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

Abstract

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 GeV/$c$ 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 $B\bar{B}$ pairs, accumulated by the Belle detector at the KEKB asymmetric $e^{+}{e}^{-}$ collider operating on the $\hat{I}\yen (4S)$ resonance. In the momentum interval ranging from 1.9 GeV/$c$ to 2.6 GeV/$c$ we measure the partial branching fraction $\hat{I}”\mathcal{B}(B\to X_{u}e\hat{I}{½}_e)=(8.47\pm 0.37\pm 1.53)\times 10^{-4}$, where the first error is statistical and the second is systematic. A prediction of the partial rate $R=(21.69\pm 3.62_{-1.98}^{+2.18})|V_{ub}{|}^2{\mathrm{ps}}^{-1}$ in this momentum interval based on theory is calculated with input HQET parameters that have been derived from Belle's measurement of the $B\to X_s{\hat{I}}^3$ photon energy spectrum, where the first error is due to the uncertainty on HQET parameters and the second error is from theory. Using both $\hat{I}”\mathcal{B}(B\to X_{u}e\hat{I}{½}_e)$ and $R$ we find $|V_{ub}|=(5.08\pm 0.47\pm 0.42_{-0.23}^{+0.26})\times 10^{-3}$, where the first error is from the partial branching fraction, and the second and third errors are from uncertainties in $R$.

22 pages, 2 figures, 4 tables and to appear in Physics Letters B