Almost 2000 physicists gathered last week (July 27-31, 2021) for the bi-annual meeting of the High Energy and Particle Physics Division of the European Physics Society (EPS-HEP 2021). The conference brought together experimentalists and theorists from across the particle physics community to discuss their findings.

ATLAS’ vibrant physics programme was on full display, with members presenting 26 new physics analyses among other key recent results. These were shown in 63 presentations and 45 posters over the five days.

Discoveries of new rare processes

The large amount of data from the second operation of the LHC (Run 2) continues to make it possible for physicists to observe new, very rare processes. The most recent of these to be observed is the production of three W bosons, announced by the ATLAS Collaboration during EPS-HEP 2021.

Less than a hundred thousand LHC collisions at the ATLAS detector are expected to have produced three W bosons, compared to 26 billion events with one W boson. A new ATLAS analysis focused on cases where two of the W bosons decay to a pair of leptons with the same sign of electric charge and the third W boson decays to a pair of jets of hadrons, or where all three W bosons decay to leptons. Figure 1 shows an example of such an event with two positrons and a muon with large missing transverse momentum due to the presence of neutrinos.

The ATLAS Collaboration has observed this process with a statistical significance of 8.2 standard deviations. The result is a first step towards a deeper understanding of how W bosons interact with themselves.

Delving further into Higgs boson interactions

Since its discovery nine years ago, researchers have made a lot of progress in understanding the physics of the Higgs boson. At EPS-HEP 2021, ATLAS researchers presented a variety of new results on the Higgs boson, shedding more light on this unique particle.

One new result used the eight million Higgs bosons produced in ATLAS to study its decay into a pair of tau leptons. The tau lepton is the heaviest lepton in nature and the one with the strongest interaction with the Higgs boson. However, it produces a challenging experimental signature, since it includes neutrinos from the decays of tau leptons that escape detection. This makes it difficult to distinguish the Higgs boson signal from other background processes that can also produce a pair of tau leptons. Z bosons – which are produced a thousand times more often than Higgs bosons in the LHC collisions – also decay to a pair of tau leptons and are the main background for this measurement. ATLAS’ new result focused on the Higgs bosons produced in the fusion of vector bosons. This is the most sensitive channel through which to study Higgs boson decays to tau leptons, as it produces a striking signature of two forward particle “jets” that help distinguish it from Z boson decays.

ATLAS researchers also presented new studies of Higgs-boson-pair production, allowing them to examine how the Higgs boson interacts with itself. The production of two Higgs bosons is extremely rare, with an expected rate in LHC collisions that is a thousand times smaller than that for single Higgs bosons. One new search focused on the case where one Higgs boson decays to a pair of tau leptons and the other to a pair of bottom quarks. Figure 2 shows an example of such an event with two b-jets, a tau lepton decaying to a muon and neutrinos, and another tau lepton decaying to a hadron and a neutrino. The search in this channel is currently the most sensitive and is already sensitive to a signal smaller than five times the Standard Model prediction. Another search focused on the Higgs boson’s most common decay mode, where both Higgs bosons decay to a pair of bottom quarks. The ATLAS search for signals with four bottom quarks is also the most sensitive probe yet of new heavy particles that decay to pairs of Higgs bosons.

Probing new interactions with the Higgs boson

The Higgs boson has now been observed through several decay modes and the search continues for additional decays, including new modes predicted by models of new phenomena such as dark matter. In models where dark matter interactions are very feeble, the Higgs boson may be the only portal to this new physics phenomenon.

ATLAS physicists presented a new search for dark matter interactions mediated by new light particles, with masses less than half of that of the Higgs boson. In such models, the Higgs boson can decay to a pair of these new particles, each then decaying to a pair of leptons (electrons or muons). The four-lepton signature is very similar to the four-lepton “golden channel” that was used in the discovery of the Higgs boson in 2012, but can be distinguished because of the low mass and spin of the new particles.

The new search considered a large range of masses – all the way down to 1 GeV – and both the cases when the new particle has a spin of 0 or 1. As shown in Figure 3, no such signal was observed and new constraints were set for models in which these exotic Higgs boson decays could be as rare as 1 in each 100,000 Higgs bosons produced in the LHC.

Searches for new physics

The as-yet unresolved mysteries of particle physics continue to motivate new searches by the ATLAS Collaboration. At EPS-HEP 2021, researchers presented several new results seeking indications of new and rare phenomena. One new result searched for indications of lepton-flavour violation in decays of the Z boson. In the Standard Model, Z bosons are expected to decay to a pair of leptons of the same flavour (electron, muon, tau or neutrinos). New physics phenomena could result in decays with two leptons of different flavour. ATLAS’ new analysis searched for Z boson decays to an electron and a muon by looking for a “bump'' in the invariant mass distribution of the electron and muon, as shown in Figure 4. This complements previous ATLAS searches that focused on Z boson decays to an electron or a muon, and a hadronically decaying tau. Physicists did not see an indication of such a signal and the results were used to constrain the size of possible new physics effects to levels that are much smaller than previous experiments, particularly those at CERN's Large Electron-Positron (LEP) Collider.

The exploration continues

The wealth of data collected by the ATLAS experiment during Run 2 of the LHC (2015–2018) continues to yield exciting new results. Many other important new analyses that were not described in this article can be found here. So far, the ATLAS Collaboration has released 132 results using the full Run-2 dataset and many more results are expected in the coming months, stay tuned!

Learn more

Results

• Observation of WWW production in proton–proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2021-039)
• Measurements of Higgs boson production cross-sections in the H→τ⁺τ^- decay channel in proton–proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2021-044)
• Search for resonant pair production of Higgs bosons in the bbbb final state using proton–proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2021-035)
• Search for resonant and non-resonant Higgs boson pair production in the bbτ⁺τ⁻ decay channel using 13 TeV proton–proton collision data from the ATLAS detector (ATLAS-CONF-2021-030)
• Search for Higgs bosons decaying to new spin-0 or spin-1 particles in four-lepton final states at the ATLAS detector with 139 fb⁻¹ of proton–proton collision data at 13 TeV (ATLAS-CONF-2021-034)
• Search for the Charged-Lepton-Flavor Violating decay Z→eμ in proton–proton collisions at 13 TeV with the ATLAS detector (ATLAS-CONF-2021-042)
• See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.

EPS-HEP Physics Briefings

• ATLAS reports first observation of WWW production, Physics Briefing, 26 July
• Probing new physics with pairs of Higgs bosons, Physics Briefing, 28 July
• Two Higgs bosons are better than one, Physics Briefing, 30 July
• ATLAS measures key Higgs boson interaction with high precision, Physics Briefing, 2 August
• Hunting for forbidden decays of the Z boson, Physics Briefing, 3 August
• Summary of new ATLAS results from EPS-HEP 2021, 26 July