Charming Penguins and Lepton Universality Violation in $b \to s \ell^+ \ell^-$ decays

TL;DR

Recent LHCb results reinforce evidence of lepton universality violation in B meson decays, emphasizing the importance of hadronic uncertainties and charming penguins in interpreting potential new physics signals.

Contribution

This work introduces a conservative approach to hadronic uncertainties, especially charming penguins, to better interpret lepton universality violation in B decays and explore new physics implications.

Findings

Data suggest a significant q^2 and helicity dependence of charm loop amplitudes.

Conservative new physics solutions favor left-handed or axial lepton couplings.

Hadronic uncertainties are crucial for assessing deviations from the Standard Model.

Abstract

The LHCb experiment has very recently presented new results on Lepton Universality Violation (LUV) in $B \to K^{(*)} \ell^+ \ell^-$ decays involving $K_S$ in the final state, which strengthens the recent evidence of LUV obtained in $B^+ \to K^{+} \ell^+ \ell^-$ decays and the previous measurements of $B \to K^{*0} \ell^+ \ell^-$. While LUV observables in the Standard Model are theoretically clean, their predictions in New Physics scenarios are sensitive to the details of the hadronic dynamics, and in particular of the charming penguin contribution. In this work, we show how a conservative treatment of hadronic uncertainties is crucial not only to assess the significance of deviations from the Standard Model but also to obtain a conservative picture of the New Physics responsible for LUV. Adopting a very general parameterization of charming penguins, we find that: $i)$ current data hint at a sizable $q^2$ and helicity dependence of charm loop amplitudes; $ii)$ conservative NP solutions to $B$ anomalies favour a left-handed or an axial lepton coupling rather than a vector one.