vertical stroke Vub vertical stroke determination in lattice QCD

The 2012 PDG reports a tension at the level of 3σ between two exclusive determinations of vertical stroke Vub vertical stroke. They are obtained by combining the experimental branching ratios of B→τν and B→πlν (respectively) with a theoretical computation of the hadronic matrix elements fB and the B→π form factor f+(q2). To understand the tension, improved precision and a careful analysis of the systematics involved are necessary. We report the results of the ALPHA collaboration for fB from the lattice with 2 flavors of O(a) improved Wilson fermions. We employ HQET, including 1/mb corrections, with pion masses ranging down to ∼190 MeV. Renormalization and matching were performed non-perturbatively, and three lattice spacings reaching a-1∼4.1 GeV are used in the continuum extrapolation. We also present progress towards a computation of f+(q2), to directly compare two independent exclusive determinations of vertical stroke Vub vertical stroke with each other and with inclusive determinations. Additionally, we report on preliminary results for fBs, needed for the analysis of Bs→μ+μ-.