Measurement of |V_ub| near the endpoint of the electron momentum spectrum from semileptonic B-meson decays

We report measurements of partial branching fractions of inclusive charmless semileptonic B-meson decays at the endpoint of the electron momentum spectrum. The measurements are made in six overlapping momentum intervals that have lower bounds ranging from 1.9 to 2.4 GeV/c and a common upper bound of 2.6 GeV/c, as measured in the centre of mass frame. The results are based on a sample of 29 million BB̄ pairs, accumulated by the Belle detector at the KEKB asymmetric e+e- collider operating on the Υ(4S) resonance. In the momentum interval ranging from 1.9 to 2.6 GeV/c we measure the partial branching fraction ΔB(B→Xueνe)=(8.47±0.37±1.53)×10-4, where the first error is statistical and the second is systematic. A prediction of the partial rate R=(21.69±3.62-1.98+2.18)|V_ub|² ps-1 in this momentum interval based on theory is calculated with input HQET parameters that have been derived from Belle's measurement of the B→Xsγ photon energy spectrum, where the first error is due to the uncertainty on HQET parameters and the second error is from theory. Using both ΔB(B→Xueνe) and R we find |V _ub|=(5.08±0.47±0.42-0.23+0.26)×10-3, where the first error is from the partial branching fraction, and the second and third errors are from uncertainties in R.