UBC Theses and Dissertations
Searches for new resonances decaying to top-antitop quark pairs with the ATLAS detector in sqrt(s) = 7 TeV proton-proton collisions Swedish, Stephen
This work presents the results of two analyses searching for massive resonances decaying to top-antitop quark pairs in sqrt(s) = 7 TeV proton-proton collisions measured with the ATLAS detector at the CERN Large Hadron Collider. The peculiar position of the top-quark within the Standard Model, as the most massive particle, motivates many theories predicting the existence of new massive particles that couple strongly to top-quarks. These particles may manifest themselves at the LHC as new massive resonances decaying to top-antitop quark pairs. An analysis conducted on the first 2.04/fb^-¹ of data collected in 2011 searched for resonances in the subset of top-pair events where the W-bosons from each top decay decayed leptonically to a final state electron or muon. A second analysis conducted on the full 4.7/fb^-¹ of 2011 data searched for resonances in the subset of top-quark pair events where W-bosons from each top decay decayed hadronically to a diquark pair. The analysis focused on the case were each top-quark is sufficiently boosted such that the resultant fully hadronic system is highly collimated, and reconstructable as a single hadronic jet with large angular separation. In each analysis, the leptophobic Z' from topcolor assisted technicolor models, and the KK-gluon from Randall-Sundrum warped extra-dimension models were chosen as benchmarks to test for the presence of narrow and wide resonances respectively. No significant deviation from the Standard Model was observed in either analysis. The results were used to set 95% C.L. upper limits on the cross-section times branching ratio to top-quark pairs as a function of resonance mass for each benchmark model. The existence of a Randall-Sundrum KK-gluon was excluded at the 95% C.L. over the mass range 500 to 1620 GeV.
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