An extension of the SM with vector-like quarks and hypothetical heavy bosons: model independent parametrisation at NLO order

TL;DR

This paper develops a model-independent framework for extending the Standard Model with vector-like quarks, heavy gauge bosons, and scalars, analyzing their production at the LHC with NLO accuracy and comparing predictions with experimental data.

Contribution

It introduces a comprehensive parametrization of the extended model, including NLO calculations and comparison of different computational schemes, highlighting the importance of off-shell effects.

Findings

Off-shell effects can be significant even with small width-to-mass ratios.

The complex mass scheme provides more reliable predictions than the narrow-width approximation.

Lower bounds on heavy gauge boson masses are derived from CMS data.

Abstract

A model independent parametrization of an extension of the Standard Model including vector-like quarks, new heavy gauge bosons and an extra scalar, is introduced. Theoretical constraints on the model couplings and hypothetical particle masses are established by making use of perturbative unitarity. The consistency of the model renormalization and implementation, within the complex mass scheme for narrow widths, is verified. The production of heavy charged and neutral gauge bosons which subsequently decay to vector-like quarks at the LHC are investigated at both leading-order and next-to-leading order. The predictions of the model are obtained by using three approaches: the narrow-width-approximation, the complex mass scheme and a mix of both, where the advantages and weaknesses of each method are emphasized. This study demonstrates that, for certain configurations, off-shell effects remain significant even when the width-to-mass ratios of the unstable particles are sufficiently small, rendering the narrow-width approximation unreliable and necessitating the use of the complex mass scheme. The predictions are compared with CMS {\tt run II} data, where lower bounds on the charged heavy gauge boson mass are set as function of the free parameter normalizing their coupling to quarks.