Probing effective operators in single top quark production in association with a lepton-neutrino pair

Abstract: We study single top quark production in association with a lepton-neutrino pair at the LHC within the framework of the Standard Model Effective Field Theory (SMEFT). We focus on relevant two-quark-two-lepton operators ($O^{3}_{lq}$, $O^{1}_{lequ}$, and $O^{3}_{lequ}$). It is known that these operators have tiny effects on the inclusive cross section of standard model tW production and are thus typically ignored in the SMEFT searches. However, we show that by employing smart observables, such as $m_{T2}$, the pp$\rightarrow$t$\ell\nu$ process is significantly sensitive to these operators. We set the most stringent limit on the coupling strength of the $O^{3}_{lq}$ operator by reinterpreting the results of a search for new phenomena with two opposite-charge leptons, jets and missing transverse momentum at $\sqrt{s}$ = 13 TeV performed by the ATLAS collaboration. The limits derived on the $O^{1}_{lequ}$ and $O^{3}_{lequ}$ operators are comparable to those obtained from probing EFT effects in pp$\rightarrow$t$\bar{\rm t}\ell\nu$ and pp$\rightarrow$t$\bar{\rm t}\ell\ell$ processes at the LHC. Consequently, we propose to include the effects of these three operators on the pp$\rightarrow$t$\ell\nu$ process in future global SMEFT analyses to increase the sensitivity and to reduce possible degeneracies.