Searches for exclusive Higgs and Z boson decays into J/ψγ,ψ(2S)γ,and Υ(nS)γ at √s=13 TeV with the ATLAS detector

Searches for the exclusive decays of the Higgs and Z bosons into a J/ψ,ψ(2S), or Υ(nS)(n=1,2,3) meson and a photon are performed with a pp collision data sample corresponding to an integrated luminosity of 36.1 fb −1 collected at √s =13 TeV with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above the expected backgrounds, and 95% confidence-level upper limits on the branching fractions of the Higgs boson decays to J/ψγ, ψ(2S)γ,and Υ(nS)γ of 3.5×10 −4, 2.0×10−3,and(4.9,5.9,5.7)×10 −4,respectively, are obtained assuming Standard Model production. The corresponding 95% confidence-level upper limits for the branching fractions of the Z boson decays are 2.3×10 −6, 4.5×10 −6 and (2.8,1.7,4.8)×10 −6, respectively

Search for vector-boson resonances decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at √s=13TeV with the ATLAS detector

A search for new charged massive gauge bosons, W_, is performed with the ATLAS detector at the LHC. Data were collected in proton–proton collisions at a center-of-mass energy of √s=13TeVand correspond to an integrated luminosity of 36.1fb−1. This analysis searches for W_bosons in the W_→t¯bdecay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0TeVand considers right-handed W_bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the W_→t¯bcross section times branching ratio and the W_boson effective couplings as a function of the W_boson mass. For right-handed W_bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15TeVare excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for W_→t¯bin the fully hadronic final state. Using the combined searches, right-handed W_bosons with masses below 3.25TeVare excluded at the 95% confidence level

Search for pair and single production of vectorlike quarks in final states with at least one Z boson decaying into a pair of electrons or muons in pp collision data collected with the ATLAS detector at √s=13 TeV

A search for vectorlike quarks is presented, which targets their decay into a Z boson and a third-generation Standard Model quark. In the case of a vectorlike quark T (B) with charge +2/3e (−1/3e), the decay searched for is T→Zt (B→Zb). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1  fb−1 of pp collisions at √s=13  TeV. The final state used is characterized by the presence of b-tagged jets, as well as a Z boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as—in the case of the single-production selections—the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of mT>1030  GeV (mT>1210  GeV) and mB>1010  GeV (mB>1140  GeV) in the singlet (doublet) model. In the case of 100% branching ratio for T→Zt (B→Zb), the limits are mT>1340  GeV (mB>1220  GeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses

Prompt and non-prompt J/ψ elliptic flow in Pb+Pb collisions at √sNN =5.02 TeV with the ATLAS detector

The elliptic flow of prompt and non-prompt J/ψ was measured in the dimuon decay channel in Pb+Pb collisions at √sNN = 5.02 TeV with an integrated luminosity of 0.42 nb −1 with the ATLAS detector at the LHC. The prompt and non-prompt signals are separated using a two-dimensional simultaneous fit of the invariant mass and pseudo-proper decay time of the dimuon system from the J/ψ decay. The measurement is performed in the kinematic range of dimuon transverse momentum and rapidity 9<p T <30 GeV,|y|<2 , and 0-60% collision centrality. The elliptic flow coefficient,v2, is evaluated relative to the event plane and the results are presented as a function of transverse momentum, rapidity and centrality. It is found that prompt and non-prompt J/ψ mesons have non-zero elliptic flow. Prompt J/ψ v 2 decreases as a function of p T , while non-prompt J/ψ it is, with limited statistical significance, consistent with a flat behaviour over the studied kinematic region. There is no observed dependence on rapidity or centrality

Search for Higgs boson decays into a pair of light bosons in the bbμμ final state in pp collision at √s=13 TeV with the ATLAS detector

A search for decays of the Higgs boson into a pair of new spin-zero particles, H→aa, where the a-bosons decay into a b-quark pair and a muon pair, is presented. The search uses 36.1fb−1of proton–proton collision data at √s=13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% confidence level are placed on the branching ratio (σH/σSM) ×B(H→aa →bbμμ), ranging from 1.2 ×10−4to 8.4 ×10−4in the a-boson mass range of 20–60GeV. Model-independent limits are set on the visible production cross-section times the branching ratio to the bbμμ final state for new physics, σvis(X) ×B(X→bbμμ), ranging from 0.1fb to 0.73fb for mμμ between 18 and 62GeV

Observation of centrality-dependent acoplanarity for muon pairs produced via two-photon scattering in Pb+Pb collisions at √sNN=5.02 TeV with the ATLAS detector

This Letter presents a measurement of γγ→μ+μ− production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at √sNN=5.02 TeV with an integrated luminosity of 0.49 nb−1. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision

Measurement of photon–jet transverse momentum correlations in 5.02TeV Pb+Pb and pp collisions with ATLAS

For abstract see published article

Search for charged Higgs bosons decaying via H± → τ±ντ in the τ+jets and τ+lepton final states with 36 fb−1 of pp collision data recorded at √s = 13 TeV with the ATLAS experiment

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 36.1 fb−1 of proton-proton collision data at √ s = 13 TeV recorded with the ATLAS detector. Depending on whether the top-quark produced together with H± decays hadronically or leptonically, the search targets τ+jets and τ+lepton final states, in both cases with a hadronically decaying τ -lepton. No evidence of a charged Higgs boson is found. For the mass range of mH± = 90–2000 GeV, upper limits at the 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 4.2–0.0025 pb. In the mass range 90–160 GeV, assuming the Standard Model cross-section for tt¯production, this corresponds to upper limits between 0.25% and 0.031% for the branching fraction B(t → bH±) × B(H± → τ ±ντ )