Search for Second Generation Leptoquarks with ATLAS at the LHC

The Large Hadron Collider will collide protons with protons at a center-of-mass energy of up to 14 TeV. New physics phenomena and new particles are predicted to be detectable with the ATLAS detector at the Large Hadron Collider. One of these predicted new particles beyond the Standard Model are leptoquarks. This thesis deals with the search for scalar second generation leptoquarks produced in pairs. Second generation leptoquarks decay into a muon-type lepton and a quark. In this thesis the decay of both second generation leptoquarks into a muon and a quark is considered. Since pair production is studied the final state consists of two high-energetic muons and two high-energetic jets. This thesis studies second generation leptoquarks with masses of mLQ = 300 GeV, mLQ = 400 GeV, mLQ = 600 GeV and mLQ = 800 GeV. The best cut variables for the discrimination between the signal and the main Standard Model backgrounds tt̄ and Z/γ∗ found in this analysis are the pT of the muons, ST (the scalar sum of the transverse momenta of the two selected muons and the transverse energies of the two selected jets), the mass of the selected dimuon system and the reconstructed leptoquark mass. The latter three cut variables have been optimized for a discovery with a 5σ significance including the systematic uncertainties and trigger efficiencies. Second generation leptoquarks have been excluded up to the mass of∼300 GeV with a 95% confidence level at present experiments. The expected integrated luminosities needed for a 5σ discovery of the tested second generation leptoquark masses with the ATLAS detector have been calculated. This thesis shows that for a disocvery with 5σ significance of a second generation leptoquark with mLQ = 300 GeV and mLQ = 400 GeV an expected integrated luminosity of 1.51 pb−1 and 7.42 pb−1 is needed respectively; this corresponds to a very early phase, i.e. the first few months, of the Large Hadron Collider run. For the discovery with a 5σ significance of second generation leptoquarks with masses of mLQ = 600 GeV and mLQ = 800 GeV an expected integrated luminosity of 103.3 pb−1 and 663 pb−1 is needed respectively; this corresponds to several months and about half a year to a year of the Large Hadron Collider run respectively.