New physics in $b\to s\ell \ell$ transitions at one loop

Abstract

We investigate new-physics contributions to $b\to s\ell \ell$ transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient, $C/{\Lambda }^2\sim 1/{\mathrm{TeV}}^2$, induces a substantial shift in the lepton flavour universality ratios $R_K$ and $R_{K^{\ast }}$ at one loop, while evading $Z$-pole precision tests, collider bounds, and other flavour constraints. Good fits to the present data are achieved by a left-handed current operator with quark-flavour indices $(2,2)$ or $(3,3)$, hitherto overlooked. Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory 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.

arXiv commentv2: 15 pages, 6 figures; Section III.2 extended, Table II and Fig. 6 correspondingly updated, references added

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