Measurement of the ratio $\mathcal{B}(B^{+}\to Xe\nu )/\mathcal{B}(B^0\to Xe\nu )$

We report measurements of the inclusive electron momentum spectra in decays of charged and neutral $B$ mesons, and of the ratio of semileptonic branching fractions $\mathcal{B}(B^{+}\to Xe\nu )$ and $\mathcal{B}(B^0\to Xe\nu )$. These were performed on a sample of $231\times {10}^6$ $B\overline{B}$ events recorded with the BABAR detector at the ${\rm Y}(4S)$ resonance. Events are selected by fully reconstructing a hadronic decay of one $B$ meson and identifying an electron among the decay products of the recoiling $\overline{B}$ meson. We obtain $\mathcal{B}(B^{+}\to Xe\nu )/\mathcal{B}(B^0\to Xe\nu )=1.074\pm {0.041}_{(\mathrm{stat})}\pm {0.026}_{(\mathrm{syst})}$.

Figure 1

Fits of Eq. (1) to distributions of the energy substituted mass for (a) neutral and (b) charged $B_{\mathrm{tag}}$ candidates. The dotted and dashed curves indicate the fitted contributions of continuum and combinatorial $B_{\mathrm{tag}}$ candidates. The gray histogram displays the contribution of $D^{*-}\leftrightarrow {\overline{D}}^{*0}$ background.

Figure 2

$m_{\mathrm{ES}}$ distributions for (a) neutral and (b) charged $B_{\mathrm{tag}}$ candidates in events with a right-sign electron.

Figure 3

Total measured spectrum (points) and estimated backgrounds (histograms) for electron candidates in events with a charged $B_{\mathrm{tag}}$ candidate, for (a) the right-sign sample, and (b) the wrong-sign sample.

Figure 4

(a) Normalized momentum spectra of primary electrons after all efficiency corrections and (b) their ratio $r_{+/0}=N_{B^0}/N_{B^{+}}(d{N}_{B^{+}\to Xe\nu }/dp)/(d{N}_{B^0\to Xe\nu }/dp)$.