Search for Vector-like Quarks using trilepton and same-sign dilepton events in 20.3 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

A search is presented for vector-like quarks using 20.3 ​fb$^{-1}$​ of proton-proton collisions at a center-of-mass energy of ​$\sqrt{s}$​ = 8 TeV collected with the ATLAS detector​ ​in 2012 at the CERN Large Hadron Collider. Vector-like quarks are predicted to​ ​exist in many theories beyond the Standard Model of particle physics that attempt​ ​to resolve the hierarchy problem. Events are selected containing jets including at​ ​​least one b-jet, sizable missing transverse momentum, large scalar sum of the jet​ ​and lepton transverse momenta, and either three leptons or two leptons with the​ ​same electric charge. Standard Model processes rarely produce this ​final state and​ ​production of vector-like quarks would lead to an enhanced rate of such events. The​ ​data are interpreted in the context of a variety of models including pair production​ ​of vector-like quarks T and B, which have the same electric charge as the corresponding Standard Model quarks and can appear in either SU(2) weak isospin singlets,​ ​doublets or triplets. In addition single and pair production of the vector-like quark​ ​T​$_{5/3}$​, which appears in an SU(2) weak isospin doublet or triplet and has electric​ ​charge 5/3, is considered. A moderate excess of data above the SM background expectation is observed with a signi​fi​cance of less than two standard deviations. The​ ​data are used to set limits at 95% Con​fi​dence Level (CL) on the new heavy quark​ ​mass for the various VLQ models considered using the CLS method. The vector-like​ ​​quarks T and B in the singlet model are excluded at 95% CL below a mass of 0.59​ ​TeV and 0.62 TeV respectively. The T​$_{5/3}$​ is excluded at 95% CL below a mass of​ ​​0.74 TeV when only pair production is considered and below 0.75 TeV when both​ ​pair and single production are considered.