A search for the neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM) is reported. The analysis is performed on data from proton-proton collisions at a centre-of-mass ...energy of 8 TeV collected with the ATLAS detector at the Large Hadron Collider. The samples used for this search were collected in 2012 and correspond to integrated luminosities in the range 19.5-20.3 fb−1. The MSSM Higgs bosons are searched for in the τ τ final state. No significant excess over the expected background is observed, and exclusion limits are derived for the production cross section times branching fraction of a scalar particle as a function of its mass. The results are also interpreted in the MSSM parameter space for various benchmark scenarios.
This thesis describes two measurement of the tt¯ production cross-section, the inclusive σtt¯j and σtt¯j in association with at least one jet in lepton + jets channel with ATLAS detector. The ...measurements employ multivariate technique with a binned maximum likelihood template fit. Several kinematic variables of the reconstructed collision events are selected to discriminate tt¯ signal events from the various background sources. The inclusive σtt¯ measurement uses approximately 35.3 pb−1 of ATLAS data collected in 2010 and selecting events with one isolated high-pT electron or muon, large missing transverse energy and two jet bins of 3 or at least 4 high-pT jets. The measured tt¯ cross section assuming the top quark mass of 172.5 GeV yields: special characters omitted The measured value is in a good agreement with the Standard Model prediction. The second measurement is performed on ATLAS data collected in 2011 corresponding to an integrated luminosity of 4.7 fb−1. The same analysis technique and event selection employed as the first measurement but in 4th and 5th jet bins. The cross-section for σtt¯ with at least one additional jet is found to be: special characters omitted Finally the results of study on PiN diodes radiation hardness are presented. Results were performed with several irradiation beams to investigate the proper optical components for ATLAS Pixel upgrade.
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at sNN−−−√=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The ...anisotropy parameter, v2 , was measured in the pseudorapidity range |η|≤2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT , thus reducing the uncertainty in v2 integrated over pT , a 1 μb−1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v2 is compared to other measurements obtained with higher pT thresholds. The integrated elliptic flow is weakly decreasing with |η| . The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.
This paper presents a search for the t-channel exchange of an R-parity violating scalar top quark (tilde-(t)) in the e{sup {+-}} {mu}{sup {+-}} continuum using 2.1 fb{sup -1} of data collected by the ...ATLAS detector in {radical}s=7 TeV pp collisions at the Large Hadron Collider. Data are found to be consistent with the expectation from the Standard Model backgrounds. Limits on R-parity-violating couplings at 95% C.L. are calculated as a function of the scalar top mass (m{sub tilde-(t)}). The upper limits on the production cross section for pp{yields}e{mu}X, through the t-channel exchange of a scalar top quark, ranges from 170 fb for m{sub tilde-(t)}=95 GeV to 30 fb for m{sub tilde (t)}=1000 GeV.
The ATLAS detector is used to search for high-mass states, such as heavy charged gauge bosons (W{prime},W*), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented ...based on the analysis of ppcollisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 36 pb{sup -1}. No excess beyond standard model expectations is observed. A W{prime} with sequential standard model couplings is excluded at 95% confidence level for masses below 1.49 TeV, and a W* (charged chiral boson) for masses below 1.35 TeV.
A search for resonances produced in 7 TeV proton-proton collisions and decaying into top-quark pairs is described. In this Letter events where the top-quark decay produces two massive jets with large ...transverse momenta recorded with the ATLAS detector at the Large Hadron Collider are considered. Two techniques that rely on jet substructure are used to separate top-quark jets from those arising from light quarks and gluons. In addition, each massive jet is required to have evidence of an associated bottom-quark decay. The data are consistent with the Standard Model, and limits can be set on the production cross section times branching fraction of a Z' boson and a Kaluza-Klein gluon resonance. These limits exclude, at the 95% credibility level, Z' bosons with masses 0.70-1.00 TeV as well as 1.28-1.32 TeV and Kaluza-Klein gluons with masses 0.70-1.62 TeV.
A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is performed in p p collisions at root s = 7 TeV using 4.7 fb(-1) of data collected with the ATLAS ...experiment at the LHC. In these models, the lightest chargino is predicted to have a lifetime long enough to be detected in the tracking detectors of collider experiments. This analysis explores such models by searching for chargino decays that result in tracks with few associated hits in the outer region of the tracking system. The transverse-momentum spectrum of candidate tracks is found to be consistent with the expectation from the Standard Model background processes and constraints on chargino properties are obtained.
The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at root s = 7 TeV. The full data set ...collected in 2011, corresponding to an integrated luminosity of 4.9 fb(-1), is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (vertical bar eta vertical bar < 1.37 and 1.52 < vertical bar eta vertical bar 2.37) and with an angular separation Delta R > 0.4, is 44.0(-4.2)(+3.2) pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins-Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.
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