Abstract
We consider top quark decay in the Standard Model Effective Field Theory (SMEFT). We present a calculation of the total decay width and the $W$-boson helicity fractions at next-to-leading order (NLO) in SMEFT. Our result includes the complete set of contributing four-fermion operators in addition to QCD dipole operators and bottom-mass suppressed effects. We show that operators that first appear at NLO in the SMEFT can be bounded by the current data as well as future data from both a high-luminosity LHC and a potential $e^+e^-$ collider, demonstrating the importance of going beyond leading order when studying the SMEFT. We discuss technical aspects of our calculation that we believe will be useful in future higher-order studies of the SMEFT, in particular the treatment of $\gamma_5$ in loop diagrams.
Highlights
The standard model (SM) has so far been remarkably successful in describing all data coming from the LHC
In this manuscript we have studied the next-to-leading order corrections to top quark decays within the SM effective field theory (SMEFT)
This work is a step toward a complete calculation of next-to-leading order (NLO) effects within SMEFT, which we believe will eventually be required by the experimental uncertainties
Summary
The standard model (SM) has so far been remarkably successful in describing all data coming from the LHC. (i) We extend the previous calculations of higher-order corrections to top quark decays in the SMEFT to include the bottom-quark chromomagnetic dipole operator and all contributing four-fermion operators, both four-quark and semileptonic types This is a further step toward a complete next-to-leading order. We consider several different schemes for the treatment of γ5 in dimensional regularization and demonstrate how imposing chiral Ward identities renders them consistent We believe that this discussion will be useful in the future as higher-order effects in the SMEFT are further studied. Such effects go like mb=mt in the SMEFT at LO due to the chiral structure of the contributing dimension-6 operators, unlike in the SM where they go as ðmb=mtÞ2 This leads to significant constraints on these operators from current data.
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