Rare $B$ decays using lattice QCD form factors

TL;DR

This paper reviews and updates lattice QCD calculations of form factors for rare B decays, improving theoretical predictions for observables relevant to testing the Standard Model.

Contribution

It provides refined unquenched lattice QCD form factors for B decays, incorporating additional constraints and correlations, enhancing the precision of Standard Model predictions.

Findings

Improved form factor fits with better kinematic constraints

Enhanced predictions for the longitudinal polarization fraction F_L

Little change in most other decay observables

Abstract

In this write-up we review and update our recent lattice QCD calculation of $B \to K^*$, $B_s \to \phi$, and $B_s \to K^*$ form factors [arXiv:1310.3722]. These unquenched calculations, performed in the low-recoil kinematic regime, provide a significant improvement over the use of extrapolated light cone sum rule results. The fits presented here include further kinematic constraints and estimates of additional correlations between the different form factor shape parameters. We use these form factors along with Standard Model determinations of Wilson coefficients to give Standard Model predictions for several observables [arXiv:1310.3887]. The modest improvements to the form factor fits lead to improved determinations of $F_L$, the fraction of longitudinally polarized vector mesons, but have little effect on most other observables.