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

TL;DR

This paper explores one-loop new physics contributions to $b\to s\ell\ell$ transitions within an effective field theory framework, identifying scenarios that explain anomalies while satisfying experimental constraints and linking them to minimal mediator models.

Contribution

It identifies novel one-loop scenarios with specific Wilson coefficients that can explain flavor anomalies and connects these to minimal leptoquark models, considering experimental constraints.

Findings

Single Wilson coefficient scenarios can significantly affect $R_K$ and $R_{K^*}$.

Left-handed current operators with specific quark-flavour indices fit current data.

Viable leptoquark models with one coupling are identified.

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, $C/\Lambda^2 \sim 1/{\rm TeV}^2$, induces a substantial shift in the lepton flavour universality ratios $R_K$ and $R_{K^*}$ 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.