Identification of high transverse momentum top quarks in pp collisions at √s = 8 TeV with the ATLAS detector

This paper presents studies of the performance of several jet-substructure techniques, which are used to identify hadronically decaying top quarks with high transverse momentum contained in large-radius jets. The efficiency of identifying top quarks is measured using a sample of top-quark pairs and the rate of wrongly identifying jets from other quarks or gluons as top quarks is measured using multijet events collected with the ATLAS experiment in 20.3 fb−1 of 8 TeV proton-proton collisions at the Large Hadron Collider. Predictions from Monte Carlo simulations are found to provide an accurate description of the performance. The techniques are compared in terms of signal efficiency and background rejection using simulations, covering a larger range in jet transverse momenta than accessible in the dataset. Additionally, a novel technique is developed that is optimized to reconstruct top quarks in events with many jets

Searches for Higgs boson pair production in the hh → bbττ, γγWW*, γγbb, bbbb channels with the ATLAS detector

We present the calculation of the hyperon forward spin polarizability γ0 using manifestly Lorentz-covariant baryon chiral perturbation theory including the intermediate contribution of the spin-3/2 states. As at the considered order the extraction of γ0 is a pure prediction of chiral perturbation theory, the obtained values are a good test for this theory. After including explicitly the decuplet states, our SU(2) results have a very good agreement with the experimental data and we extend our framework to SU(3) to give predictions for the hyperons’ γ0 values. Prominent are the Σ− and Ξ− baryons as their photon transition to the decuplet is forbidden in SU(3) symmetry and therefore they are not sensitive to the explicit inclusion of the decuplet in the theory

Measurement of the production of neighbouring jets in lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector

This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at View the MathML source using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 nb−1 of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, RΔR, of “neighbouring” jets that accompany “test” jets within a given range of angular distance, ΔR , in the pseudorapidity–azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-kt algorithm with radius parameters d=0.2, 0.3, and 0.4. RΔR was measured in different Pb+Pb collision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. A centrality dependence of RΔR is observed for all three jet radii with RΔR found to be lower in central collisions than in peripheral collisions. The ratios formed by the RΔR values in different centrality bins and the values in the 40–80% centrality bin are presented

Search for the Standard Model Higgs boson decaying into bb¯ produced in association with top quarks decaying hadronically in pp collisions at √s = 8 TeV with the ATLAS detector

A search for Higgs boson production in association with a pair of top quarks (tt¯H) is performed, where the Higgs boson decays to bb¯¯, and both top quarks decay hadronically. The data used correspond to an integrated luminosity of 20.3 fb−1 of pp collisions at s√=8 TeV collected with the ATLAS detector at the Large Hadron Collider. The search selects events with at least six energetic jets and uses a boosted decision tree algorithm to discriminate between signal and Standard Model background. The dominant multijet background is estimated using a dedicated data-driven technique. For a Higgs boson mass of 125 GeV, an upper limit of 6.4 (5.4) times the Standard Model cross section is observed (expected) at 95% confidence level. The best-fit value for the signal strength is μ = 1.6 ± 2.6 times the Standard Model expectation for mH = 125 GeV. Combining all tt¯H searches carried out by ATLAS at s√=8 and 7 TeV, an observed (expected) upper limit of 3.1 (1.4) times the Standard Model expectation is obtained at 95% confidence level, with a signal strength μ = 1.7 ± 0.8

Performance of the ATLAS trigger system in 2015

During 2015 the ATLAS experiment recorded 3.8 fb−1 of proton–proton collision data at a centre-of-mass energy of 13 TeV. The ATLAS trigger system is a crucial component of the experiment, responsible for selecting events of interest at a recording rate of approximately 1 kHz from up to 40 MHz of collisions. This paper presents a short overview of the changes to the trigger and data acquisition systems during the first long shutdown of the LHC and shows the performance of the trigger system and its components based on the 2015 proton–proton collision dat

Search for the Standard Model Higgs boson produced in association with top quarks and decaying into bb in pp collisions at √s = 8 TeV with the ATLAS detector

A search for the Standard Model Higgs boson produced in association with a top-quark pair, t ¯tH, is presented. The analysis uses 20.3 fb−1 of pp collision data at √s = 8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H b ¯b decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by t ¯t+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible t ¯t+b ¯b background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95% confidence level. The ratio of the measured t ¯tH signal cross section to the Standard Model expectation is found to be μ = 1.5±1.1 assuming a Higgs boson mass of 125 GeV

Search for dark matter in association with a Higgs boson decaying to b-quarks in pp collisions at √s=13 TeV with the ATLAS detector

A search for dark matter pair production in association with a Higgs boson decaying to a pair of bottom quarks is presented, using 3.2 fb−1 of pp collisions at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. The decay of the Higgs boson is reconstructed as a high-momentum View the MathML sourcebb¯ system with either a pair of small-radius jets, or a single large-radius jet with substructure. The observed data are found to be consistent with the expected backgrounds. Results are interpreted using a simplified model with a Z′Z′ gauge boson mediating the interaction between dark matter and the Standard Model as well as a two-Higgs-doublet model containing an additional Z′Z′ boson which decays to a Standard Model Higgs boson and a new pseudoscalar Higgs boson, the latter decaying into a pair of dark matter particles

Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at √s = 8 TeV with the ATLAS detector

A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T), the decay targeted is T → Zt, while the decay targeted for a new charge −1/3 quark (B) is B → Zb. The search is performed with a dataset corresponding to 20.3 fb−1 of pp collisions at √ s = 8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters

Search for lepton-flavour-violating H → μτ decays of the Higgs boson with the ATLAS detector

A direct search for lepton-flavour-violating H → μτ decays of the recently discovered Higgs boson with the ATLAS detector at the LHC is presented. The analysis is performed in the H → μτ had channel, where τ had is a hadronically decaying τ -lepton. The search is based on the data sample of proton-proton collisions collected by the ATLAS experiment corresponding to an integrated luminosity of 20.3 fb−1 at a centre-of-mass energy of s√=8s=8 TeV. No statistically significant excess of data over the predicted background is observed. The observed (expected) 95% confidence-level upper limit on the branching fraction, Br(H → μτ ), is 1.85% (1.24%)