Observation of B+ to p Lambdabar pi+ pi- at Belle

We study charmless $B^{+}$ meson decays to the $p\bar{\mathrm{\Lambda }}{\pi }^{+}{\pi }^{-}$ final state using a $605{\mathrm{f}\mathrm{b}}^{-1}$ data sample collected at the $\mathrm{{\rm Y}}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. There are significant signals found with the $p\bar{\mathrm{\Lambda }}$ mass peaking near threshold. The observed branching fraction for non-resonant B^+ \to p \bar\Lambda} \pi^+ \pi^- is $(5.92_{-0.84}^{+0.88}(stat\mathrm{.})\pm 0.69(syst\mathrm{.}))\times {10}^{-6}$ with a significance of 9.1 standard deviations. We also observe the intermediate three-body decay $B^{+}\to p\bar{\mathrm{\Lambda }}{\rho }^0$ with a branching fraction of $(4.78_{-0.64}^{+0.67}(stat\mathrm{.})\pm 0.60(syst\mathrm{.}))\times {10}^{-6}$ and a significance of 9.5 standard deviations, and find a hint of a $B^+ \to p \bar{\Lambda} f_2(1270)$ signal. No other intermediate three-body decay is found in this study.