A search for pair produced vector-like T quarks in dilepton and multi-jet final states at quare root of s = 13 TeV

Abstract

The Standard Model fermions are known to be chiral in nature. Several new theories like composite Higgs models, Randall-Sundrum model, GUTs etc. however predict the existence of new heavy vector-like fermions. In this thesis, we explore the possibility of finding heavy vector-like partners of the top quark, namely the Tprime (T) quark, in pp collisions at the LHC at a centre of mass energy of 13 TeV. We have assumed that the T decays to a top quark and a Z boson with 100 % branching ratio (T!tZ). We look for pair produced T quarks decaying to a dilepton and a multi-jet final state. The T quark, if found in nature, is expected to be very massive. Its decay products will, therefore, have a considerable boost. We have used the properties of boosted decay topologies in designing a 2 algorithm that chooses the best possible candidates for the decay products of the T quark in an event. Using candidates selected by the 2 algorithm, we reconstruct an invariant mass for the T quark.We set expected and observed 95% CL upper limits on the TT production cross-section using the Asymptotic CLs method. We were able to set an expected 95 % CL lower limit of 1170 GeV on the T quark mass. We also used the 2 algorithm on the data (integrated luminosity of 35.9 fb−1) and were able to exclude T quark masses below 1095 GeV with 95% CL.