Searches for top-antitop resonances in proton-proton collisions at a center of mass energy of 7 TeV with the ATLAS detector

The LHC and the ATLAS detector offer an unprecedented opportunity to test theories beyond the StandardModel of particle physics. Some of these theories predict heavy particles that would decay predominantly into a top and an antitop quark. This thesis presents a technique to experimentally measure the invariant mass (Mt¯t) of top-antitop pairs, event-by-event, allowing for a complete reconstruction of the Mt¯t spectrum. Two different models of heavy narrow resonances were considered: a particle X with a negligible mass width, and a Z′ particle. Under these models, no resonances in the Mt¯t spectrum were found in 35.3 pb−1 of 7 TeV centre of mass proton-proton collision data. An upper limit on the production cross section times the branching ratio of the heavy particle decaying into a top-antitop (σup) as a function of its mass (MX or MZ′) was established at a 95% confidence level. Including systematic errors, the observed (expected) σup at 95% varies from 3.2 pb (27.9+11.3 −19.9 pb) to 77.7 pb (9.8+10.7 −5.6 pb) forMX ranging from 760 GeV to 1000 GeV, and from 2.9 pb (55.9+56.9 −47.0 pb) to 43.4 pb (7.8+8.9 −4.7 pb) for MZ′ ranging from 700 GeV to 1000 GeV.