NLO QCD+EW predictions for $tHj$ and $tZj$ production at the LHC

TL;DR

This paper presents NLO QCD+EW calculations for single top quark production with Higgs or Z bosons at the LHC, including off-shell effects and scheme uncertainties, providing more accurate theoretical predictions.

Contribution

It offers the first combined NLO QCD+EW predictions for $tHj$ and $tZj$ production, including off-shell effects and a detailed comparison of flavor schemes and shower effects.

Findings

NLO EW corrections are within QCD uncertainties when scheme differences are considered.

Including off-shell effects improves the realism of the predictions.

Differences between NLO QCD+EW and QCD+shower predictions are analyzed.

Abstract

In this work we calculate the cross sections for the hadroproduction of a single top quark or antiquark in association with a Higgs ($tHj$) or $Z$ boson ($tZj$) at NLO QCD+EW accuracy. In the case of $tZj$ production we consider both the case of the $Z$ boson undecayed and the complete final state $t \ell^+ \ell^- j$, including off-shell and non-resonant effects. We perform our calculation in the five-flavour-scheme (5FS), without selecting any specific production channel ($s$-, $t$- or $tW$ associated). Moreover, we provide a more realistic estimate of the theory uncertainty by carefully including the differences between the four-flavour-scheme (4FS) and 5FS predictions. The difficulties underlying this procedure in the presence of EW corrections are discussed in detail. We find that NLO EW corrections are in general within the NLO QCD theory uncertainties only if the flavour scheme uncertainty (4FS vs.~5FS) is taken into account. For the case of $t \ell^+ \ell^- j$ production we also investigate differences between NLO QCD+EW predictions and NLO QCD predictions matched with a parton shower simulation including multiple photon emissions.