Probing supergravity models with the b-->s gamma microscope.

Abstract

We present a calculation of the branching ratio $B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})$ in two well-motivated supersymmetric models: the minimal SU(5) and the no-scale flipped SU(5) supergravity models. We find that the improved CLEO upper bound [$B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})<5.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ at 95% C.L.] does not yet constrain the minimal SU(5) supergravity model, where $B{(b\ensuremath{\rightarrow}s\ensuremath{\gamma})}_{\mathrm{minimal}}=(2.3\ensuremath{-}3.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. In the flipped SU(5) model the CLEO bound is constraining, although still not very significantly, even for light charged Higgs boson masses. An improvement in sensitivity by a factor of 2 will probe all (more than half) of the parameter space of the minimal (flipped) SU(5) supergravity model. This "resolution" of the $b\ensuremath{\rightarrow}s\ensuremath{\gamma}$ microscope far surpasses that of present collider experiments. In the flipped model there exists a significant region of parameter space where $B(b\ensuremath{\rightarrow}s\ensuremath{\gamma})$ is highly suppressed due to a new phenomenon involving a complicated cancellation against the QCD correction parameter.