The ATLAS detector at the Large Hadron Collider is used to search for high-mass resonances decaying to dielectron or dimuon final states. Results are presented from an analysis of proton-proton (pp) collisions at a center-of-mass energy of 8 TeV corresponding to an integrated luminosity of 20.3 fb −1 in the dimuon channel. A narrow resonance with Standard Model Z couplings to fermions is excluded at 95% confidence level for masses less than 2.79 TeV in the dielectron channel, 2.53 TeV in the dimuon channel, and 2.90 TeV in the two channels combined. Limits on other model interpretations are also presented, including a grand-unification model based on the E 6 gauge group, Z Ã bosons, minimal Z 0 models, a spin-2 graviton excitation from Randall-Sundrum models, quantum black holes, and a minimal walking technicolor model with a composite Higgs boson.

Using 1:8 fb À1 of pp collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Çð1S; 2S; 3SÞ mesons. Ç mesons are reconstructed using the dimuon decay mode. Total production cross sections for p T < 70 GeV and in the rapidity interval jy Ç j <

The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb −1 , collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, f (D) DP , has been measured through the p T balance between the two jets and amounts to f (D) DP = 0.08 ± 0.01 (stat.) ± 0.02 (sys.) for jets with transverse momentum p T > 20 GeV and rapidity |y| < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of σ eff = 15 ± 3 (stat.) +5 −3 (sys.) mb.

This paper presents a summary of beam-induced backgrounds observed in the ATLAS detector and discusses methods to tag and remove background contaminated events in data. Triggerrate based monitoring of beam-related backgrounds is presented. The correlations of backgrounds with machine conditions, such as residual pressure in the beam-pipe, are discussed. Results from dedicated beam-background simulations are shown, and their qualitative agreement with data is evaluated. Data taken during the passage of unpaired, i.e. non-colliding, proton bunches is used to obtain background-enriched data samples. These are used to identify characteristic features of beam-induced backgrounds, which then are exploited to develop dedicated background tagging tools. These tools, based on observables in the Pixel detector, the muon spectrometer and the calorimeters, are described in detail and their efficiencies are evaluated. Finally an example of an application of these techniques to a monojet analysis is given, which demonstrates the importance of such event cleaning techniques for some new physics searches.

Abstract:The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy √ s = 8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb −1 . No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c ) of approximately 30. Keywords: Hadron-Hadron Scattering JHEP04(2015)116The ATLAS collaboration 58 IntroductionSupersymmetry (SUSY) [1][2][3][4][5][6][7][8][9] postulates the existence of particles (sparticles) which differ by half a unit of spin from their Standard Model (SM) partners. The squarks (q L andq R ) and sleptons (˜ L and˜ R ) are the scalar partners of the left-handed and right-handed quarks and leptons, the gluinos (g) are the fermionic partners of the gluons, and the charginos (χ ± i with i = 1, 2) and neutralinos (χ 0 i with i = 1, 2, 3, 4) are the mass eigenstates (ordered from the lightest to the heaviest) formed from the linear superpositions of the SUSY partners of the Higgs and electroweak gauge bosons. An attractive feature of SUSY is that it can solve the SM hierarchy problem [10][11][12][13][14][15] if the gluino, higgsino and top squark masses are not much higher than the TeV scale.If strongly interacting sparticles exist at the TeV scale, they should be accessible at the Large Hadron Collider (LHC). In the minimal supersymmetric extension of the SM such particles decay into jets, possibly leptons, and the lightest sparticle (LSP). If the LSP is stable owing to R-parity conservation [15][16][17][18][19] and only weakly interacting, it escapes detection, leading to missing transverse momentum (p miss T and its magnitude E miss T ) in the final state. In this scenario, the LSP can be a dark-matter candidate. Significant E miss T can also arise in R-parity-violating scenarios in which the LSP decays to final states containing neutrinos or in scenarios where neutrinos are present in the cascade decay chains of the produced sparticles.This paper presents a search with the ATLAS detector [20,21] for SUSY in final states containing jets, at least one isolated lepton (electron or muon) and large E miss T . Different search channels are used in order to cover a broad parameter space: the events are selected by different requirements on the transverse momentum (p T ) of the leptons, either using low-p T leptons (referred to as the "soft" lepton selection), or high-p T leptons (referred to as the "hard" lepton selection). Each of these categories is further subdivided i...