Extraction of bare form factors for B s → K l ν decays in nonperturbative HQET

Abstract

We discuss the extraction of the ground state (K(p)|Vμ(0)|Bs(0)) 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-flavor simulations and the final state momentum is |p| = 0.5 GeV. The b-quark is treated in HQET including the 1/mb 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/mb 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.