Form Factors for B -> pi l nu-bar_l and B -> K* gamma Decays on the Lattice

TL;DR

This paper introduces a unified lattice QCD method to analyze form factors in B meson decays, providing the first q^2 dependence at the B scale and highlighting uncertainties in decay rate calculations.

Contribution

It presents a novel unified approach for analyzing form factors in B decays, enabling consistency checks and first-time q^2 dependence determination at the B scale.

Findings

Pole fits may not match q^2 behavior of form factors, causing decay rate uncertainties.

The hadronisation ratio R_{K^*} is approximately 35% or 15%, depending on form factor behavior.

First determination of q^2 dependence of form factors at the B scale.

Abstract

We present a unified method for analysing form factors in B -> pi l nu-bar_l and B -> K* gamma decays. The analysis provides consistency checks on the q^2 and 1/M extrapolations necessary to obtain the physical decay rates. For the first time the q^2 dependence of the form factors is obtained at the B scale. In the B -> pi l nu-bar_l case, we show that pole fits to f^+ may not be consistent with the q^2 behaviour of f^0, leading to a possible factor of two uncertainty in the decay rate and hence in the value of |V_{ub}|^2 deduced from it. For B -> K* gamma, from the combined analysis of form factors T_1 and T_2, we find the hadronisation ratio R_{K^*} of the exclusive B -> K* gamma to the inclusive b -> s gamma rates is of order 35% or 15% for constant and pole-type behaviour of T_2 respectively.