Study On The Selection Of Semileptonic Top Events In ATLAS

It is a an evolving scheme throughout the history of mankind to find out the fundamental constituents of matter, which resulted in the development of high energy particle physics in twentieth century. Since the foundation of quantum mechanics, great progress had been made in the search for the ultimate components of matter and in the investigation on the forces experimented by such elementary particles. The continuous progress of discoveries culminated with the theory, called Standard Model .After the first proposal of the Standard Model, a lot of experiments were carried out and all these experiments had shown impressive agreement with the Standard Model predictions. All the particles foreseen by the Standard Model have been directly observed excepted the Higgs boson. In the currently accepted theoretical picture the Higgs boson is a very massive and low interacting particle which is the quantum of the Higgs field, giving mass to all known massive particles.A great proton-proton collider LHC(Large Hadron Collider) with 14-TeV center of mass energy is construction in CERN(European Organization for Nuclear Research) in order to search for the Higgs boson. The LHC will be characterized by its very high luminosity, needed to cope with the extremely small Higgs production cross-section.ATLAS is a huge general-purpose particle detector which will be installed on one colliding point of LHC. It was designed to exploit the full discovery potential of LHC, to search for Higgs particle and heavy Wand Z- objects, for super-symmetric particles, for compositeness of the fundamental fermions, and to investigate the CP violation in B-physics.The ability to cope with a broad variety of possible physics processes is expected to maximize the detector's potential for discovery of new unexpected physics.Because of its high mass, intriguingly close to the electroweak symmetry breaking scale, the top quark raises interesting questions and its sector is an ideal place to look for new physics.The Large Hadron Collider (LHC) will be a top factory, producing more than 8 millions of ttbar events per year .The production will occur through the gg->ttbar(90%) and qq->ttbar(10%) hard processes. Depending on the Wdecay modes, the ttbar events can be classified into three channels: the semileptonic channel(tt-> WWbb- >lnj1j2bb), the dileptonic channel (tt->WWbb->lnInbb) and the all hadronic channel (tt->WWbb->j1j2j3j4bb).This work is to choose and reconstruct Semileptonic signal events from ATLAS simulation samples.Semileptonic signal events are characterized by one isolated lepton, at least 4 jets of which 2 are b-jets, and missing energy. They are selected by requiring an isolated lepton from first and second level trigger. The offline kinematic cut on the lepton is directly given by the trigger threshold which is set to pT>20 GeV . Moreover, at least four jets with pT>30 GeV are required, among which at least two are b-tagged. This prcut is a good compromise between a low combinatorial background and a good statistics. The 20 GeV cut on the missing transverse energy is standard for events with one neutrino and rejects almost no signal events. Their resulting efficiency on signal events is 9.1%.After kinematic cuts, the full event topology is reconstructed. On the top hadronic decay side, the two non b-jets with Mjj closest and within 20 GeV to the known Mw are selected. On the leptonic side, the missing transverse momentum is used to evaluate the neutrino Pt and then get Px Py and Pz. Lepton .and neutrino are combined to Mw. At last,top gets its weight from W plusing bjet.According the selection conditions of semilepton channel of Top decay, We can get selecting efficiencies as follows:efficiency of asking 1 lepton 59.470016%efficiency of asking 1 electron 50.936070%efficiency of asking 1 muon 67.615798%efficiency of asking 4+jets 41.929659%efficiency of 2 btags 42.275226%pTtniss cut 103.803269%efficiency of kinematical cuts 9.165463% (7298 evts)electron reconstruction efficiency: 69.207652%electron efficiency: 50.934171%electron purity: 98.649536%muon reconstruction efficiency: 94.138335%muon efficiency: 67.577283%muon purity 98.996181 %According to the results of this work,we can select the semilepton channel efficiency and reject background envents well. These information will help the selection of semilepton top decay events when LHC start to run.