A Search for First Generation Leptoquarks at the ATLAS detector
Caputo, Regina Marie
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Similarities between quarks and leptons, which are elementary particles, suggest an additional symmetry or communication between the two families. Leptoquarks are hypothetical particles that carry both lepton and baryon number and would represent this additional symmetry. They are proposed to exist in several extensions to the Standard Model such as Grand Unification Theories (GUTs) and technicolor models. This work reports on the search for first generation scalar leptoquarks at the ATLAS detector using an integrated luminosity of 35 pb<super>-1</super> collected during the 2010 LHC running. Leptoquarks are produced in pairs and each leptoquark decays into a lepton/quark pair. One resulting event topology is two high energy jets, one high energy electron and missing transverse energy arising from a neutrino. The background, predominantly from associated production of vector bosons with jets and top quarks, is estimated using Standard Model simulated data, normalized and checked against observations in control regions. Multijet (QCD) background is estimated using data driven methods, primarily the Matrix Method for shape determination and the Fitting Method for normalization. The number of events observed is in good agreement with background predictions. First generation leptoquarks with a mass less than 319 GeV at excluded at a 95% CL for the branching fraction, Β, of a leptoquark to an electron and quark of 0.5. Weaker limits are derrived for other branching fraction values.