Search for a beyond the standard model resonance decaying to top quark pair in the dilepton final state with the ATLAS experiment

Top quark pair production (tt̄) can be studied at the LHC with unprecented statistics. In this thesis, studies performed at the ATLAS experiment exploiting the dilepton channel, i.e. events where both top quarks decaying to t→Wb have a subsequent W^{\pm}→\ell^{\pm}\nu (\ell^{\pm} = e^{\pm} , \mu^{\pm}) decay, are presented. A search for a Beyond Standard Model (BSM) resonance is perfomed considering the full pp collision dataset collected by ATLAS at √s = 13 TeV, using two different invariant mass variables (m_{\ell\ellbb} and m^{NW}_{tt̄}), each combined with a spin-sensitive variable (∆φ ). Expected mass limits for three different BSM models, at 95% confidence level, are obtained. A neutral heavy gauge boson Z' with a mass smaller than 1.8 TeV (1.85 TeV) can be excluded if using m_{\ell\ellbb} (m^{NW}_{tt̄}). A Randall-Sundrum graviton G, with a mass smaller than 0.8 TeV (0.8 TeV) can be excluded if using m_{\ell\ellbb} (m^{NW}_{tt̄}). A Kaluza-Klein gluon g_{KK} , with a mass smaller than 3.1 TeV (3.25 TeV) can be excluded if using m_{\ell\ellbb} (m^{NW}_{tt̄}).