Rare Decays of Heavy Quarks - Searching Ground for New Physics

The search for new physics beyond the standard model is proceeding nowadays intensively along experimental and theoretical lines. We review here the sector of charm radiative decays in this context. The calculation of $D\to V\gamma$, $D\to \ell^+\ell^-\gamma$ transitions reveals their unequivocal dominance by long-distance contributions. On the other hand, the beauty-conserving charm-changing electroweak transition $B_c\to B_u^*\gamma$ is shown to have unique properties which make it a promising avenue in the search for new physics. We describe a calculation of short- and long-distance contributions to this decay which finds them to be of comparable size. The branching ratio of this decay in the standard model is estimated to be $\simeq 10^{-8}$.