For over 50 years, results shown at the Rencontres de Moriond Electroweak and Unified Theories conference have provided a contemporary snapshot of the fields of collider physics, neutrino physics, dark matter and astroparticle physics. This conference is not only an important jour fixe within the annual conference calendar, it also aims to promote effective collaboration between experimentalists and theorists, with a unique aspect being the focus on discussions on (as well as off) the wonderful ski slopes of the Alps. After two challenging COVID-19-impacted years (the 2020 edition was cancelled and the 2021 edition was held online only), the conference welcomed back nearly 150 participants for its 56^th edition.

As one of the most exciting and promising experiments for new crucial discoveries in the field of high-energy particle physics, ATLAS contributed many novel results presented in dedicated talks. Contributions spanned a wide range of topics, from precision measurements and searches for new phenomena to detector performance, exploiting the full LHC Run-2 dataset, recorded from 2015 to 2018. Select highlights are discussed below, hinting at exciting prospects for follow-up investigations with the upcoming LHC Run-3 dataset.

First observation of rare associated single-top photon production

As the heaviest particle ever discovered, the top quark plays an important role in many beyond-the-Standard-Model theories. Its large mass may indicate that it plays a fundamental role in the electroweak symmetry-breaking mechanism, and it may have enhanced interactions to many new particles. It is thus imperative that physicists perform accurate measurements of its properties with rare processes.

One such rare process is the production of a single top quark in association with a photon. The ATLAS Collaboration has observed this process with a significance of 9.1 standard deviations using the full LHC Run-2 dataset. This observation is compatible with the Standard Model prediction and was made possible using novel, sophisticated machine-learning approaches as shown in Figure 1 for the most sensitive search region. Going forward, this exciting, rare process can be used to find new particles that could alter the top-photon interaction.

Higgs-boson physics, nearly 10 years after discovery

Since discovering a new particle with an approximate mass of 125 GeV, the ATLAS Collaboration has extensively studied its properties, which indicate that it is consistent with the Standard-Model Higgs boson. The Higgs boson is predicted to interact with matter particles with a strength proportional to their mass. By measuring these interaction strengths in the ATLAS experiment, physicists can test the Standard Model and gain new insight into the nature of particle generations.

One important interaction with matter particles has not yet been found: the Higgs boson decay into a charm quark-antiquark pair. This channel is challenging due to its tiny predicted branching fraction (2.89%) and experimental difficulties in identifying charm quarks within the collimated sprays of particles that are created when quarks hadronize, called ‘jets’.

In a new result presented at Moriond, ATLAS physicists combined two measurements of the Higgs-boson interaction strength: one with pairs of beauty quarks and one with pairs of charm quarks. This new combination allowed them to exclude the hypothesis that the Higgs-boson interaction with charm quarks is stronger or equal than that with beauty quarks. This underlines that the Higgs-boson coupling is smaller for charm quarks than it is for beauty quarks, and consequently the Higgs boson interacts differently with quarks of the second and third generation.

Searching for new physics using unusual signatures shows exciting promise

Thus far, most searches for new physics at the LHC have been performed under the assumption that the new particles decay promptly, i.e. very close to the collision point. However, beyond-the-Standard-Model theories sometimes predict the existence of new particles with relatively long lifetimes. For particles moving close to the speed of light, this can lead to macroscopic, detectable displacements between the production and decay points of a new, unstable long-lived particle (LLP).

LLPs can leave a rich variety of experimental signatures in the ATLAS detector; these are generally very different from signals left by Standard-Model processes. For example, LLP signatures can include tracks with unusual ionisation and propagation properties or displaced vertices within the different sub-detectors. Despite being excellent prospects for the discovery of new physics at particle colliders, standard reconstruction techniques often reject events containing LLPs precisely because of their unusual nature. ATLAS physicists have developed several dedicated techniques to uncover LLP signals, and are thus starting to probe signatures that could soon reveal new phenomena. The following two new ATLAS results were enthusiastically discussed during the conference.

First, ATLAS physicists used the high rate of W bosons produced in LHC collisions to search for new “heavy neutral leptons” (HNL) in the mass range below the W-boson mass. Specifically, they looked for decays of a W boson into a Standard-Model lepton – which would appear “promptly” at the LHC collision point – and a heavy neutral lepton – which would decay into a Standard-Model lepton pair, displaced a few centimetres from the collision point inside the ATLAS inner tracking detector. The prompt lepton from the W decay was used for triggering, and the requirement of a displaced di-lepton vertex helped reduce the background to negligible levels. For the first time at the LHC, this analysis reports not only on heavy-neutral-lepton decays to electron-muon pairs in the one-HNL model (1SFH), but also explores the simplest realistic model with two quasi-degenerate HNLs (2QDH). The realistic 2QDH benchmark models are motivated by a recent global fit to neutrino oscillation data with the data assuming both inverted and normal neutrino mass hierarchy, respectively. This search allows to exclude the squared mixing parameters of the HNL with the Standard-Model neutrino states for these models as low as 2x10^-7 (Figure 2).

Second, new results for a search for heavy, detector-stable, charged LLPs were presented for the first time. If created in LHC collisions, such LLPs would be directly detectable via tracks left behind the ATLAS inner detector. ATLAS’ search targeted new particles that are much heavier than the proton, which would result in its speed being markedly lower than that of any other particle of the same momentum. The analysis exploited the specific ionisation of isolated high-momentum particles as recorded in the ATLAS pixel detector. Figure 3 shows that the observed data agree with the expected Standard-Model background, except for the excess of events above a particle mass of 1 TeV as detected within one search region. Despite this intriguing excess with a global significance of 3.3 standard deviations, the time-of-flight measurements in outer sub-detectors indicate that none of the candidate tracks match the heavy and slow-moving charged-particle hypothesis. It will be exciting to see what further investigation using the future Run-3 dataset will reveal.

New LHC data-taking period about to start – the exploration continues

The ATLAS Collaboration released 21 exciting new results for the 56th Rencontres de Moriond Electroweak and Unified Theories conference. Intriguing hints in these data will be further scrutinised in coming years, as ATLAS scientists eagerly look forward to the start of the LHC Run-3 data-taking period this year.

Learn more

• Summary of new ATLAS results from 2022 Winter Conferences, ATLAS News, March 2022
• See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.