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 tℓ+ℓ−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 tℓ+ℓ−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.
Highlights
The tHj rate is about 10% of the ttH one, and the process has been searched for at the LHC [5,6,7]
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
We examine whether NLO EW corrections, when they are dominated by QED final-state radiation (FSR), can be equivalently simulated by allowing photon emissions within the QCD shower
Summary
We describe the calculation setup, which is common for the three processes considered in this work:. Unless it is differently specified, with the notation tHj, tZj and t + −j we will understand both the final states with top quarks and antiquarks. At the inclusive and differential level, at NLO QCD+EW accuracy in the 5FS. [17,18,19,20,21,22,23,24,25], the different contributions to any differential or inclusive cross section Σ can be denoted as: ΣLO(αs, α) = α3+kΣ3+k,0 ≡ LO1 , ΣNLO(αs, α) = αsα3+kΣ4+k,0 + α4+kΣ4+k,1 ≡ NLO1 + NLO2 ,. One should note that no additional perturbative orders are present when all possible SM tree-level and one-loop diagrams contributing to these processes are taken into account. Quark physics, such as, e.g., ttW and ttttfor which the two approximations are different and lead to sizeable numerical differences due to contributions that are formally suppressed w.r.t. the NLOEW in the (αs/α) power counting [23, 24, 26, 27]
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