Abstract
We study single top quark production in association with a WZ pair at the LHC in the context of the Standard Model (SM) and the Standard Model Effective Field Theory (SMEFT). A significant advantage of tWZ compared to other EW top production processes is its sensitivity to unitarity-violating behaviour induced in its 2 → 2 sub-amplitudes through modified EW interactions. At NLO in QCD, tWZ interferes with t overline{t} Z and t overline{t} and a method to meaningfully separate it from these overlapping processes needs to be employed. In order to define tWZ production for total rates and differential distributions, we consider the approaches proposed in the literature for similar cases and find that diagram-removal procedures provide reliable results both for the SM and the SMEFT in a suitably defined phase-space region. We provide robust results for total and differential cross sections for tWZ at 13 TeV, including the six relevant dimension-6 operators ( mathcal{O} tW, mathcal{O} tZ, mathcal{O} tG, {mathcal{O}}_{varphi Q}^{left(-right)} , {mathcal{O}}_{varphi Q}^{(3)} , mathcal{O} φt), also matching short-distance events to parton shower.
Highlights
In order to define tW Z production for total rates and differential distributions, we consider the approaches proposed in the literature for similar cases and find that diagram-removal procedures provide reliable results both for the Standard Model (SM) and the Standard Model Effective Field Theory (EFT) (SMEFT) in a suitably defined phase-space region
All in all we find the tW Z process to be very stable under parton shower (PS), which is expected from the fact that it is mostly driven by EW interactions, and that our results serve a proof of principle for the realistic generation of precise SMEFT predictions that can be used in future measurements and new physics searches via this process
We have presented a dedicated study of the tW Z and its sensitivity to indirect new physics effects via the SMEFT
Summary
Being a rare EW top production process, tW Z is a potential probe of the weak couplings of the top quark that are, to date, relatively poorly measured. What is not immediately obvious is that such modifications lead to an unacceptable growth with energy, beyond the maximal E0 dependence admitted by perturbative unitarity for a 2 → 2 scattering amplitude This indicates that the theory has a finite range of validity in energy, beyond which new, unitarising physics must appear, and clearly motivates the use of SMEFT to classify the possible deviations from SM interactions. In most cases, when the scattering states involve longitudinal polarisations of gauge bosons, the energy-growing SMEFT helicity configurations coincide with the leading high-energy SM ones, such that their interference grows with energy This provides an enhanced, leading-order sensitivity to the coefficients, motivating searches that exploit these channels
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