This paper reports a detailed study of techniques for identifying boosted, hadronically decaying
bosons using 20.3 fbFormula: see text of proton-proton collision data collected by the ATLAS detector ...at the LHC at a centre-of-mass energy Formula: see text. A range of techniques for optimising the signal jet mass resolution are combined with various jet substructure variables. The results of these studies in Monte Carlo simulations show that a simple pairwise combination of groomed jet mass and one substructure variable can provide a 50 % efficiency for identifying
bosons with transverse momenta larger than 200 GeV while maintaining multijet background efficiencies of 2-4 % for jets with the same transverse momentum. These signal and background efficiencies are confirmed in data for a selection of tagging techniques.
Combined analyses of the Higgs boson production and decay rates as well as its coupling strengths to vector bosons and fermions are presented. The combinations include the results of the analyses of ...the Formula: see text and Formula: see text decay modes, and the constraints on the associated production with a pair of top quarks and on the off-shell coupling strengths of the Higgs boson. The results are based on the LHC proton-proton collision datasets, with integrated luminosities of up to 4.7 Formula: see text at Formula: see text TeV and 20.3 Formula: see text at Formula: see text TeV, recorded by the ATLAS detector in 2011 and 2012. Combining all production modes and decay channels, the measured signal yield, normalised to the Standard Model expectation, is Formula: see text. The observed Higgs boson production and decay rates are interpreted in a leading-order coupling framework, exploring a wide range of benchmark coupling models both with and without assumptions on the Higgs boson width and on the Standard Model particle content in loop processes. The data are found to be compatible with the Standard Model expectations for a Higgs boson at a mass of 125.36 GeV for all models considered.
Distributions of transverse momentum Formula: see text and the related angular variable Formula: see text of Drell–Yan lepton pairs are measured in 20.3 fbFormula: see text of proton–proton ...collisions at Formula: see text TeV with the ATLAS detector at the LHC. Measurements in electron-pair and muon-pair final states are corrected for detector effects and combined. Compared to previous measurements in proton–proton collisions at Formula: see text TeV, these new measurements benefit from a larger data sample and improved control of systematic uncertainties. Measurements are performed in bins of lepton-pair mass above, around and below the
-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of Formula: see text the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of Formula: see text this is not the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of Formula: see text while the fixed-order prediction of Dynnlo falls below the data at high values of Formula: see text. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the Formula: see text and Formula: see text distributions as a function of lepton-pair mass and rapidity than the basic shape of the data.
A study of the decays Formula: see text and Formula: see text has been performed using data corresponding to an integrated luminosity of 25 fbFormula: see text of 7 and 8 TeV proton-proton collisions ...collected with the ATLAS detector during the LHC Run 1. For the Formula: see text dimuon decay, an upper limit on the branching fraction is set at Formula: see text at 95 % confidence level. For Formula: see text, the branching fraction Formula: see text is measured. The results are consistent with the Standard Model expectation with a
value of 4.8 %, corresponding to 2.0 standard deviations.
Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, Formula: see text system and event-level kinematic observables in ...proton-proton collisions at a centre-of-mass energy of Formula: see text. The observables have been chosen to emphasize the Formula: see text production process and to be sensitive to effects of initial- and final-state radiation, to the different parton distribution functions, and to non-resonant processes and higher-order corrections. The dataset corresponds to an integrated luminosity of 20.3 fbFormula: see text, recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one charged lepton and at least four jets with at least two of the jets tagged as originating from a
-quark. The measured spectra are corrected for detector effects and are compared to several Monte Carlo simulations. The results are in fair agreement with the predictions over a wide kinematic range. Nevertheless, most generators predict a harder top-quark transverse momentum distribution at high values than what is observed in the data. Predictions beyond NLO accuracy improve the agreement with data at high top-quark transverse momenta. Using the current settings and parton distribution functions, the rapidity distributions are not well modelled by any generator under consideration. However, the level of agreement is improved when more recent sets of parton distribution functions are used.
A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fbFormula: see text of ...proton-proton collision data at Formula: see text. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at Formula: see text confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the Formula: see text LEP limit to the top squark mass, lower limits between 490 and Formula: see text are placed on the top squark mass within the model considered.
The decays Formula: see text and Formula: see text are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fbFormula: see text of
...collisions collected at centre-of-mass energies Formula: see text TeV and 8 TeV, respectively. Signal candidates are identified through Formula: see text and Formula: see text decays. With a two-dimensional likelihood fit involving the Formula: see text reconstructed invariant mass and an angle between the Formula: see text and Formula: see text candidate momenta in the muon pair rest frame, the yields of Formula: see text and Formula: see text, and the transverse polarisation fraction in Formula: see text decay are measured. The transverse polarisation fraction is determined to be Formula: see text, and the derived ratio of the branching fractions of the two modes is Formula: see text, where the first error is statistical and the second is systematic. Finally, a sample of Formula: see text decays is used to derive the ratios of branching fractions Formula: see text and Formula: see text, where the third error corresponds to the uncertainty of the branching fraction of Formula: see text decay. The available theoretical predictions are generally consistent with the measurement.
A search has been made for supersymmetry in a final state containing two photons and missing transverse momentum using the ATLAS detector at the Large Hadron Collider. The search makes use of ...Formula: see text of proton-proton collision data collected at a centre-of-mass energy of 13 TeV in 2015. Using a combination of data-driven and Monte-Carlo-based approaches, the Standard Model background is estimated to be Formula: see text events. No events are observed in the signal region; considering the expected background and its uncertainty, this observation implies a model-independent 95 % CL upper limit of 0.93 fb (3.0 events) on the visible cross section due to physics beyond the Standard Model. In the context of a generalized model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle, this leads to a lower limit of 1650 GeV on the mass of a degenerate octet of gluino states, independent of the mass of the lighter bino-like neutralino.
Fiducial cross-sections for Formula: see text production with one or two additional
-jets are reported, using an integrated luminosity of 20.3 fbFormula: see text of proton-proton collisions at a ...centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for Formula: see text events with at least one additional
-jet is measured to be 950 Formula: see text 70 (stat.) Formula: see text (syst.) fb in the lepton-plus-jets channel and 50 Formula: see text 10 (stat.) Formula: see text (syst.) fb in the Formula: see text channel. The cross-section times branching ratio for events with at least two additional
-jets is measured to be 19.3 Formula: see text 3.5 (stat.) Formula: see text 5.7 (syst.) fb in the dilepton channel (Formula: see text, Formula: see text, and
) using a method based on tight selection criteria, and 13.5 Formula: see text 3.3 (stat.) Formula: see text 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 Formula: see text 0.33 (stat.) Formula: see text 0.28 (syst.)% for the ratio of Formula: see text production with two additional
-jets to Formula: see text production with any two additional jets. All measurements are in good agreement with recent theory predictions.
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