Form factors for ${\mathrm{B}}_{\mathrm{s}}\to \mathrm{K}\mathrm{\ell }\nu$ decays in Lattice QCD

We present the current status of the computation of the form factor $f_+ (q^2)$forthesemi-leptonicdecay$\mathrm B_\mathrm s \to \mathrm K \ell \nu$ by the ALPHA collaboration. We use gauge configurations which were generated as part of the Coordinated Lattice Simulations (CLS) effort. They have $N_\mathrm f=2$non-perturbatively$O(a)$ improved Wilson fermions, and pion masses down to $\approx 250\mathrm{M}\mathrm{e}\mathrm{V}$ with $m_{\pi }L\ge 4$. The heavy quark is treated in non-perturbative Heavy Quark Effective Theory (HQET). We discuss how to extract the form factors from the correlation functions and present first results for the form factor at $q^2=21.23{\mathrm{G}\mathrm{e}\mathrm{V}}^2$ extrapolated to the continuum. Next-to-leading order terms in HQET and the chiral extrapolation still need to be included in the analysis.