Tree-level contributions to B -> Xs gamma

Understand

Weak radiative decay B -> X_s gamma is known to be a loop-generated process.

• However, it does receive tree-level contributions from CKM-suppressed b -> u ubar s gamma transitions.
• In the present paper, we evaluate such contributions together with similar ones from the QCD penguin operators.
• For a low value of the photon energy cutoff E_0 ~ m_b/20 that has often been used in the literature, they can enhance the inclusive branching ratio by more than 10%.