Browsing by Author "Khalek, S. A."
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Item Measurement of the Underlying Event in Jet Events from 7 TeV Proton−Proton Collisions with the ATLAS Detector(Springer New York LLC, 2014-08-12) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. A.; Abdinov, O.; Aben, R.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouDistributions sensitive to the underlying event in QCD jet events have been measured with the ATLAS detector at the LHC, based on 37 pb⁻¹ of proton-proton collision data collected at a centre-of-mass energy of 7 TeV. Charged-particle mean p_T and densities of all-particle E_T and charged-particle multiplicity and p_T have been measured in regions azimuthally transverse to the hardest jet in each event. These are presented both as one-dimensional distributions and with their mean values as functions of the leading-jet transverse momentum from 20 to 800 GeV. The correlation of charged-particle mean p_T with charged-particle multiplicity is also studied, and the E_T densities include the forward rapidity region; these features provide extra data constraints for Monte Carlo modelling of colour reconnection and beam-remnant effects respectively. For the first time, underlying event observables have been computed separately for inclusive jet and exclusive dijet event selections, allowing more detailed study of the interplay of multiple partonic scattering and QCD radiation contributions to the underlying event. Comparisons to the predictions of different Monte Carlo models show a need for further model tuning, but the standard approach is found to generally reproduce the features of the underlying event in both types of event selection.Item Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in pp Collision Data at √s̅ = 7 TeV with the ATLAS Detector(American Physical Society, 2013-01-03) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. A.; Abdelalim, A. A.; Abdinov, O.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengResults of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at √s̅ = 7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4.6 fb(-1) are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates.Item Search for Direct Third-Generation Squark Pair Production in Final States with Missing Transverse Momentum and Two B-Jets in pp Collisions with the ATLAS Detector(Springer, 2013-10-29) Aad, G.; Abajyan, T.; Abbott, B.; Abdallah, J.; Khalek, S. A.; Abdinov, O.; Aben, R.; Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-Cheng; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); |Ahsan, Mahsana; Izen, Joseph M.; Lou, Xinchou; Reeves, Kendall; Wong, Wei-ChengThe results of a search for pair production of supersymmetric partners of the Standard Model third-generation quarks are reported. This search uses 20.1 fb-1of pp collisions at √s̅ = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. The lightest bottom and top squarks (b∼1 and t∼1 respectively) are searched for in a final state with large missing transverse momentum and two jets identified as originating from b-quarks. No excess of events above the expected level of Standard Model background is found. The results are used to set upper limits on the visible cross section for processes beyond the Standard Model. Exclusion limits at the 95% confidence level on the masses of the third-generation squarks are derived in phenomenological supersymmetric R-parityconserving models in which either the bottom or the top squark is the lightest squark. The b∼1is assumed to decay via b∼1→ bχ∼1 and the t∼1 via t∼1→ bχ∼ 1±, with undetectable products of the subsequent decay of the χ∼1± due to the small mass splitting between the χ∼1± and the χ∼0 1.