A search is presented for single production of a vector-like quark (T) decaying to a Z boson and a top quark, with the Z boson decaying leptonically and the top quark decaying hadronically. The search uses data collected by the CMS experiment in protonproton collisions at a center-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb −1 . The presence of forward jets is a particular characteristic of single production of vector-like quarks that is used in the analysis. For the first time, different T quark width hypotheses are studied, from negligibly small to 30% of the new particle mass. At the 95% confidence level, the product of cross section and branching fraction is excluded above values in the range 0.26-0.04 pb for T quark masses in the range 0.7-1.7 TeV, assuming a negligible width. A similar sensitivity is observed for widths of up to 30% of the T quark mass. The production of a heavy Z boson decaying to Tt, with T → tZ, is also searched for, and limits on the product of cross section and branching fractions for this process are set between 0.13 and 0.06 pb for Z boson masses in the range from 1.5 to 2.5 TeV.

This paper explores the effects of both finite width and interference (with background) in the pair production and decay of extra heavy quarks with charge 2/3 at the Large Hadron Collider (LHC). This dynamics is normally ignored in standard experimental searches and we assess herein the regions of validity of current approaches, also evaluating the performances of a set of current experimental analyses at 8 and 13 TeV for the deterimination of the excluded regions in the (MVLQ, ΓVLQ) plane, MVLQ being the mass of the VLQ and ΓVLQ its width. Further, we discuss the configurations of masses, widths and couplings where the latter breaks down.1 It has to be specified here that new chiral quarks have been excluded in the context of a minimal extension of the SM where an extra quark would be the only new particle. If the Higgs sector is also enlarged to contain new states [5][6][7], or if more than one quark multiplet is introduced [8], new chiral quarks can indeed be accomodated. We will not discuss such non-minimal extensions in this context.

Vectorlike quarks (VLQs) are predicted by several theoretical scenarios of new physics and having colour quantum numbers they can copiously be produced at the Large Hadron Collider (LHC) so long that their mass is within the testable kinematic regime of this machine. While it would be convenient to assume that such objects are narrow and can be treated in the so-called narrow width approximation (NWA), this is not always possible: non-minimal new physics scenarios can predict VLQs with potentially large couplings and/or exotic decays, which can contribute to generate a large decay width for such extra quarks without being constrained by current bounds. In this analysis we consider single production of VLQs, which is rapidly becoming a channel of choice in experimental searches owing to the ever increasing limits on their mass, and we describe an approach to treat the large width regime which retains to a large extent a degree of model independence. We also consider the role of potentially sizable and model-dependent interference effects between different VLQ production and decay channels as well as between these and the corresponding irreducible background.