#vectorquarks

A search is presented for exotic processes that result in final states containing jets including at least one  jet, sizable missing transverse momentum, and a pair of leptons with the same electric charge. There are several models that predict an enhanced rate of production of such events beyond the expectations of the Standard Model (SM); the ones considered in this note are pair production of chiral  quarks, pair production of vector-like quarks, enhanced four top quark production and production of two positively-charged top quarks. Using a sample of 14.3 fb of  collisions at  TeV recorded by the ATLAS detector at the Large Hadron Collider, with selection criteria optimised for each signal, no significant excess of events over the background expectation is observed. This observation is interpreted as constraining the signal hypotheses, and it is found at 95% confidence level that: the mass of the , assuming 100% branching fraction to , must be  TeV; the mass of a vector-like  () quark, assuming branching ratios to , , and  decay modes consistent with the  or  being a singlet, must be  0.59 (0.54) TeV; the four top production cross section must be  fb in the SM and  fb for production via a contact interaction; the mass of an sgluon must be  TeV; in the context of models with two universal extra dimensions the inverse size of the extra dimensions must be  TeV; and the cross section for production of two positively-charged top quarks must be  fb.

A search is presented for the production of a new heavy quark with its antiparticle, assuming the new quark has a significant branching ratio to decay into a  boson and a third generation Standard Model quark. In the case of a new charge  quark () the decay targeted is , while for a new charge  quark () the decay targeted is . The search uses a dataset corresponding to  of  collisions at  TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum  boson candidate reconstructed from a pair of oppositely charged electrons or muons. Additionally, the presence of at least two jets possessing properties consistent with the decay of a  hadron is required, as well as large total transverse momentum of all central jets in the event. No significant excess of events above the Standard Model expectation is observed, and upper limits are derived for vector-like quarks of various masses in a two-dimensional plane of branching ratios. Under branching ratio assumptions corresponding to a weak-isospin singlet scenario, a  () quark with mass lower than 585 (645) GeV is excluded at the 95% confidence level. Under branching ratio assumptions corresponding to a particular weak-isospin doublet scenario, a  () quark with mass lower than 680 (725) GeV is excluded at the 95% confidence level.