ATLAS highlights from the Moriond 2021 conferences

Traditionally held on the Alpine slopes of La Thuile in Italy, this year’s editions of the Moriond EW and QCD conferences followed an unusual yet successful @Home format.

Moriond EW took a new approach, with “plenary” talks pre-recorded and released daily, and live discussion sessions following a day later. This gave physicists ample time to watch the talks, regardless of their time zone, and prepare a barrage of questions for speakers well in advance. Moriond QCD held a more traditional online conference, with attendees tuning in at the same time to hear talks live and engage directly with participants.

The ATLAS Collaboration presented a host of brand-new results spanning a broad range of subjects, from new insight into Higgs boson interactions and further tests of the Standard Model to searches for new phenomena motivated by as-yet unresolved mysteries of particle physics. Several of the highlights were featured in special ATLAS Physics Briefings and are thus not discussed below.

Further verifications of the Standard Model

Physics,ATLAS — Figure 1: The measured differential cross section as a function of the transverse momentum of the most energetic jet in the event. The measured values (full black dots) are compared to the Standard Model predictions obtained with several event generators (empty coloured markers) and show excellent agreement over the full range of the jet transverse-momentum spectrum. (Image: ATLAS Collaboration/CERN)

Physicists are keenly probing Standard Model predictions, increasing the precision of their measurements and their confidence in the theory. While Standard Model measurements, including those of Higgs-boson properties, have shown very good agreement with the predictions of this established physics model, they still leave plenty of space for new physics.

In one new analysis, ATLAS researchers precisely measured a cross section of W-boson pair production in association with at least one jet. The measurement, in a previously unexplored topology, was performed on events where the W-bosons decay to electrons or muons and their neutrino counterparts.

Researchers measured the cross section as a function of several kinematic variables – such as the dilepton invariant mass, the lepton and jet transverse momenta, and the angular separations between the jets and the leptons – and found very good agreement between the data and Standard Model predictions. As an example, Figure 1 shows the measured cross section as a function of the transverse momentum of the most energetic jet in the event, with an excellent agreement between the data and the Standard Model all the way up to the highest jet momentum.

Looking for new physics in Higgs-boson events

Ever since discovering the Higgs boson, researchers have been trying to better understand its properties, in particular its couplings with Standard Model particles. The Higgs boson couples to particles through so-called Yukawa couplings, the size of which is proportional to a particle's mass. Therefore, any massive particle should, in principle, couple to the Higgs boson – even if it does not interact with Standard Model particles through the electromagnetic, weak and strong forces. This observation is particularly interesting in the context of dark matter searches.

At this year’s Moriond, the ATLAS Collaboration presented a search for Higgs-boson decays to dark matter. The final state in the analysis – explored for the first time in ATLAS – balances the large missing transverse energy from invisible dark matter particles with two energetic particle jets in the forward detector region and a photon. The new analysis takes advantage of the jet topology typical of the vector-boson fusion process and the presence of a photon. This allowed physicists to better reject background processes and improve their signal reconstruction efficiency compared to, for example, typical monojet searches.

No hints of new physics were observed, and the analysis set the upper limit on the branching fraction (or decay probability) of the Higgs boson to invisible final states at 37%. ATLAS researchers also interpreted the search for the case where the Higgs boson decays to a Standard Model photon and a dark photon that escapes detection. The upper limit set on the branching fraction of the Higgs boson to a photon plus invisible particle(s) amounts to 1.4% and is the most stringent limit to date.

ATLAS physicists also presented several searches for Higgs-boson decays to new light bosons (denoted as “a”) that decay to b-quarks or muons. Such light bosons are interesting because they could provide a channel for dark-matter annihilation into Standard Model particles and could be related to the observed excess of gamma rays from the centre of our galaxy.

In a new analysis of events with two b-quarks and two muons, physicists searched for Higgs-boson decays to two a-bosons with a mass between 16 and 62 GeV. Since a-bosons are expected to decay to b-quarks most of the time, these events provide a good balance between a high branching fraction to b-quarks and a clean signature from a narrow di-muon resonance from the a-boson decays to two muons. Researchers observed a moderate excess of events above the Standard Model prediction at an a-mass of 52 GeV, as can be seen in Figure 2. With a level of 3.3 sigma (local significance), the excess is still far too low to claim any evidence of new physics. But it is certainly enough to provoke physicists’ imagination and to add to the suspense while waiting for other related searches to be released by ATLAS and CMS.

Researchers continuously strive to cover the full kinematic spectrum of new physics, to prevent any new particles from escaping detection. For instance, to date ATLAS physicists have looked for a-bosons with a range of different possible lifetimes: from promptly decaying a-bosons to a-bosons that travel more than a metre before decaying in the ATLAS calorimeter or the muon spectrometer.

Another new analysis presented at this year’s Moriond focused on cases where a-bosons live long enough to travel up to 30 cm before decaying inside the ATLAS tracking detectors. Scientists looked for Higgs-boson decays to two of these a-bosons in a final state with four b-quarks. Searches for long-lived particles – i.e. particles that do not decay promptly at the ATLAS interaction point – are challenging as they often require specialised reconstruction techniques. For their new search, ATLAS researchers developed dedicated tracking and vertexing methods to reconstruct vertices in the tracker associated with b-quark-initiated jets. Figure 3 shows the results in terms of upper limits set on the branching fraction of the Higgs boson to two a-bosons, as a function of the a-particle lifetime. The effort successfully covered the sensitivity gap between the two previous searches.

Searching for long-lived supersymmetry

ATLAS’ search for long-lived particles also extends into supersymmetric models. Researchers presented a new search for the long-lived supersymmetric partners of charged weak bosons (charginos) that focused on detector signatures with disappearing charged-particle tracks.

During its shorter-than-a-nanosecond proper lifetime, the chargino would travel about 10 cm before decaying inside the ATLAS inner tracking detector. It would transform into very low-energy pions and undetectable particles – namely, the neutral, lightest supersymmetric particles that could be responsible for dark matter. The identification of a disappearing track from a chargino is illustrated in Figure 4. The chargino track is reconstructed from at least four hits in the innermost pixel detector, and is characterised by a lack of hits in the outermost silicon trackers and no calorimeter activity. In cases where charginos are produced in electroweak interactions, ATLAS physicists were able to exclude charginos with masses up to 210 GeV or 660 GeV, depending on the exact model assumptions.

Next year, back to the Alps!

The wide range of new results presented at Moriond, including several not highlighted here, all point to a wealth of interesting problems for physicists to work on. Look forward to new insights from the ATLAS Collaboration in the coming years – and to discussing these results in person when Moriond returns to the Alps!

About the event display: ATLAS 13 TeV collision event with large missing transverse energy, a photon and two jets with large invariant mass. The signature is consistent with the signal hypotheses of both the Higgs boson decay to a Standard Model photon and a dark photon, and the Higgs boson decay to invisible particles.