much anticipated result was an update on RK, the ratio of rare decays of a B+ to electrons and muons, using data taken at energies of 7, 8 and 13 TeV. These decays are predicted to occur at the same rate to within 1%; previous data collected are consistent with this prediction but favour a lower value, and the latest LHCb results continue to support this picture. Together with other measurements, these results paint an intriguing picture of possible new physics (p33) that was explored in several talks by theorists.

Run-2 results

The LHC experiments presented many new results based on data collected during Run 2. ATLAS and CMS have measured most of the Higgs boson’s main production and decay modes with high statistical significance and carried out searches for new, additional Higgs bosons. From a combination of all Higgs-boson measurements, ATLAS obtained new constraints on the important Higgs self-coupling, while CMS presented updated results on the Higgs decay to two Z bosons and its coupling to top quarks.

Precision SM studies continued with first evidence from ATLAS for the simultaneous production of three W or Z bosons, and CMS presented first evidence for the production of two W bosons in two simultaneous interactions between colliding partons. The very large new dataset has also allowed ATLAS and CMS to expand their searches for new physics, setting stronger lower limits on the allowed mass ranges of supersymmetric and other hypothetical particles (see Boosting searches for fourth-generation quarks and Pushing the limits on supersymmetry). These also include new limits from CMS on the parameters describing slowly moving heavy particles, and constraints from both collaborations on the production rate of Z bosons. ATLAS, using the results of lead–ion collisions taken in 2018, also reported the observation of light-by-light scattering – a very rare process that is forbidden by classical electrodynamics.

New results and prospects in the neutrino sector were communicated, including Daya Bay and the reactor antineutrino flux anomaly, searches for neutrinoless double-beta decay, and the reach of T2K and NOvA in tackling the neutrino mass hierarchy and leptonic CP violation. Dark matter, axions and cosmology also featured prominently. New results from experiments such as XENON1T, ABRACADABRA, SuperCDMS and ATLAS and CMS illustrate the power of multi-prong dark-matter searches – not just for WIMPs but also very light or exotic candidates. Cosmologist Lisa Randall gave a broad-reaching talk about “post-modern cosmology”, in which she argued that – as in particle physics – the easy times are probably over and that astronomers need to look at more subtle effects to break the impasse.

Moriond electroweak also introduced a new session: “feeble interactions”, which was designed to reflect the growing interest in very weak processes at the LHC and future experiments.

LHCb continued to enjoy the limelight during Moriond’s QCD session, announcing the discovery of a new five-quark hadron, named Pc(4312)+, which decays to a proton and a J/ψ and is a lighter companion of the pentaquark structures revealed by LHCb in 2015 (p15). The result is expected to motivate deeper studies of the structure of these and other exotic hadrons. Another powerful way to delve into the depths of QCD, addressed during the second week of the conference, is via the Bc meson family. Following the observation of the Bc(2S) by ATLAS in 2014, CMS reported the existence of a two-peak feature in data corresponding to the Bc(2S) and the Bc*(2S) – supported by new results from LHCb based on its full 2011–2018 data sample. Independent measurements of CP violation in the Bs system reported by ATLAS and LHCb during the electroweak session were also combined to yield the most precise measurement yet, which is consistent with the small value predicted by the SM.

A charmed life

In the heavy-ion arena, ALICE highlighted its observation that baryons containing charm quarks are produced more often in proton–proton collisions than in electron–positron collisions. Initial measurements in lead–lead collisions suggest an even higher production rate for charmed baryons, similar to what has been observed for strange baryons. These results indicate that the presence of quarks in the colliding beams affects the hadron production rate. The collaboration also presented the first measurement of the triangle-shaped flow of J/ψ particles in lead–lead collisions, showing that even heavy quarks are affected by the quarks and gluons in the quark–gluon plasma and retain some memory of the collisions’ initial geometry.

The SM still stands strong after Moriond 2019, and the observation of CP violation in D mesons represents another victory, concluded Shahram Rahatlou of Sapienza University of Rome in the experimental summary. “But the flavour anomaly is still there to be pursued at low and high mass.”

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