Extraction of bare Form Factors for ${\mathrm{B}}_{\mathrm{s}}\to \mathrm{K}\mathrm{\ell }\nu$ Decays in non-perturbative HQET

We discuss the extraction of the ground state $\langle $\mathrm{K}$ ({\bf p})|V_\mu(0)|$\mathrm{B}$ ({\bf 0})\rangle$ matrix elements from Euclidean lattice correlation functions. The emphasis is on the elimination of excited state contributions. Two typical gauge-field ensembles with lattice spacings $0.075,0.05$ fm and pion masses $330,270$ MeV are used from the O($a$)- improved CLS 2-flavour simulations and the final state momentum is $|{\bf p}|=0.5,{\rm GeV}$\mathrm{.}Theb-quarkistreatedinHQETincludingthe$1/m_$\mathrm{b}$$ corrections. Fits to two-point and three-point correlation functions and suitable ratios including summed ratios are used, yielding consistent results with precision of around 2% which is $not$ limited by the $1\mathrm{/}{m}_{\mathrm{b}}$ corrections but by the dominating static form factors. Excited state contributions are under reasonable control but are the bottleneck towards precision. We do not yet include a specific investigation of multi-hadron contaminations, a gap in the literature which ought to be filled soon.