Annihilation Type Radiative Decays of $B$ Meson in Perturbative QCD Approach

With the perturbative QCD approach based on $k_T$ factorization, we study the pure annihilation type radiative decays $B^0\to \varphi \gamma$ and $B^0\to J/\psi \gamma$\mathrm{.}Wefindthatthebranchingratioof$B^0 \to \phi\gamma$is$(2.7^{+0.3+1.2}{-0.6-0.6})\times10^{-11}$, which is too small to be measured in the current $B$ factories of BaBar and Belle. The branching ratio of $B^0\to J/\psi \gamma$is$({4.5^{+0.6+0.7}{-0.5-0.6}})\times10^{-7}$, which is just at the corner of being observable in the $B$ factories. A larger branching ratio $BR(B_s^0\to J\mathrm{/}\psi \gamma )\simeq 5\times {10}^{-6}$ is also predicted. These decay modes will help us testing the standard model and searching for new physics signals.