Study of $B^{+}\to p\overline{\Lambda }\gamma$, $p\overline{\Lambda }{\pi }^0$ and $B^0\to p\overline{\Lambda }{\pi }^{-}$

We study the following charmless baryonic three-body decays of $B$ mesons: $B^{+}\to p\overline{\Lambda }\gamma$, $B^{+}\to p\overline{\Lambda }{\pi }^0$, and $B^0\to p\overline{\Lambda }{\pi }^{-}$. The partial branching fractions as a function of the baryon-antibaryon mass and the polar angle distributions of the proton in the baryon-antibaryon system are presented. This study includes the first observation of $B^{+}\to p\overline{\Lambda }{\pi }^0$, which is measured to have a branching fraction of $({3.00}_{-0.53}^{+0.61}\pm 0.33)\times {10}^{-6}$. We also set upper limits on branching fractions of the two-body decays $B^0\to p{\overline{\Sigma }}^{*-}$, $B^0\to {\Delta }^0\overline{\Lambda }$, $B^{+}\to p{\overline{\Sigma }}^{*0}$, and $B^{+}\to {\Delta }^{+}\overline{\Lambda }$ at the 90% confidence level. These results are obtained from a $414{\mathrm{fb}}^{-1}$ data sample collected near the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider.

Figure 1

A $b\to s$ penguin diagram for $B^{+}\to p\overline{\Lambda }\gamma$.

Figure 2

The $\Delta E$ and $M_{\mathrm{bc}}$ distributions for (a),(b) $p\overline{\Lambda }\gamma$, (c),(d) $p\overline{\Lambda }{\pi }^0$, and (e),(f) $p\overline{\Lambda }{\pi }^{-}$ modes with the requirement of baryon-antibaryon mass $<2.8\mathrm{GeV}/c^2$. The solid curve represents the fit projection, which is the sum of signal (dash-dotted peak) and background (dashed curve) estimations.

Figure 3

Differential branching fractions for (a) $p\overline{\Lambda }\gamma$, (b) $p\overline{\Lambda }{\pi }^0$, and (c) $p\overline{\Lambda }{\pi }^{-}$ modes as a function of baryon-antibaryon pair mass. The shaded distribution shows the expectation from a phase-space MC simulation. The theoretical predicted curves from Ref. [11] for the $p\overline{\Lambda }\gamma$ mode and from Ref. [10] for the $p\overline{\Lambda }{\pi }^{-}$ mode are overlaid for comparison. The area of the shaded distributions and areas under the theoretical curves are scaled to match the measured branching fractions from data. The uncertainties are statistical only.

Figure 4

Differential branching fractions versus $\cos {\theta }_p$ for (a) $p\overline{\Lambda }\gamma$, (b) $p\overline{\Lambda }{\pi }^0$, and (c) $p\overline{\Lambda }{\pi }^{-}$ modes in the region near threshold (baryon-antibaryon mass $<2.8\mathrm{GeV}/c^2$). The uncertainties are statistical only.

Figure 5

Anisotropy parameter $\overline{\alpha }$ vs $E_{\Lambda }$ in the $B$ rest frame for $p\overline{\Lambda }\gamma$, $p\overline{\Lambda }{\pi }^0$, and $p\overline{\Lambda }{\pi }^{-}$ modes. The energy spread for each decay mode is represented by the horizontal error bar. The theoretical prediction by Ref. [12] is shown as a solid curve.