Browsing by Author "Ashan, Mahsana"
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Item Measurement of Dijet Cross-Sections in pp Collisions at 7 TeV Centre-of-Mass Energy Using the ATLAS Detector(Springer, 2014-05-014) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Ashan, Mahsana; Haleem, M.; Izen, Joseph M.; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, Xinchou; Wong, Wei-ChengDouble-differential dijet cross-sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and half the rapidity separation of the two highest-p₋T jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5 fb⁻¹, recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross-sections are presented at the particle level. Cross-sections are measured up to 5 TeV dijet mass using jets reconstructed with the anti-k₋t algorithm for values of the jet radius parameter of 0.4 and 0.6. The cross-sections are compared with next-to-leading-order perturbative QCD calculations by NLOJet++ corrected to account for non-perturbative effects. Comparisons with POWHEG predictions, using a next-to-leading-order matrix element calculation interfaced to a parton-shower Monte Carlo simulation, are also shown. Electroweak effects are accounted for in both cases. The quantitative comparison of data and theoretical predictions obtained using various parameterizations of the parton distribution functions is performed using a frequentist method. In general, good agreement with data is observed for the NLOJet++ theoretical predictions when using the CT10, NNPDF2.1 and MSTW 2008 PDF sets. Disagreement is observed when using the ABM11 and HERAPDF1.5 PDF sets for some ranges of dijet mass and half the rapidity separation. An example setting a lower limit on the compositeness scale for a model of contact interactions is presented, showing that the unfolded results can be used to constrain contributions to dijet production beyond that predicted by the Standard Model.Item Search for a Supersymmetric Partner to the Top Quark in Final States with Jets and Missing Transverse Momentum at √s̅ = 7TeV with the ATLAS Detector(American Physical Society, 2012-11-20) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; Ashan, Mahsana; 250674117 (Izen, JM); 250674117 (Izen, JM); Ashan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-chengA search for direct pair production of supersymmetric top squarks (t 1) is presented, assuming the t1 decays into a top quark and the lightest supersymmetric particle, χ10, and that both top quarks decay to purely hadronic final states. A total of 16 (4) events are observed compared to a predicted standard model background of 13.5-3.6+3.7(4.4-1.3+1.7) events in two signal regions based on Ldt=4.7fb-1 of pp collision data taken at √s=7TeV with the ATLAS detector at the LHC. An exclusion region in the t1 versus χ10 mass plane is evaluated: 370Item Search for Direct Top Squark Pair Production in Final States with One Isolated Lepton, Jets, and Missing Transverse Momentum in √s= 7 TeV pp Collisions Using 4.7 fb⁻¹ of ATLAS Data(American Physical Society) Aad, G.; Abbott, B.; Abdallah, J.; Abdinov, O.; Aben, R.; Abolins, M.; AbouZeid, O. S.|Ashan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-cheng; ATLAS Collaboration; 250674117 (Izen, JM); 250674117 (Izen, JM); Ashan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-chengA search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at √s=7 TeV. The measurement is based on 4.7 fb⁻¹ of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.Item Study of Heavy-Flavor Quarks Produced in Association with Top-Quark Pairs at √s̅=7 TeV using the ATLAS Detector(American Physical Society, 2014-04-21) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. Abdel; Abdelalim, A. A.; Abdinov, O.; Ashan, Mahsana; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; Izen, Joseph M.; Lou, Xinchou; 250674117 (Izen, JM); 250674117 (Izen, JM); Ashan, Mahsana; Namasivayam, Harisankar; Reeves, Kendall; Wong, Wei-Cheng; Izen, Joseph M.; Lou, XinchouUsing a sample of dilepton top-quark pair (tt̅) candidate events, a study is performed of the production of top-quark pairs together with heavy-flavor (HF) quarks, the sum of tt̅ + b + X and tt̅ + c + X, collectively referred to as tt̅ + HF. The data set used corresponds to an integrated luminosity of 4.7 fb⁻¹ of proton-proton collisions at a center-of-mass energy of 7 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF (b or c) quarks in the tt̅ sample is inferred by looking for events with at least three b-tagged jets, where two are attributed to the b quarks from the tt̅ decays and the third to additional HF production. The dominant background to tt̅ + HF in this sample is tt̅ + jet events in which a light-flavor jet is misidentified as a heavy-flavor jet. To determine the heavy-and light-flavor content of the additional b-tagged jets, a fit to the vertex mass distribution of b-tagged jets in the sample is performed. The result of the fit shows that 79 ± 14 (stat) ± 22 (syst) of the 105 selected extra b-tagged jets originate from HF quarks, 3 standard deviations away from the hypothesis of zero + HF production. The result for extra HF production is quoted as a ratio (R-HF) of the cross section for tt̅ + HF production to the cross section for tt̅ production with at least one additional jet. Both cross sections are measured in a fiducial kinematic region within the ATLAS acceptance. R-HF is measured to be [6.2 ± 1.1(stat) ± 1.8 (syst)]% for jets with p_T > 25 GeV and |η| < 2.5, in agreement with the expectations from Monte Carlo generators.