Measurement of the branching fraction and $\overline{\Lambda }$ polarization in $B^0\to \overline{\Lambda }p{\pi }^{-}$

We present a measurement of the $B^0\to \overline{\Lambda }p{\pi }^{-}$ branching fraction performed using the BABAR detector at the PEP-II asymmetric $e^{+}{e}^{-}$ collider. Based on a sample of $467\times {10}^6$ $B\overline{B}$ pairs we measure $\mathcal{B}(B^0\to \overline{\Lambda }p{\pi }^{-})=[3.07\pm 0.31(\mathrm{stat})\pm 0.23(\mathrm{syst})]\times {10}^{-6}$. The measured differential spectrum as a function of the dibaryon invariant mass $m(\overline{\Lambda }p)$ shows a near-threshold enhancement similar to that observed in other baryonic $B$ decays. We study the $\overline{\Lambda }$ polarization as a function of $\overline{\Lambda }$ energy in the $B^0$ rest frame ($E_{\overline{\Lambda }}^{*}$) and compare it with theoretical expectations of fully longitudinally right-polarized $\overline{\Lambda }$ at large $E_{\overline{\Lambda }}^{*}$.

Figure 1

Reconstruction efficiency measured on the Monte Carlo signal sample, as a function of $\cos {\theta }_{\mathrm{H}}$ and $E_{\overline{\Lambda }}^{*}$. In this plot ${\theta }_{\mathrm{H}}$ is the angle between the antiproton direction, in the $\overline{\Lambda }$ rest frame, and $\widehat{L}$, the unit vector in the direction of the $\overline{\Lambda }$ in the $B^0$ rest frame.

Figure 2

Upper plot: $m_{\mathrm{ES}}$ distribution of candidates with $|\Delta E|<27\mathrm{MeV}$. Lower plot: $\Delta E$ distribution of candidates with $m_{\mathrm{ES}}>5.274\mathrm{GeV}/c^2$. The projections of the two-dimensional fit PDF are shown superimposed.

Figure 3

Upper plot: ${}_s\mathcal{P}\mathrm{lot}$ of the $m(\overline{\Lambda }p)$ event distribution with efficiency corrections applied. Lower plot: ${}_s\mathcal{P}\mathrm{lot}$ of the $E_{\overline{\Lambda }}^{*}$ distribution with efficiency corrections applied. Horizontal bars represent bin ranges.

Figure 4

The product of $\overline{\Lambda }$ longitudinal polarization and ${\alpha }_{\overline{\Lambda }}$ as a function of $E_{\overline{\Lambda }}^{*}$. Horizontal bars represent bin ranges.