Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in √s=8 TeV pp collisions with the ATLAS detector

A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb⁻¹ of LHC proton-proton collision data at √s=8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be −0.8 ± 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis

Search for pair production of Higgs bosons in the bbbb final state using proton--proton collisions at √s = 13 TeV with the ATLAS detector

A search for Higgs-boson pair production in the bbbb final state is carried out with 3.2 fb⁻¹ of proton--proton collision data collected at √s=13 TeV with the ATLAS detector. The data are consistent with the estimated background and are used to set upper limits on the production cross section of Higgs-boson pairs times branching ratio to bbbb for both nonresonant and resonant production. In the case of resonant production of Kaluza--Klein gravitons within the Randall--Sundrum model, upper limits in the 24 to 91 fb range are obtained for masses between 600 and 3000 GeV, at the 95% confidence level. The production cross section times branching ratio for nonresonant Higgs-boson pairs is also constrained to be less than 1.22 pb, at the 95% confidence level

Measurement of the bb dijet cross section in pp collisions at √s = 7 TeV with the ATLAS detector

The dijet production cross section for jets containing a b-hadron (b-jets) has been measured in proton–proton collisions with a centre-of-mass energy of √s =7 TeV, using the ATLAS detector at the LHC. The data used correspond to an integrated luminosity of 4.2fb⁻¹. The cross section is measured for events with two identified b-jets with a transverse momentum pT>20GeV and a minimum separation in the η–ϕ plane of ΔR=0.4. At least one of the jets in the event is required to have p T >270 GeV. The cross section is measured differentially as a function of dijet invariant mass, dijet transverse momentum, boost of the dijet system, and the rapidity difference, azimuthal angle and angular distance between the b-jets. The results are compared to different predictions of leading order and next-to-leading order perturbative quantum chromodynamics matrix elements supplemented with models for parton-showers and hadronization

Search for high-mass new phenomena in the dilepton final state using proton-proton collisions at √s=13 TeV with the ATLAS detector

A search is conducted for both resonant and non-resonant high-mass new phenomena in dielectron and dimuon final states. The search uses 3.2 fb⁻¹ of proton-proton collision data, collected at √s=13 TeV by the ATLAS experiment at the LHC in 2015. The dilepton invariant mass is used as the discriminating variable. No significant deviation from the Standard Model prediction is observed; therefore limits are set on the signal model parameters of interest at 95% credibility level. Upper limits are set on the cross-section times branching ratio for resonances decaying to dileptons, and the limits are converted into lower limits on the resonance mass, ranging between 2.74 TeV and 3.36 TeV, depending on the model. Lower limits on the ℓℓqq contact interaction scale are set between 16.7 TeV and 25.2 TeV, also depending on the model

Transverse momentum, rapidity, and centrality dependence of inclusive charged-particle production in √sNN=5.02 TeV p + Pb collisions measured by the ATLAS experiment

Measurements of the per-event charged-particle yield as a function of the charged-particle transverse momentum and rapidity are performed using p+ Pb collision data collected by the ATLAS experiment at the LHC at a centre-of-mass energy of √sNN=5.02 TeV. Charged particles are reconstructed over pseudorapidity |η|< 2.3 and transverse momentum between 0.1 GeV and 22 GeV in a dataset corresponding to an integrated luminosity of 1 μb⁻¹. The results are presented in the form of charged-particle nuclear modification factors, where the p+ Pb charged-particle multiplicities are compared between central and peripheral p+ Pb collisions as well as to charged-particle cross sections measured in pp collisions. The p+ Pb collision centrality is characterized by the total transverse energy measured in −4.9 < η < −3.1 , which is in the direction of the outgoing lead beam. Three different estimations of the number of nucleons participating in the p+ Pb collision are carried out using the Glauber model and two Glauber-Gribov colour-fluctuation extensions to the Glauber model. The values of the nuclear modification factors are found to vary significantly as a function of rapidity and transverse momentum. A broad peak is observed for all centralities and rapidities in the nuclear modification factors for charged-particle transverse momentum values around 3 GeV. The magnitude of the peak increases for more central collisions as well as rapidity ranges closer to the direction of the outgoing lead nucleus

Dark matter interpretations of ATLAS searches for the electroweak production of supersymmetric particles in √s=8 TeV proton-proton collisions

A selection of searches by the ATLAS experiment at the LHC for the electroweak production of SUSY particles are used to study their impact on the constraints on dark matter candidates. The searches use 20fb⁻¹ of proton-proton collision data at √s=8 TeV. A likelihood-driven scan of a five-dimensional effective model focusing on the gaugino--higgsino and Higgs sector of the phenomenological minimal supersymmetric Standard Model is performed. This scan uses data from direct dark matter detection experiments, the relic dark matter density and precision flavour physics results. Further constraints from the ATLAS Higgs mass measurement and SUSY searches at LEP are also applied. A subset of models selected from this scan are used to assess the impact of the selected ATLAS searches in this five-dimensional parameter space. These ATLAS searches substantially impact those models for which the mass m(χ0/1) of the lightest neutralino is less than 65 GeV, excluding 86% of such models. The searches have limited impact on models with larger m(χ0/1) due to either heavy electroweakinos or compressed mass spectra where the mass splittings between the produced particles and the lightest supersymmetric particle is small

Measurement of the transverse momentum and φ *η distributions of Drell–Yan lepton pairs in proton–proton collisions at √s=8 TeV with the ATLAS detector

Distributions of transverse momentum p ll T and the angular variable ϕ ∗ η of Drell--Yan lepton pairs are measured in 20.3 fb⁻¹ of proton--proton collisions at √s=8 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 √s=7 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 Z-boson mass peak. The data are compared to predictions from perturbative and resummed QCD calculations. For values of ϕ ∗ η <1 the predictions from the Monte Carlo generator ResBos are generally consistent with the data within the theoretical uncertainties. However, at larger values of ϕ ∗ η this is not generally the case. Monte Carlo generators based on the parton-shower approach are unable to describe the data over the full range of p ll T and the fixed-order prediction of DYNNLO falls below the data at high values of p ll T. ResBos and the parton-shower Monte Carlo generators provide a much better description of the evolution of the ϕ ∗ η and p ll T distributions as a function of lepton-pair mass and rapidity

Measurements of top-quark pair differential cross-sections in the lepton+jets channel in pp collisions at √s=8 TeV using the ATLAS detector

Measurements of normalized differential cross-sections of top-quark pair production are presented as a function of the top-quark, tt system and event-level kinematic observables in proton-proton collisions at a centre-of-mass energy of √s=8 TeV. The observables have been chosen to emphasize the tt 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 fb⁻¹, 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 b-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

Search for charged Higgs bosons in the H± → tb decay channel in pp collisions at √s=8 TeV using the ATLAS detector

Charged Higgs bosons heavier than the top quark and decaying via H ± →tb are searched for in proton--proton collisions measured with the ATLAS experiment at √s=8 TeV corresponding to an integrated luminosity of 20.3 fb⁻¹. The production of a charged Higgs boson in association with a top quark, gb→tH ±, is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background-only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the gb→tH ± production cross section times the branching fraction BR(H ± →tb) . Additionally, the complementary s-channel production, qq ′ →H±, is investigated through a reinterpretation of W ′ →tb searches in ATLAS. Final states with one electron or muon are relevant for H ± masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the qq ′ →H ± production cross section times the branching fraction BR(H ± →tb)