Evidence for Semileptonic B- to p pbar l nubar Decays

We find evidence for the semileptonic baryonic decay $B^-\to p\bar p\ell^-\bar\nu_\ell$($\ell=e,\mu$),basedonadatasampleof772million$B\bar B$pairscollectedatthe$\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy electron-positron collider. A neural-network based hadronic $B$-meson tagging method is used in this study. The branching fraction of $B^{-}\to p\bar{p}{\mathrm{\ell }}^{-}{\bar{\nu }}_{\mathrm{\ell }}$ is measured to be $(5.8_{-2.1}^{+2.4}\text{(stat.)}\pm 0.9\text{(syst.)})\times {10}^{-6}$ with a significance of 3.2$\sigma$, where lepton universality is assumed. We also estimate the corresponding upper limit: $$\mathcal{B}$(B^-\to p\bar p\ell^-\bar\nu_\ell) < 9.6\times 10^{-6}$ at the 90% confidence level. This measurement helps constrain the baryonic transition form factor in B decays.