A combination of searches for the single production of vectorlike top quarks (𝑇) is presented. These analyses are based on proton-proton collisions at √𝑠 =13  TeV recorded in 2015–2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139  fb⁻¹. The 𝑇 decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a 𝑍 boson (𝑇 →𝐻⁡𝑡 and 𝑇→𝑍⁢𝑡). The individual searches used in the combination are differentiated by the number of leptons (𝑒, 𝜇) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vectorlike top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vectorlike top quark SU(2) singlet with electroweak coupling, 𝜅, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.

This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying τ-lepton pair with low transverse momentum. When two τ-leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the τ-leptons. A dedicated treatment attempting to tag the τ-lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from τ⁺τ⁻ systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in Zγ events using proton–proton collision data at √s=13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb⁻¹. The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.

A search is conducted for a new scalar boson S, with a mass distinct from that of the Higgs boson, decaying promptly into four leptons (ℓ=e, μ) via an intermediate state containing two on-shell, promptly decaying new spin-1 bosons Z_d: S→Z_dZ_d→4ℓ, where the Z_d boson has a mass between 15 and 300 GeV, and the S boson has a mass between either 30 and 115 GeV or 130 and 800 GeV. The search uses proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider with an integrated luminosity of 139 fb⁻¹ at a centre-of-mass energy of √s=13 TeV. No significant excess above the Standard Model background expectation is observed. Upper limits at 95% confidence level are set on the production cross-section times branching ratio, σ(gg→S)×B(S→Z_dZ_d→4ℓ), as a function of the mass of both particles, m_S and m_Zd.

Charged Higgs bosons produced either in top-quark decays or in association with a top quark, subsequently decaying via 𝐻± →𝜏±⁢𝜈𝜏, are searched for in 140  fb−1 of proton-proton collision data at √𝑠 =13  TeV recorded with the ATLAS detector. Depending on whether the top quark is produced together with the 𝐻± decays hadronically or semileptonically, the search targets 𝜏 +jets or 𝜏 +lepton final states, in both cases with a 𝜏-lepton decaying into a neutrino and hadrons. No significant excess over the Standard Model background expectation is observed. For the mass range of 80 ≤𝑚𝐻± ≤3000  GeV, upper limits at 95% confidence level are set on the production cross section of the charged Higgs boson times the branching fraction ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏) in the range 4.5 pb–0.4 fb. In the mass range 80–160 GeV, assuming the Standard Model cross section for 𝑡⁢¯𝑡 production, this corresponds to upper limits between 0.27% and 0.02% on ℬ⁡(𝑡 →𝑏⁢𝐻±) ×ℬ⁡(𝐻± →𝜏±⁢𝜈𝜏).

A search is performed for dark matter particles produced in association with a resonantly produced pair of 𝑏-quarks with 30<𝑚𝑏⁢𝑏<150  GeV using 140  fb−1 of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the LHC. This signature is expected in extensions of the standard model predicting the production of dark matter particles, in particular those containing a dark Higgs boson 𝑠 that decays into 𝑏⁢¯𝑏. The highly boosted 𝑠 →𝑏⁢¯𝑏 topology is reconstructed using jet reclustering and a new identification algorithm. This search places stringent constraints across regions of the dark Higgs model parameter space that satisfy the observed relic density, excluding dark Higgs bosons with masses between 30 and 150 GeV in benchmark scenarios with 𝑍′ mediator masses up to 4.8 TeV at 95% confidence level.

This Letter presents a search for highly ionizing magnetic monopoles in 262  μ⁢b⁻¹ of ultraperipheral Pb+Pb collision data at √𝑠_NN=5.36  TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20–150 GeV. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80–120 GeV are excluded.

This Letter presents a search for highly ionizing magnetic monopoles in 262  μ⁢b⁻¹ of ultraperipheral Pb+Pb collision data at √𝑠_NN=5.36  TeV collected by the ATLAS detector at the LHC. A new methodology that exploits the properties of clusters of hits reconstructed in the innermost silicon detector layers is introduced to study highly ionizing particles in heavy-ion data. No significant excess above the background, which is estimated using a data-driven technique, is observed. Using a nonperturbative semiclassical model, upper limits at 95% confidence level are set on the cross section for pair production of monopoles with a single Dirac magnetic charge in the mass range of 20–150 GeV. Depending on the model, monopoles with a single Dirac magnetic charge and mass below 80–120 GeV are excluded.

Quantum field theory provides the foundation for understanding fundamental interactions through gauge theories. While the Standard Model successfully describes numerous subatomic processes, it remains incomplete, lacking explanations for gravity, dark matter, and other open questions, driving the search for new physics.

This thesis presents two analyses using datasets from the ATLAS experiment, including the full Run-2 and partial Run-3 proton–proton collision data. The electroweak ttWj (ttWj EW) analysis studies tW scattering at a centre-of-mass energy of 13 TeV, utilising an integrated luminosity of 140.1 fb^-1. Background contributions are constrained through data-driven methods. This work provides the first experimental direct constraints on the ttWj EW process, establishing an observed (expected) upper limit at the 95% confidence level (CL_s) of 250 (232) fb, compared to the Standard Model prediction of 47.7 fb. Furthermore, a Standard Model Effective Field Theory (SMEFT) interpretation constrains the two dimension-6 operators relevant to ttWj EW production, highlighting the interplay between ttWj EW and ttZ processes in resolving parameter space degeneracies.

The X → SH → bbγγ search explores a new scalar boson X decaying into a Higgs boson H and a lighter scalar S, with S → bb and H → γγ. Combining the ATLAS Run-2 and partial Run-3 datasets, the analysis sets expected 95% upper limits (CL_s) on the signal strength ranging from 0.09 to 7.6 - representing a 19 ~ 60% improvement over the ATLAS Run-2 results published in 2024 (JHEP 11 (2024) 047). This enhancement is primarily driven by additional Run-3 data and an improved b-tagging algorithm, particularly benefiting the low-mass region.

With the ongoing ATLAS Run-3 and the upcoming High-Luminosity LHC, increasing data availability will improve precision tests of the Standard Model and sensitivity to new physics. In particular, SMEFT studies will benefit from higher statistics, allowing for stronger constraints on Higgs, fermion, and vector boson interactions.

This paper presents for the first time a precise measurement of the production properties of the Z boson in the full phase space of the decay leptons. This is in contrast to the many previous precise unfolded measurements performed in the fiducial phase space of the decay leptons. The measurement is obtained from proton–proton collision data collected by the ATLAS experiment in 2012 at s=8 TeV at the LHC and corresponding to an integrated luminosity of 20.2 fb-1. The results, based on a total of 15.3 million Z-boson decays to electron and muon pairs, extend and improve a previous measurement of the full set of angular coefficients describing Z-boson decay. The double-differential cross-section distributions in Z-boson transverse momentum pT and rapidity y are measured in the pole region, defined as 80<mℓℓ<100 GeV, over the range |y|<3.6. The total uncertainty of the normalised cross-section measurements in the peak region of the pT distribution is dominated by statistical uncertainties over the full range and increases as a function of rapidity from 0.5–1.0% for |y|<2.0 to 2-7% at higher rapidities. The results for the rapidity-dependent transverse momentum distributions are compared to state-of-the-art QCD predictions, which combine in the best cases approximate N4LL resummation with N3LO fixed-order perturbative calculations. The differential rapidity distributions integrated over pT are even more precise, with accuracies from 0.2–0.3% for |y|<2.0 to 0.4–0.9% at higher rapidities, and are compared to fixed-order QCD predictions using the most recent parton distribution functions. The agreement between data and predictions is quite good in most cases.

The identification of top quark decays where the top quark has a large momentum transverse to the beam axis, known as top tagging, is a crucial component in many measurements of Standard Model processes and searches for beyond the Standard Model physics at the Large Hadron Collider. Machine learning techniques have improved the performance of top tagging algorithms, but the size of the systematic uncertainties for all proposed algorithms has not been systematically studied. This paper presents the performance of several machine learning based top tagging algorithms on a dataset constructed from simulated proton-proton collision events measured with the ATLAS detector at √_s = 13 TeV. The systematic uncertainties associated with these algorithms are estimated through an approximate procedure that is not meant to be used in a physics analysis, but is appropriate for the level of precision required for this study. The most performant algorithms are found to have the largest uncertainties, motivating the development of methods to reduce these uncertainties without compromising performance. To enable such efforts in the wider scientific community, the datasets used in this paper are made publicly available.