The Rumble in the Meson: a leptoquark versus a Z′ to fit b → sμ+μ− anomalies including 2022 LHCb R_K( ) measurements

A bstract We juxtapose global fits of two bottom-up models (an S 3 scalar leptoquark model and a B 3 – L 2 Z′ model) of b → sμ + μ − anomalies to flavour data in order to quantify statistical preference or lack thereof. The leptoquark model couples directly to left-handed di-muon pairs, whereas the Z′ model couples to di-muon pairs with a vector-like coupling. Bₛ-Bₛ̄ B s − B s ¯ mixing is a focus because it is typically expected to disfavour Z′ explanations. In two-parameter fits to 247 flavour observables, including B s / d → μ + μ − branching ratios for which we provide an updated combination and LHCb R_K( ) R K ∗ measurements from December 2022, we show that each model provides a similar improvement in quality-of-fit of √Δ χ² ∆ χ 2 = 3 . 6 with respect to the Standard Model. The main effect of the Bₛ-Bₛ̄ B s − B s ¯ mixing constraint in the Z′ model is to disfavour values of the s L – b L mixing angle greater than about 5| V cb |. This limit is rather loose, meaning that a good fit to data does not require ‘alignment’ in either quark Yukawa matrix. No curtailment of the s L − b L mixing angle is evident in the S 3 model.