Using a $492{\mathrm{fb}}^{-1}$ data sample collected near the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider, we observe the decay $B^0\to p\overline{p}{K}^{*0}$ with a branching fraction of $\mathbf{(}{1.18}_{-0.25}^{+0.29}(\mathrm{stat})\pm 0.11(\mathrm{syst})\mathbf{)}\times {10}^{-6}$. We study the decay dynamics of $B^0\to p\overline{p}{K}^{*0}$ and compare with $B^{+}\to p\overline{p}{K}^{*+}$. The $K^{*0}$ meson is found to be almost 100% polarized (with a fraction of $(101\pm 13\pm 3)\%$ in the helicity zero state), while the $K^{*+}$ meson has a $(32\pm 17\pm 9)\%$ fraction in the helicity zero state. The direct $CP$ asymmetries for $B^0\to p\overline{p}{K}^{*0}$ and $B^{+}\to p\overline{p}{K}^{*+}$ are measured to be $-0.08\pm 0.20\pm 0.02$ and $-0.01\pm 0.19\pm 0.02$, respectively. In addition, we report improved measurements of the branching fractions $\mathcal{B}(B^{+}\to p\overline{p}{K}^{*+})=({3.38}_{-0.60}^{+0.73}\pm 0.39)\times {10}^{-6}$ and $\mathcal{B}(B^0\to p\overline{p}{K}^0)=({2.51}_{-0.29}^{+0.35}\pm 0.21)\times {10}^{-6}$, which supersede our previous measurements.

• Received 4 February 2008

DOI:https://doi.org/10.1103/PhysRevLett.100.251801