Abstract
A search for vectorlike quarks is presented, which targets their decay into a π boson and a third-generation Standard Model quark. In the case of a vectorlike quark π (π΅) with charge +2/3β’π (β1/3β’π), the decay searched for is π βπβ’π‘ (π΅ βπβ’π). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1ββfbβ1 of πβ’π collisions at βπ =13ββTeV. The final state used is characterized by the presence of π-tagged jets, as well as a π boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well asβin the case of the single-production selectionsβthe presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of ππ >1030ββGeV (ππ >1210ββGeV) and ππ΅ >1010ββGeV (ππ΅ >1140ββGeV) in the singlet (doublet) model. In the case of 100% branching ratio for π βπβ’π‘ (π΅ βπβ’π), the limits are ππ >1340ββGeV (ππ΅ >1220ββGeV). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses.