Extraction of $B_s \to D^{(*)}_s$ form factors from ${ N_f}=2$ lattice QCD

TL;DR

This paper presents a two-flavour lattice QCD study of $B_s$ to $D_s^{(*)}$ transitions, calculating form factors relevant for testing lepton flavour universality and aiding future analyses with more realistic quark masses.

Contribution

It introduces a method for extracting $b o c$ form factors from lattice QCD with two flavors, providing initial results and insights for future studies with more realistic simulations.

Findings

Calculated zero-recoil form factors: ${\

G}^{B_s o D_s}(1)=1.03(14)$ and $h^{B_s o D^*_s}(1)=0.85(16)$.

Developed a methodology to control systematic effects in lattice form factor extraction.

Abstract

We report on a two-flavour lattice QCD study of the $B_s \to D_s$ and $B_s \to D^*_s$ transitions parameterized, in the heavy quark limit, by the form factors ${\cal G}$, and $h_{A_1}$, $h_{A_2}$ and $h_{A_3}$, respectively. In the search for New Physics through tests of lepton flavour universality, $B_s$ decay channels are complementary to the $B$ decays widely studied at $B$ factories and LHCb, while on the theory side they can be better controlled than the $B_c$ and $\Lambda_b$ decays. The purpose of this exploratory two-flavour study is, in preparation for future analyses of lattice QCD simulations with ${N_f>2}$ and physical quark-masses, to gain experience on a suitable method for a lattice extraction of form factors associated with $b \to c$ currents that may yield tighter control over systematic effects like contamination from excited states and cut-off effects. We obtain the zero-recoil values ${\cal G}^{B_s \to D_s}(1)=1.03(14)$ and $h^{B_s \to D^*_s}(1)=0.85(16)$.