Sure it can, but it has to proceed via the weak interaction, and it's suppressed by the GIM mechanism. For example, D⁰→γγ (c̅u or cu̅ annihilation) is rare and

Sure it can, but it has to proceed via the weak interaction, and it's suppressed by the GIM mechanism. For example, D⁰→γγ (c̅u or cu̅ annihilation) is rare and not yet observed, but it's allowed.