103 research outputs found

    Reconstruction of primary vertices in pppp collisions at energies of 900 GeV and 7 TeV with the ATLAS detector

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    The Large Hadron Collider (LHC) of the European Organisation for Nuclear Research (CERN) started its operation in Autumn of 2009. The initial run at a centre-of-mass energy of 900 GeV, has been followed by the on-going run at the energy of 7 TeV. While initially the probability of several proton-proton collisions to happen within the same bunch-crossing was approximately 10^-5, the level of the pile-up grows steadily with better focusing and squeezing of the LHC beams at collision point. Presented in this contribution is the performance of the primary vertex reconstruction algorithms used for analysis of the first collisions at the LHC. Different approaches used for the reconstruction of primary vertices in 900 GeV and 7 TeV collisions are presented. The efficiencies of the primary vertex reconstruction used for the first physics analyses of ATLAS are shown. The resolutions on positions of the reconstructed primary vertices are investigated by studying the distributions of pulls of distance between artificially created half-vertices. Implications of the ATLAS performancewith respect to primary vertex reconstruction for the on-going and future physics analyses are discussed.Comment: 5 Pages, 3 Figures, Presented at Hadron Collider Physics Symposium 2010, Toronto, Canad

    Simulation of Heavily Irradiated Silicon Pixel Sensors and Comparison with Test Beam Measurements

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    Charge collection measurements performed on heavily irradiated p-spray DOFZ pixel sensors with a grazing angle hadron beam provide a sensitive determination of the electric field within the detectors. The data are compared with a complete charge transport simulation of the sensor which includes signal trapping and charge induction effects. A linearly varying electric field based upon the standard picture of a constant type-inverted effective doping density is inconsistent with the data. A two-trap double junction model implemented in the ISE TCAD software can be tuned to produce a doubly-peaked electric field which describes the data reasonably well. The modeled field differs somewhat from previous determinations based upon the transient current technique. The model can also account for the level of charge trapping observed in the data.Comment: 8 pages, 11 figures. Talk presented at the 2004 IEEE Nuclear Science Symposium, October 18-21, Rome, Italy. Submitted to IEEE Transactions on Nuclear Scienc

    Search for dark matter produced in association with bottom or top quarks in ‚ąös = 13 TeV pp collisions with the ATLAS detector

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    A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb‚ąí1 of proton‚Äďproton collision data recorded by the ATLAS experiment at ‚ąös = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

    Measurements of top-quark pair differential cross-sections in the eőľe\mu channel in pppp collisions at s=13\sqrt{s} = 13 TeV using the ATLAS detector