PQCD Calculation for $\Lambda_{b}\to \Lambda \gamma$ in the Standard Model

We calculate the branching ratio of $\Lambda_b \to \Lambda \gamma$ in the standard model using the PQCD method. The predicted branching ratio $B(\Lambda_b \to \Lambda \gamma)$ can be as large as $1\times 10^{-7}$, with reasonable parameter ranges in the heavy baryon distribution amplitude. This branching ratio is much smaller than those obtained in other hadronic model calculations. Future experimental data can provide important information on the PQCD method for heavy baryon radiative decay and also information about possible new physics beyond the standard model