New physics in b→sll transitions at one loop

We investigate new-physics contributions to b. s transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the fewscenarioswhere a singleWilson coefficient, C/2 1/TeV2, induces a substantial shift in the lepton flavour universality ratios RK and RK* at one loop, while evading Z-pole precision tests, collider bounds, and other flavour constraints. Novel fits to the present data are achieved by a left-handed current operator with quarkflavour indices (2, 2) or (3, 3). Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effectivetheory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to fermions only. If the anomalies were confirmed, it appears that one-loop explanations have good prospects of being directly tested at the LHC.