Production of extra quarks at the Large Hadron Collider beyond the narrow width approximation

Abstract

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.

Low- and high-energy phenomenology of a doubly charged scalar

“…Furthermore, in the DY processes, for values of Γ S /m S above ∼1% the relative difference is negative for m S < ∼ 1300 GeV and positive for larger masses. Around m S ∼ 1300 GeV a cancellation between effects can be observed, due to a different scaling of the phase space and the PDFs with the transferred energy in the process depending on the width of the S. This effect has been observed and described in [93] for a different process. Of course, and analogously to what was found in [93], for values of m S corresponding to a cancellation at the level of integrated cross section differences in the kinematics of the final state still appear at differential level and affects the efficiency of a specific set of experimental cuts.…”

Single production of vectorlike quarks with large width at the Large Hadron Collider

“…The total width of the VLQ, however, can be increased not only by increasing the couplings of the VLQ with SM quarks and bosons, but also by including further decay channels to other new physics states. In the case of a VLQ with large width, the latter hypothesis is preferable from a phenomenological point of view, as large couplings of VLQs with SM quarks are in general strongly constrained by oblique observables and Higgs couplings [48,52].…”

“…These relations are not satisfied in the large width regime. Minimal extensions of the SM with one unique VLQ representation where the VLQ has large couplings are excluded by other constraints (such as EW precision tests or corrections to SM couplings) [48,52]. In our case study the large width regime will be achieved by considering the total width as a free parameter but imposing for VLQ singlets and doublets identical relations between the κ parameters as in the NWA.…”

“…On the other hand, VLQs can have a large width, both in virtue of their mass (much larger than the top quark one) and the variety of decay channels they can undergo, including neutral current ones: contributions from large widths can in turn lead to significant effects, also induced by interferences with the irreducible background, onto the sensitivity of current experimental analyses. Analyses of large width effects in the case of VLQ pair production, i.e., even when the VLQ production stage is due to QCD interactions only, have been performed in [48,49] (see also [50,51]).…”

Summary of the working group on Higgs physics and physics beyond the Standard Model

“…Other searches assume a preferred mediator coupling to the third generation and search for dark matter in association with tt, t, or bb [45]. Such searches are sensitive to the presence of vector-like quarks coupling to dark matter, with the sensitivity depending on the width of the vector-like quark [46].…”