Production Cross Section Measurement of Top-Antitop Pairs in the Dimuon Decay Channel at $\sqrt{s} = 7TeV$ with the CMS Experiment

The LHC started to produce first proton–proton collisions at a centre of mass energy of 7TeV in spring 2010 opening up a new energy scale for particle physics. During the first data taking period, the CMS detector has recorded 35.9 pb−1 of data. One of the first goals is the rediscovery and precision measurement of the already known standard model. Especially the top quark is interesting since it is the heaviest particle observed by now. In this work, an early measurement of the top–antitop cross section using the first CMS data is presented, which is based on 28 selected candidate events. The dimuon decay channel tt → bb μ+μ−+ neutrinos has been chosen for this study since the two isolated muons give a clear signature to distinguish signal events from the QCD background. The contribution of events with muons from heavy flavour decays and misidentified muons is expected to be the least understood background in first data. Most important background, however, is the Drell–Yan production of a muon pair accompanied by additional hard jets. For this reason, both backgrounds are estimated from data. Several approaches are studied for the measurement of the cross section. The default selection of two muons, two jets from the hadronised b quarks, and missing transverse energy is improved by applying a kinematic reconstruction algorithm for the top mass. Both attempts give consistent results. In addition, two b tagging algorithms are tested to improve the significance. They lead to compatible results for the cross section. The value obtained by the mass reconstruction method, which is taken as final result, is σtt = 150 ± 48 (stat.) ± 35 (syst.) ± 6 (lumi.) pb.