Fast b-tagging at the high-level trigger of the ATLAS experiment in LHC Run 3

The ATLAS experiment relies on real-time hadronic jet reconstruction and b-tagging to record fully hadronic events containing b-jets. These algorithms require track reconstruction, which is computationally expensive and could overwhelm the high-level-trigger farm, even at the reduced event rate that passes the ATLAS first stage hardware-based trigger. In LHC Run 3, ATLAS has mitigated these computational demands by introducing a fast neural-network-based b-tagger, which acts as a low-precision filter using input from hadronic jets and tracks. It runs after a hardware trigger and before the remaining high-level-trigger reconstruction. This design relies on the negligible cost of neural-network inference as compared to track reconstruction, and the cost reduction from limiting tracking to specific regions of the detector. In the case of Standard Model HH → bb̅bb̅, a key signature relying on b-jet triggers, the filter lowers the input rate to the remaining high-level trigger by a factor of five at the small cost of reducing the overall signal efficiency by roughly 2%

Search for flavour-changing neutral-current couplings between the top quark and the Higgs boson in multi-lepton final states in 13 TeV collisions with the ATLAS detector

A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark ( = ,) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two bosons, two bosons or a ⁺⁻ pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are ̄ and production. For the ̄ production, one top quark decays via → . The proton–proton collision data set analysed amounts to (140 fb⁻¹) at (√ = 13 TeV). No significant excess beyond Standard Model expectations is observed and upper limits are set on the → branching ratios at 95 % confidence level, amounting to observed (expected) limits of ( → ) < 2.8(3.0) × 10⁻⁴ and ( → ) < 3.3 (3.8) × 10⁻⁴. Combining this search with other searches for flavour-changing neutral-current interactions previously conducted by ATLAS, considering → ̄ and → decays, as well as → +−decays with one or two hadronically decaying -leptons, yields observed (expected) upper limits on the branching ratios of ( → ) < 2.6 (1.8) × 10⁻⁴and ( → ) < 3.4 (2.3) × 10⁻⁴

Precise test of lepton flavour universality in -boson decays into muons and electrons in collisions at √s = 13 TeV with the ATLAS detector

Abstract The ratio of branching ratios of the boson to muons and electrons, _∧/ = ( → )/( → ) , has been measured using 140 fb⁻¹ of collision data at √s = 13 TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the ̄ production cross-section in the , and dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio _∧/ for the boson measured in inclusive → and → events. By using the precise value of _∧/ determined from ⁺ ⁻ colliders, the ratio _∧ / is determined to be _∧/ = 0.9995 ± 0.0022 (stat) ± 0.0036 (syst) ± 0.0014 (ext). The three uncertainties correspond to data statistics, experimental systematics and the external measurement of _∧/, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in -boson decays at the 0.5% level

Precise measurements of - and -boson transverse momentum spectra with the ATLAS detector using collisions at √s = 5.02 TeV and13 TeV

Abstract This paper describes measurements of the transverse momentum spectra of and bosons produced in proton–proton collisions at centre-of-mass energies of √s = 5.02 TeV and √s = 13 TeV with the ATLAS experiment at the Large Hadron Collider. Measurements are performed in the electron and muon channels, → and → ( = or ), and for events further separated by charge. The data were collected in 2017 and 2018, in dedicated runs with reduced instantaneous luminosity, and correspond to 255 and 338 pb⁻¹ at √s = 5.02 TeV and 13 TeV, respectively. These conditions optimise the reconstruction of the -boson transverse momentum. The distributions observed in the electron and muon channels are unfolded, combined, and compared to QCD calculations based on parton shower Monte Carlo event generators and analytical resummation. The description of the transverse momentum distributions by Monte Carlo event generators is imperfect and shows significant differences largely common to ⁻, ⁺ and production. The agreement is better at s = 5.02 TeV, especially for predictions that were tuned to production data at √s = 7 TeV. Higher-order, resummed predictions based on DYTurbo generally match the data best across the spectra. Distribution ratios are also presented and test the understanding of differences between the production processes

Search for new phenomena with top-quark pairs and large missing transverse momentum using 140 fb⁻¹ of collision data at √s = 13 TeV with the ATLAS detector

A search is conducted for new phenomena in events with a top quark pair and large missing transverse momentum, where the top quark pair is reconstructed in final states with one isolated electron or muon and multiple jets. The search is performed using the Large Hadron Collider proton-proton collision data sample at a centre-of-mass energy of √s = 13 TeV recorded by the ATLAS detector that corresponds to an integrated luminosity of 140 fb⁻¹. An analysis based on neural network classifiers is optimised to search for directly produced pairs of supersymmetric partners of the top quark (stop), and to search for spin-0 mediators, produced in association with a pair of top quarks, that decay into dark-matter particles. In the stop search, the analysis is designed to target models in which the mass difference between the stop and the neutralino from the stop decay is close to the top quark mass. This new search is combined with previously published searches in final states with different lepton multiplicities. No significant excess above the Standard Model background is observed, and limits at 95% confidence level are set. Models with neutralinos with masses up to 570 GeV are excluded, while for small neutralino masses models are excluded for stop masses up to 1230 GeV. Scalar (pseudoscalar) dark matter mediator masses as large as 350 (370) GeV are excluded when the coupling strengths of the mediator to Standard Model and dark-matter particles are both set to one. At lower mediator masses, models with production cross-sections as small as 0.15 (0.16) times the nominal predictions are excluded. Results of this search are also used to set constraints on effective four-fermion contact interactions between top quarks and neutrinos

Search for short- and long-lived axion-like particles in → → 4 decays with the ATLAS experiment at the LHC

Presented is the search for anomalous Higgs boson decays into two axion-like particles (ALPs) using the full Run 2 data set of 140 fb⁻¹ of proton-proton collisions at a centre-of-mass energy of √ = 13 TeV recorded by the ATLAS experiment. The ALPs are assumed to decay into two photons, providing sensitivity to recently proposed models that could explain the ( − 2) discrepancy. This analysis covers an ALP mass range from 100 to 62{GeV} and ALP-photon couplings in the range 10⁻⁷ TeV⁻¹ <_ / <1 TeV⁻¹, eV⁻¹, and therefore includes signatures with significantly displaced vertices and highly collinear photons. No significant excess of events above the Standard Model background is observed. Upper limits at 95% confidence level are placed on the branching ratio of the Higgs boson to two ALPs in the four-photon final state, and are in the range of 10⁻⁵ to 3 × 10⁻², depending on the hypothesized ALP mass and ALP-photon coupling strength

Search for dark mesons decaying to top and bottom quarks in proton-proton collisions at √ = 13 TeV with the ATLAS detector

A search for dark mesons originating from strongly-coupled, SU(2) dark flavor symmetry conserving models and decaying gauge phobically to pure Standard Model final states containing top and bottom quarks is presented. The search targets fully hadronic final states and final states with exactly one electron or muon and multiple jets. The analyzed data sample corresponds to an integrated luminosity of 140 fb⁻¹ of proton-proton collisions collected at √ = 13 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess over the Standard Model background expectation is observed and the results are used to set the first direct constraints on this type of model. The two-dimensional signal space of dark pion masses and dark rho-meson masses is scanned. For / = 0.45, dark pions with masses < 940 GeV are excluded at the 95% CL, while for / = 0.25 masses < 740 GeV are excluded

Search for new particles in final states with a boosted top quark and missing transverse momentum in proton-proton collisions at √s = 13 TeV with the ATLAS detector

A search for events with one top quark and missing transverse momentum in the final state is presented. The fully hadronic decay of the top quark is explored by selecting events with a reconstructed boosted top-quark topology produced in association with large missing transverse momentum. The analysis uses 139 fb⁻¹ of proton-proton collision data at a centre-of-mass energy of √s = 13 TeV recorded during 2015-2018 by the ATLAS detector at the Large Hadron Collider. The results are interpreted in the context of simplified models for Dark Matter particle production and the single production of a vector-like quark. Without significant excess relative to the Standard Model expectations, 95% confidence-level upper limits on the corresponding cross-sections are obtained. The production of Dark Matter particles in association with a single top quark is excluded for masses of a scalar (vector) mediator up to 4.3 (2.3) TeV, assuming χ = 1 GeV and the model couplings _ = 0.6 and χ = 0.4 ( = 0.5 and χ = 1). The production of a single vector-like quark is excluded for masses below 1.8 TeV assuming a coupling to the top quark _ = 0.5 and a branching ratio for → of 25%

Search for resonant production of dark quarks in the dijet final state with the ATLAS detector

This paper presents a search for a new ′ resonance decaying into a pair of dark quarks which hadronise into dark hadrons before promptly decaying back as Standard Model particles. This analysis is based on proton-proton collision data recorded at √ = 13 TeV with the ATLAS detector at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of 139 fb⁻¹. After selecting events containing large-radius jets with high track multiplicity, the invariant mass distribution of the two highest-transverse-momentum jets is scanned to look for an excess above a data-driven estimate of the Standard Model multijet background. No significant excess of events is observed and the results are thus used to set 95% confidence-level upper limits on the production cross-section times branching ratio of the ′ to dark quarks as a function of the ′ mass for various dark-quark scenarios

Observation of ̄ production in the lepton+jets and dilepton channels in +Pb collisions at √_NN = 8.16 TeV with the ATLAS detector

Abstract This paper reports the observation of top-quark pair production in proton-lead collisions in the ATLAS experiment at the Large Hadron Collider. The measurement is performed using 165 nb⁻¹ of +Pb data collected at √_NN = 8.16 TeV in 2016. Events are categorised in two analysis channels, consisting of either events with exactly one lepton (electron or muon) and at least four jets, or events with two opposite-charge leptons and at least two jets. In both channels at least one -tagged jet is also required. Top-quark pair production is observed with a significance over five standard deviations in each channel. The top-quark pair production cross-section is measured to be _̄ = 58.1 ± 2.0 (stat.) +4.8[over]−4.4 + 4.8 (syst.) nb, with a total uncertainty of 9%. In addition, the nuclear modification factor is measured to be R p A = 1.090 ± 0.039 (stat.) +0.094[over]−0.087 +0.094 (syst.). The measurements are found to be in good agreement with theory predictions involving nuclear parton distribution functions