Constraints on tensor and scalar couplings from $B\to K\bar\mu\mu$ and $B_s\to \bar\mu\mu$

TL;DR

This paper derives the most stringent model-independent bounds on tensor and scalar couplings in B meson decays using recent experimental data, enhancing constraints on new physics beyond the standard model.

Contribution

It provides the first comprehensive constraints on all complex scalar and pseudoscalar couplings using angular observables and branching ratios in B decays.

Findings

Bounds on tensor and scalar couplings are the tightest to date.

Angular observable $F_H^$ offers complementary constraints.

Potential for large deviations in $B o K^*$ observables within LHCb sensitivity.

Abstract

The angular distribution of $B\to K\bar\ell\ell$ ($\ell = e,\,\mu,\,\tau$) depends on two parameters, the lepton forward-backward asymmetry, $A_{\rm FB}^\ell$, and the flat term, $F_H^\ell$. Both are strongly suppressed in the standard model and constitute sensitive probes of tensor and scalar contributions. We use the latest experimental results for $\ell = \mu$ in combination with the branching ratio of $B_s\to \bar\mu\mu$ to derive the strongest model-independent bounds on tensor and scalar effective couplings to date. The measurement of $F_H^\mu$ provides a complementary constraint to that of the branching ratio of $B_s\to \bar\mu\mu$ and allows us---for the first time---to constrain all complex-valued (pseudo-)scalar couplings and their chirality-flipped counterparts in one fit. Based on Bayesian fits of various scenarios, we find that our bounds even become tighter when vector couplings are allowed to deviate from the standard model and that specific combinations of angular observables in $B \to K^*$ are still allowed to be up to two orders of magnitude larger than in the standard model, which would place them in the region of LHCb's sensitivity.