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Item Open Access Constraints on the Cosmic Expansion History from GWTC–3(American Astronomical Society, 2023) Abbott, R.; Abe, H.; Acernese, F.; Ackley, K.; Adhikari, N.; Adhikari, R.X.; Adkins, V.K.; Adya, V.B.; Affeldt, C.; Agarwal, D.; Agathos, M.; Agatsuma, K.; Aggarwal, N.; Aguiar, O.D.; Aiello, L.; Ain, A.; Ajith, P.; Akutsu, T.; Albanesi, S.; Alfaidi, R.A.; et al.We use 47 gravitational wave sources from the Third LIGO–Virgo–Kamioka Gravitational Wave Detector Gravitational Wave Transient Catalog (GWTC–3) to estimate the Hubble parameter H(z), including its current value, the Hubble constant H0. Each gravitational wave (GW) signal provides the luminosity distance to the source, and we estimate the corresponding redshift using two methods: the redshifted masses and a galaxy catalog. Using the binary black hole (BBH) redshifted masses, we simultaneously infer the source mass distribution and H(z). The source mass distribution displays a peak around 34 Me, followed by a drop-off. Assuming this mass scale does not evolve with the redshift results in a H(z) measurement, yielding = - H + - - 0 68 km s Mpc 8 12 1 1 (68% credible interval) when combined with the H0 measurement from GW170817 and its electromagnetic counterpart. This represents an improvement of 17% with respect to the H0 estimate from GWTC–1. The second method associates each GW event with its probable host galaxy in the catalog GLADE+, statistically marginalizing over the redshifts of each event’s potential hosts. Assuming a fixed BBH population, we estimate a value of = - H + - - 0 68 km s Mpc 6 8 1 1 with the galaxy catalog method, an improvement of 42% with respect to our GWTC–1 result and 20% with respect to recent H0 studies using GWTC–2 events. However, we show that this result is strongly impacted by assumptions about the BBH source mass distribution; the only event which is not strongly impacted by such assumptions (and is thus informative about H0) is the well-localized event GW190814.Item Open Access Open Data from the Third Observing Run of LIGO, Virgo, KAGRA, and GEO(American Astronomical Society, 2023) Abbott, R.; Abe, H.; Acernese, F.; Ackley, K.; Adhicary, S.; Adhikari, N.; Adhikari, R.X.; Adkins, V.K.; Adya, V.B.; Affeldt, C.; Agarwal, D.; Agathos, M.; Aguiar, O.D.; Aiello, L.; Ain, A.; Ajith, P.; Akutsu, T.; Albanesi, S.; Alfaidi, R.A.; Al-Jodah, A.; et al.The global network of gravitational-wave observatories now includes five detectors, namely LIGO Hanford, LIGO Livingston, Virgo, KAGRA, and GEO 600. These detectors collected data during their third observing run, O3, composed of three phases: O3a starting in 2019 April and lasting six months, O3b starting in 2019 November and lasting five months, and O3GK starting in 2020 April and lasting two weeks. In this paper we describe these data and various other science products that can be freely accessed through the Gravitational Wave Open Science Center at https://gwosc.org. The main data set, consisting of the gravitational-wave strain time series that contains the astrophysical signals, is released together with supporting data useful for their analysis and documentation, tutorials, as well as analysis software packages.Item Open Access Feynman-Hellmann approach to transition matrix elements and quasidegenerate energy states(American Physical Society (APS), 2023) Batelaan, M.; Can, K.U.; Horsley, R.; Nakamura, Y.; Rakow, P.E.L.; Schierholz, G.; Stüben, H.; Young, R.D.; Zanotti, J.M.The Feynman-Hellmann approach to computing matrix elements in lattice QCD by first adding a perturbing operator to the action is described using the transition matrix and the Dyson expansion formalism. This perturbs the energies in the two-point baryon correlation function, from which the matrix element can be obtained. In particular at leading order in the perturbation we need to diagonalize a matrix of near-degenerate energies. While the method is general for all hadrons, we apply it here to a study of a sigma to nucleon baryon transition vector matrix element.Item Open Access Broadband Quantum Enhancement of the LIGO Detectors with Frequency-Dependent Squeezing(American Physical Society (APS), 2023) Ganapathy, D.; Jia, W.; Nakano, M.; Xu, V.; Aritomi, N.; Cullen, T.; Kijbunchoo, N.; Dwyer, S.E.; Mullavey, A.; McCuller, L.; Abbott, R.; Abouelfettouh, I.; Adhikari, R.X.; Ananyeva, A.; Appert, S.; Arai, K.; Aston, S.M.; Ball, M.; Ballmer, S.W.; Barker, D.; et al.Quantum noise imposes a fundamental limitation on the sensitivity of interferometric gravitationalwave detectors like LIGO, manifesting as shot noise and quantum radiation pressure noise. Here, we present the first realization of frequency-dependent squeezing in full-scale gravitational-wave detectors, resulting in the reduction of both shot noise and quantum radiation pressure noise, with broadband detector enhancement from tens of hertz to several kilohertz. In the LIGO Hanford detector, squeezing reduced the detector noise amplitude by a factor of 1.6 (4.0 dB) near 1 kHz; in the Livingston detector, the noise reduction was a factor of 1.9 (5.8 dB). These improvements directly impact LIGO’s scientific output for high-frequency sources (e.g., binary neutron star postmerger physics). The improved lowfrequency sensitivity, which boosted the detector range by 15%–18% with respect to no squeezing, corresponds to an increase in the astrophysical detection rate of up to 65%. Frequency-dependent squeezing was enabled by the addition of a 300-meter-long filter cavity to each detector as part of the LIGO Aþ upgrade.Item Open Access Searching for the causes of anomalous Advanced LIGO noise(AIP Publishing, 2023) Berger, B.K.; Areeda, J.S.; Barker, J.D.; Effler, A.; Goetz, E.; Helmling-Cornell, A.F.; Lantz, B.; Lundgren, A.P.; Macleod, D.M.; McIver, J.; Mittleman, R.; Nguyen, P.; Pele, A.; Pham, H.; Rangnekar, P.; Rink, K.; Schofield, R.M.S.; Smith, J.R.; Soni, S.; Warner, J.; et al.Advanced LIGO and Advanced Virgo have detected gravitational waves from astronomical sources to open a new window on the Universe. To explore this new realm requires an exquisite level of detector sensitivity, meaning that the much stronger signal from instrumental and environmental noise must be rejected. Selected examples of unwanted noise in Advanced LIGO are presented. The initial focus is on how the existence of this noise (characterized by particular frequencies or time intervals) was discovered. Then, a variety of methods are used to track down the source of the noise, e.g., a fault within the instruments or coupling from an external source. The ultimate goal of this effort is to mitigate the noise by either fixing equipment or by augmenting methods to suppress the coupling to the environment.Item Open Access Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb−¹ of ATLAS √s = 13 TeV proton–proton collision data(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Hoffman, A.C.A.; Acharya, B.S.; Achkar, B.; Bourdarios, C.A.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; Adye, T.; et al.Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the μ+μ− γ final state with a proton– proton collision data sample corresponding to an integrated luminosity of 139 fb−1 collected at √s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/ψ γ , ψ(2S) γ , and ϒ(1S, 2S, 3S) γ are found to be 2.0 × 10−4, 10.5×10−4, and (2.5, 4.2, 3.4)×10−4, respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 × 10−6, 2.4 × 10−6, and (1.1, 1.3, 2.4) × 10−6. An observed 95% CL interval of (−133, 175) is obtained for the κc/κγ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (−37, 40) is obtained for κb/κγ.Item Open Access Search in diphoton and dielectron final states for displaced production of Higgs or Z bosons with the ATLAS detector in √s =13 TeV pp collisions(American Physical Society, 2023) Amerl, M.; Filmer, E.K.; Jackson, P.; Kong, A.X.Y.; Potti, H.; Ruggeri, T.A.; Ting, E.X.L.; White, M.J.; Gingrich, D.M.; Lindon, J.H.; Nishu, N.; Pinfold, J.L.; Cakir, O.; Yildiz, H.D.; Kuday, S.; Turk Cakir, I.; Sultansoy, S.; Adam Bourdarios, C.; Berger, N.; Costanza, F.; et al.A search is presented for displaced production of Higgs bosons or Z bosons, originating from the decay of a neutral long-lived particle (LLP) and reconstructed in the decay modes H → γγ and Z → ee. The analysis uses the full Run 2 dataset of proton-proton collisions delivered by the LHC at an energy of ffiffi s p ¼ 13 TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb−1. Exploiting the capabilities of the ATLAS liquid argon calorimeter to precisely measure the arrival times and trajectories of electromagnetic objects, the analysis searches for the signature of pairs of photons or electrons which arise from a common displaced vertex and which arrive after some delay at the calorimeter. The results are interpreted in a gauge-mediated supersymmetry breaking model with pair-produced Higgsinos that decay to LLPs, and each LLP subsequently decays into either a Higgs boson or a Z boson. The final state includes at least two particles that escape direct detection, giving rise to missing transverse momentum. No significant excess is observed above the background expectation. The results are used to set upper limits on the cross section for Higgsino pair production, up to a χ˜ 0 1 mass of 369 (704) GeV for decays with 100% branching ratio of χ˜ 0 1 to Higgs (Z) bosons for a χ˜ 0 1 lifetime of 2 ns. A model-independent limit is also set on the production of pairs of photons or electrons with a significant delay in arrival at the calorimeter.Item Open Access Search for supersymmetry in final states with missing transverse momentum and three or more b-jets in 139 fb−1 of proton–proton collisions at √s = 13 TeV with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; Adye, T.; Affolder, A.A.; et al.A search for supersymmetry involving the pair production of gluinos decaying via off-shell third-generation squarks into the lightest neutralino (χ˜ 0 1 ) is reported. It exploits LHC proton–proton collision data at a centre-ofmass energy √s = 13 TeV with an integrated luminosity of 139 fb−1 collected with the ATLAS detector from 2015 to 2018. The search uses events containing large missing transverse momentum, up to one electron or muon, and several energetic jets, at least three of which must be identified as containing b-hadrons. Both a simple kinematic event selection and an event selection based upon a deep neuralnetwork are used. No significant excess above the predicted background is found. In simplified models involving the pair production of gluinos that decay via off-shell top (bottom) squarks, gluino masses less than 2.44 TeV (2.35 TeV) are excluded at 95% CL for a massless χ˜ 0 1 . Limits are also set on the gluino mass in models with variable branching ratios for gluino decays to bb¯χ˜ 0 1 , tt ¯χ˜ 0 1 and tb¯χ˜ − 1 /tb¯ χ˜ + 1 .Item Open Access Search for resonant W Z production in the fully leptonic final state in proton–proton collisions at √s = 13 TeV with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Hoffman, A.C.A.; Acharya, B.S.; Achkar, B.; Adam, L.; Bourdarios, C.A.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; et al.A search for a W Z resonance, in the fully leptonic final state (electrons or muons), is performed using 139 fb−1 of data collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. The results are interpreted in terms of a singly charged Higgs boson of the Georgi–Machacek model, produced by W Z fusion, and of a Heavy Vector Triplet, with the resonance produced by W Z fusion or the Drell–Yan process. No significant excess over the Standard Model prediction is observed and limits are set on the production cross-section times branching ratio as a function of the resonance mass for these processes.Item Open Access Search for pair-production of vector-like quarks in pp collision events at √s = 13 TeV with at least one leptonically decaying Z boson and a third-generation quark with the ATLAS detector(Elsevier BV, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D.K.; Abidi, S.H.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; et al.A search for the pair-production of vector-like quarks optimized for decays into a Z boson and a third-generation Standard Model quark is presented, using the full Run 2 dataset corresponding to 139 fb−1 of pp collisions at √s = 13 TeV, collected in 2015–2018 with the ATLAS detector at the Large Hadron Collider. The targeted final state is characterized by the presence of a Z boson with high transverse momentum, reconstructed from a pair of same-flavour leptons with opposite-sign charges, as well as by the presence of b-tagged jets and high-transverse-momentum large-radius jets reconstructed from calibrated smaller-radius jets. Events with exactly two or at least three leptons are used, which are further categorized by the presence of boosted W , Z, and Higgs bosons and top quarks. The categorization is performed using a neural-network-based boosted object tagger to enhance the sensitivity to signal relative to the background. No significant excess above the background expectation is observed and exclusion limits at 95% confidence level are set on the masses of the vector-like partners T and B of the top and bottom quarks, respectively. The limits depend on the branching ratio configurations and, in the case of 100% branching ratio for T → Z t and 100% branching ratio for B → Zb, this search sets the most stringent limits to date, allowing mT > 1.60 TeV and mB > 1.42 TeV, respectively.Item Open Access Search for pair-produced vector-like top and bottom partners in events with large missing transverse momentum in pp collisions with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; Adye, T.; Affolder, A.A.; et al.A search for pair-produced vector-like quarks using events with exactly one lepton (e or μ), at least four jets including at least one b-tagged jet, and large missing transverse momentum is presented. Data from proton–proton collisions at a centre-of-mass energy of √s =13 TeV, recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb−1, are analysed. Vector-like partners T and B of the top and bottom quarks are considered, as is a vector-like X with charge +5/3, assuming their decay into a W, Z, or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of T and B quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59 TeV assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47 TeV (1.46 TeV) for exclusive T → Zt (B/X → Wt) decays. In addition, lower limits on the T and B quark masses are derived for all possible branching ratios.Item Open Access Search for new phenomena in final states with photons, jets and missing transverse momentum in pp collisions at √s = 13 TeV with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D.K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; et al.A search for new phenomena has been performed in final states with at least one isolated high-momentum photon, jets and missing transverse momentum in proton– proton collisions at a centre-of-mass energy of √ s = 13 TeV. The data, collected by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb−1 . The experimental results are interpreted in a supersymmetric model in which pair-produced gluinos decay into neutralinos, which in turn decay into a gravitino, at least one photon, and jets. No significant deviations from the predictions of the Standard Model are observed. Upper limits are set on the visible cross section due to physics beyond the Standard Model, and lower limits are set on the masses of the gluinos and neutralinos, all at 95% confidence level. Visible cross sections greater than 0.022 fb are excluded and pair-produced gluinos with masses up to 2200 GeV are excluded for most of the NLSP masses investigated.Item Open Access Search for leptonic charge asymmetry in tt¯ W production in final states with three leptons at √s = 13 TeV(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; Adye, T.; Affolder, A.A.; et al.A search for the leptonic charge asymmetry (A `c) of top-quark-antiquark pair production in association with a W boson (ttW¯ ) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on √ s = 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015–2018, corresponding to an integrated luminosity of 139 fb−1 . A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be −0.12 ± 0.14 (stat.) ± 0.05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of −0.11 ± 0.17 (stat.) ± 0.05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are −0.084 +0.005 −0.003 (scale) ± 0.006 (MC stat.) and −0.063 +0.007 −0.004 (scale) ± 0.004 (MC stat.) respectively, and in agreement with the measurements.Item Open Access Search for high-mass W γ and Zγ resonances using hadronic W/Z boson decays from 139 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D.K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; et al.A search for high-mass charged and neutral bosons decaying to W γ and Zγ final states is presented in this paper. The analysis uses a data sample of √s = 13 TeV proton-proton collisions with an integrated luminosity of 139 fb−1 collected by the ATLAS detector during LHC Run 2 operation. The sensitivity of the search is determined using models of the production and decay of spin-1 charged bosons and spin-0/2 neutral bosons. The range of resonance masses explored extends from 1.0 TeV to 6.8 TeV. At these high resonance masses, it is beneficial to target the hadronic decays of the W and Z bosons because of their large branching fractions. The decay products of the high-momentum W/Z bosons are strongly collimated and boosted-boson tagging techniques are employed to improve the sensitivity. No evidence of a signal above the Standard Model backgrounds is observed, and upper limits on the production cross-sections of these bosons times their branching fractions to W γ and Zγ are derived for various boson production models.Item Open Access Search for heavy, long-lived, charged particles with large ionisation energy loss in pp collisions at √s = 13 TeV using the ATLAS experiment and the full Run 2 dataset(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D.K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; et al.This paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb−1 of proton–proton collisions at √s = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to O(1) ns with a mass, measured using the Bethe–Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.Item Open Access Search for Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in √s=13 TeV pp Collisions with the ATLAS Detector(American Physical Society (APS), 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abhayasinghe, D.K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; et al.A search for a long-lived, heavy neutral lepton (N ) in 139 fb−1 of √s = 13 TeV pp collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W → N μ or W → N e and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV < mN < 15 GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.Item Open Access Search for flavour-changing neutral-current couplings between the top quark and the photon with the ATLAS detector at √s = 13 TeV(Elsevier BV, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abed Abud, A.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; et al.This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly suppressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in pp collisions at √s = 13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb−1. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant excess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left- and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t → uγ ) < 0.85 (0.88+0.37 −0.25) × 10−5 and B(t → cγ ) < 4.2 (3.40+1.35 −0.95) × 10−5 for a left-handed tqγ coupling, and B(t → uγ ) < 1.2 (1.20+0.50 −0.33)×10−5 and B(t → cγ ) < 4.5 (3.70+1.47 −1.03)×10−5 for a right-handed coupling.Item Open Access Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb−1 of proton–proton collisions at √s = 13 TeV with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Hoffman, A.C.A.; Acharya, B.S.; Achkar, B.; Adam, L.; Bourdarios, C.A.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; et al.A search for pair production of doubly charged Higgs bosons (H±±), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb−1 recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, H±± → ±± where , = e, μ, τ , in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass m(H±±), at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, B(H±± → e±e±) =B(H±± → e±μ±) = B(H±± → μ±μ±) = B(H±± →e±τ ±) = B(H±± → μ±τ ±) = B(H±± → τ ±τ ±) = 1/6, the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the leftright symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee–Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of m(H±±) = 900 GeV (880 GeV).Item Open Access Search for displaced photons produced in exotic decays of the Higgs boson using 13 TeV pp collisions with the ATLAS detector(American Physical Society (APS), 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Abusleme Hoffman, A.C.; Acharya, B.S.; Achkar, B.; Adam, L.; Adam Bourdarios, C.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; et al.A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full run 2 dataset of proton-proton collisions delivered by the LHC at a center-of-mass energy of ffiffi s p ¼ 13 TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb−1. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.Item Open Access Search for dark matter produced in association with a single top quark and an energetic W boson in √s = 13 TeV pp collisions with the ATLAS detector(Springer Science and Business Media LLC, 2023) Aad, G.; Abbott, B.; Abbott, D.C.; Abeling, K.; Abidi, S.H.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Hoffman, A.C.A.; Acharya, B.S.; Achkar, B.; Adam, L.; Bourdarios, C.A.; Adamczyk, L.; Adamek, L.; Addepalli, S.V.; Adelman, J.; Adiguzel, A.; Adorni, S.; et al.This paper presents a search for dark matter, χ, using events with a single top quark and an energetic W boson. The analysis is based on proton–proton collision data collected with the ATLAS experiment at √s = 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb−1. The search considers final states with zero or one charged lepton (electron or muon), at least one b-jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended twoHiggs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, H±, arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a: H± → W±a(→ χχ). Signal models with H± masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a tan β value of 1. For masses of a of 150 (250) GeV, tan β values up to 2 are excluded for H± masses between 200 (400) GeV and 1.5 TeV. Signals with tan β values between 20 and 30 are excluded for H± masses between 500 and 800 GeV.