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 $(M_{\rm VLQ},\Gamma_{\rm VLQ})$ plane, $M_{\rm VLQ}$ being the mass of the VLQ and $\Gamma_{\rm VLQ}$ its width. Further, we discuss the configurations of masses, widths and couplings where the latter breaks down.
Highlights
Following the discovery of a Higgs boson [1,2] with essentially a Standard Model (SM) nature [3,4], the existence of a fourth generation of chiral quarks has been excluded [4].1 the same LHC data constrain vector-like quarks (VLQs) significantly less
If the T VLQ has a large width, the transferred momentum of the process can have values in a larger range than in the narrow width approximation (NWA) case, where it is constrained by the resonant production of the T pair: this means in turn that the PDFs are sampled at different scales and the cross section receives a nontrivial mass- and width-dependent contribution which results in the observed behavior
As, according to current experimental limits, the latter cannot have the V − A structure of the top quark, we have first assessed how off-shellness impacts the heavy quark decay signature common to the one of top quark pairs, i.e., W−b Wþb, showing that a V þ A chiral structure would be affected over the LHC kinematical regime for pair production of heavy quarks which can be profiled through a resonance
Summary
Following the discovery of a Higgs boson [1,2] with essentially a Standard Model (SM) nature [3,4], the existence of a fourth generation of chiral quarks (i.e., with SM-like V − A structure in gauge boson charged currents) has been excluded [4].1 the same LHC data constrain vector-like quarks (VLQs) significantly less. Differences between the case of the top quark and a VLQ due to the different structure of their couplings in the charged decay currents would play a role.2 In this connection, one should recall that, in taking the NWA, as is generally done in most Monte Carlo (MC) programs used in phenomenological and experimental analyses, one neglects off-diagonal spin effects which stem from the quark (top or vector-like) being massive and whose size is intimately related to the vector/axial (or left/right) composition of the fermionic state entering the charged decay currents and, to the value of the ratio ΓVLQ=MVLQ.
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