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

Understand

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.