81 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

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 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 measurement of the top quark pair-production cross section in the lepton + jets decay channel is presented. It is based on 4.6 fb^-1of $$\sqrt{s}$$ = 7 TeV pp collision data collected during 2011 by the ATLAS experiment at the CERN Large Hadron Collider. A three-class, multidimensional event classifier based on support vector machines is used to differentiate $$t\bar{t}$$ events from backgrounds. The $$t\bar{t}$$ production cross section is found to be σ_$$t\bar{t}$$ = 168.5 ± 0.7(stat)$$^{+6.2}_{-5.9}$$(syst)$$^{+ 3.4}_{-3.2}$$(lumi) pb. In conclusion, the result is consistent with the Standard Model prediction based on QCD calculations at next-to-next-to-leading order.

We present measurements of cross sections for production of a leptonically decaying $$Z$$ boson in association with a large-radius jet in 13 TeV proton-proton collisions at the LHC, using 36 fb^–1 of data from the ATLAS detector. Integrated and differential cross sections are measured at particle level in both a flavor inclusive and a doubly $$b$$-tagged fiducial phase space. The large-radius jet mass and transverse momentum, its kinematic relationship to the Z boson, and the angular separation of $$b$$-tagged small-radius track jets within the large-radius jet are measured. This measurement constitutes an important test of perturbative quantum chromodynamics in kinematicmore » and flavor configurations relevant to several Higgs boson and beyond-Standard-Model physics analyses. The results highlight issues with modeling of additional hadronic activity in the flavor-inclusive selection, and a distinction between flavor-number schemes in the $$b$$-tagged phase space.« less

Herein 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 139fb^–1 of pp collisions at $$\sqrt{s}$$ = 13TeV, 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 exactlymore » 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→Zt and 100% branching ratio for B→Zb, this search sets the most stringent limits to date, allowing m_T>1.60TeV and m_B>1.42TeV, respectively.« less

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton–proton collision data at a centre-of-mass energy of 13 TeV, corresponding to 139 fb^–1 of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derivedmore » respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.« less

This article presents the results of two studies of Higgs boson properties using the WW^*(→eνμν)jj final state, based on a dataset corresponding to 36.1fb^-1 of $$\sqrt{s}$$ =13 TeV proton–proton collisions recorded by the ATLAS experiment at the Large Hadron Collider. The first study targets Higgs boson production via gluon–gluon fusion and constrains the CP properties of the effective Higgs–gluon interaction. Using angular distributions and the overall rate, a value of tan(α)=0.0±0.4(stat.)±0.3(syst.) is obtained for the tangent of the mixing angle for CP-even and CP-odd contributions. The second study exploits the vector-boson fusion production mechanism to probe the Higgs boson couplingsmore » to longitudinally and transversely polarised W and Z bosons in both the production and the decay of the Higgs boson; these couplings have not been directly constrained previously. The polarisation-dependent coupling-strength scale factors are defined as the ratios of the measured polarisation-dependent coupling strengths to those predicted by the Standard Model, and are determined using rate and kinematic information to be a_L=0.91$$^{+0.10}_{-0.18}$$(stat.)$$^{+0.09}_{-0.17}$$(syst.) and a_T=1.2±0.4(stat.)$$^{+0.2}_{-0.3}$$(syst.). These coupling strengths are translated into pseudo-observables, resulting in κ_VV=0.91$$^{+0.10}_{-0.18}$$(stat.)$$^{+0.09}_{-0.17}$$(syst.) and ϵ_VV=0.13$$^{+0.28}_{-0.20}$$ (stat.)$$^{+0.08}_{-0.10}$$(syst.). All results are consistent with the Standard Model predictions.« less

A search for long-lived charginos produced either directly or in the cascade decay of heavy prompt gluino states is presented. The search is based on proton–proton collision data collected at a centre-of-mass energy of $$\sqrt{s}$$ = 13 TeV between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 136 fb^-1. Long-lived charginos are characterised by a distinct signature of a short and then disappearing track, and are reconstructed using at least four measurements in the ATLAS pixel detector, with no subsequent measurements in the silicon-microstrip tracking volume nor any associated energy deposits inmore » the calorimeter. The final state is complemented by a large missing transverse-momentum requirement for triggering purposes and at least one high-transverse-momentum jet. No excess above the expected backgrounds is observed. Exclusion limits are set at 95% confidence level on the masses of the chargino and gluino for different chargino lifetimes. Chargino masses up to 660 (210) GeV are excluded in scenarios where the chargino is a pure wino (higgsino). For charginos produced during the cascade decay of a heavy gluino, gluinos with masses below 2.1 TeV are excluded for a chargino mass of 300 GeV and a lifetime of 0.2 ns.« less

This paper presents studies of Bose–Einstein correlations (BEC) in proton–proton collisions at a centre-of-mass energy of 13 TeV, using data from the ATLAS detector at the CERN Large Hadron Collider. Data were collected in a special low-luminosity configuration with a minimum-bias trigger and a high-multiplicity track trigger, accumulating integrated luminosities of 151 μb^-1 and 8.4 nb^-1, respectively. The BEC are measured for pairs of like-sign charged particles, each with |η|<2.5, for two kinematic ranges: the first with particle p_T>100 MeV and the second with particle p_T>500 MeV. The BEC parameters, characterizing the source radius and particle correlation strength, are investigatedmore » as functions of charged-particle multiplicity (up to 300) and average transverse momentum of the pair (up to 1.5 GeV). The double-differential dependence on charged-particle multiplicity and average transverse momentum of the pair is also studied. The BEC radius is found to be independent of the charged-particle multiplicity for high charged-particle multiplicity (above 100), confirming a previous observation at lower energy. This saturation occurs independent of the transverse momentum of the pair.« less