Nonperturbative calculations of form factors for exclusive semileptonic $B_{(s)}$ decays

Precise theoretical predictions derived from the Standard Model are a key ingredient in searches for new physics in the flavor sector. The large mass and long lifetime of the $b$ quark make processes involving $b$ quarks of particular interest. We use lattice simulations to perform nonperturbative QCD calculations for semileptonic $B_{(s)}$ decays. We present results from our determinations of $B_s\to D_s \ell \nu$ and $B_s\to K \ell \nu$ semileptonic form factors and provide an outlook for our $B\to \pi\ell\nu$ calculation. In addition we discuss the determination of $R$-ratios testing lepton-flavor universality and suggest use of an improved ratio. Our calculations are based on the set of 2+1 flavor domain wall Iwasaki gauge field configurations generated by the RBC-UKQCD collaboration featuring three lattice spacings of $1/a = 1.78$, $2.38$, and $2.79\,\text{GeV}$. Heavy $b$-quarks are simulated using the relativistic heavy quark action