102 Search Results

A search for high-mass resonances decaying into a τ-lepton and a neutrino using proton-proton collisions at a center-of-mass energy of $$\sqrt{s}$$ =13 TeV is presented. The full run 2 data sample corresponding to an integrated luminosity of 139 fb^-1 recorded by the ATLAS experiment in the years 2015–2018 is analyzed. The τ-lepton is reconstructed in its hadronic decay modes and the total transverse momentum carried out by neutrinos is inferred from the reconstructed missing transverse momentum. The search for new physics is performed on the transverse mass between the τ-lepton and the missing transverse momentum. No excess of events abovemore » the Standard Model expectation is observed and upper exclusion limits are set on the W' →τ⁢v production cross section. Heavy W' vector bosons with masses up to 5.0 TeV are excluded at 95% confidence level, assuming that they have the same couplings as the Standard Model W boson. For nonuniversal couplings, W' bosons are excluded for masses less than 3.5–5.0 TeV, depending on the model parameters. In addition, model-independent limits on the visible cross section times branching ratio are determined as a function of the lower threshold on the transverse mass of the τ-lepton and missing transverse momentum.« less

The need for large-scale production of highly accurate simulated event samples for the extensive physics programme of the ATLAS experiment at the Large Hadron Collider motivates the development of new simulation techniques. Building on the recent success of deep learning algorithms, variational autoencoders and generative adversarial networks are investigated for modelling the response of the central region of the ATLAS electromagnetic calorimeter to photons of various energies. The properties of synthesised showers are compared with showers from a full detector simulation using GEANT4. Both variational autoencoders and generative adversarial networks are capable of quickly simulating electromagnetic showers with correct totalmore » energies and stochasticity, though the modelling of some shower shape distributions requires more refinement. This feasibility study demonstrates the potential of using such algorithms for ATLAS fast calorimeter simulation in the future and shows a possible way to complement current simulation techniques.« less

A search for quantum black holes in electron + jet and muon + jet invariant mass spectra is performed with 140 fb^-1 of data collected by the ATLAS detector in proton-proton collisions at $$\sqrt{s}$$ =13 TeV at the Large Hadron Collider. The observed invariant mass spectrum of lepton + jet pairs is consistent with Standard Model expectations. Upper limits are set at 95% confidence level on the production cross section times branching fractions for quantum black holes decaying into a lepton and a quark in a search region with invariant mass above 2.0 TeV. The resulting quantum black hole lowermore » mass threshold limit is 9.2 TeV in the Arkani-Hamed-Dimopoulos-Dvali model, and 6.8 TeV in the Randall-Sundrum model.« less

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2015-2018 at $$\sqrt{s}$$ = 13 TeV from pp collisions corresponding to an integrated luminosity of 139 fb^-1 are examined. Particles producing anomalously high ionization, consistent with long-lived spin-$$\frac{1}{2}$$ massive particles with electric charges from |q|=2e to |q|=7e are searched for. No statistically significant evidence of such particles is observed, and 95% confidence level cross-section upper limits are calculated and interpreted as the lower mass limits for a Drell-Yan plus photon-fusion production mode. The least stringent limit, 1060 GeV, is obtained formore » |q|=2e particles, and the most stringent one, 1600 GeV, is for |q|=6e particles.« less

This Letter reports the observation of single top quarks produced together with a photon, which directly probes the electroweak coupling of the top quark. The analysis uses 139 fb^-1 of 13 TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider. Requiring a photon with transverse momentum larger than 20 GeV and within the detector acceptance, the fiducial cross section is measured to be 688±23(stat) $$^{+75}_{-71}$$(syst) fb, to be compared with the standard model prediction of 515$$^{+36}_{-42}$$ fb at next-to-leading order in QCD.

In the version of this article initially published, the ATLAS Collaboration author names, affiliations and acknowledgements were omitted and have now been included in the HTML and PDF versions of the article.

This Letter reports the observation of τ-lepton pair production in ultraperipheral lead-lead collisions, Pb+Pb→Pb(γγ→ττ)Pb, and constraints on the τ-lepton anomalous magnetic moment, a_τ. The dataset corresponds to an integrated luminosity of 1.44 nb^-1 of LHC Pb+Pb collisions at $$\sqrt{s_{NN}}$$ =5.02 TeV recorded by the ATLAS experiment in 2018. Selected events contain one muon from a τ-lepton decay, an electron or charged-particle track(s) from the other τ-lepton decay, little additional central-detector activity, and no forward neutrons. The γγ→ττ process is observed in Pb+Pb collisions with a significance exceeding 5 standard deviations, and a signal strength of μ_ττ=1.03$$^{+0.06}_{-0.05}$$ assuming the Standard Modelmore » value for a_τ. To measure a_τ, a template fit to the muon transverse-momentum distribution from τ-lepton candidates is performed, using a dimuon (γγ→μμ) control sample to constrain systematic uncertainties. The observed 95% confidence-level interval for aτ is -0.057 < a_τ < 0.024.« less

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 $$\sqrt{s}=13$$ TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected back grounds. 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, andmore » (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 κ_c/κ_γ ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for κ_b/κ_γ.« less

A search for flavor-changing neutral-current couplings between a top quark, an up or charm quark, and a $$Z$$ boson is presented, using proton-proton collision data at $$\sqrt{s}$$ = 13 TeV collected by the ATLAS detector at the Large Hadron Collider. The analyzed data set corresponds to an integrated luminosity of 139 fb^-1. The search targets both single-top-quark events produced as $gq$ → $tZ$ (with $$q$$ = $u, c$) and top-quark-pair events, with one top quark decaying through the $$t$$ → $Zq$ channel. The analysis considers events with three leptons (electrons or muons), a $$b$$-tagged jet, possible additional jets, and missingmore » transverse momentum. The data are found to be consistent with the background-only hypothesis and 95% confidence-level limits on the $$t$$ → $Zq$ branching ratios, assuming only tensor operators of the Standard Model effective field theory framework contribute to the $tZq$ vertices. These are 6.2 x 10^-5 (13 x 10^-5) for $$t$$ → $Zu$ ($$t$$ → $Zc$) for a left-handed $tZq$ coupling, and 6.6 x 10^-5 (12 x 10^-5) in the case of a right-handed coupling. These results are interpreted as 95% CL upper limits on the strength of the corresponding couplings, yielding limits for |$${C}^{(13)*}_{uW}$$| and |$${C}^{(13)*}_{uB}$$| (|$${C}^{(31)}_{uW}$$| and |$${C}^{(31)}_{uB}$$|) of 0.15 (0.16), and limits for |$${C}^{(23)*}_{uW}$$| and |$${C}^{(23)*}_{uB}$$| (|$${C}^{(32)}_{uW}$$| and |$${C}^{(32)}_{uB}$$|) of 0.22 (0.21), assuming a new-physics energy scale Λ_NP of 1 TeV.« less

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 $$\sqrt{s}$$ = 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 125more » 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.« less