B-]S-GAMMA CONSTRAINTS ON THE MINIMAL SUPERGRAVITY MODEL WITH LARGE TAN-BETA - ART. NO. 015007

In the minimal supergravity model (MSUGRA), as the parameter tan beta increases, the charged Higgs boson and light bottom squark masses decr ease, which can potentially increase contributions from tH(+/-), g (gb ) over tilde(j) and (Z) over tilde(i)(b) over tilde(j) loops in the de cay b-->s gamma. We update a previous QCD improved b-->s gamma decay c alculation to include in addition the effects of gluino and neutralino loops. We find that, in the MSUGRA model, loops involving charginos a lso increase, and dominate over tW, tH(+/-), (gq) over tilde and (Z) o ver tilde(i)(q) over tilde contributions for tan beta greater than or similar to 5 - 10. We find for large values of tan beta similar to 35 that most of the parameter space of the MSUGRA model for mu<0 is ruled out due to too large a value of branching ratio B(b-->s gamma). For m u>0 and large tan beta, most of parameter space is allowed, although t he regions with the least fine-tuning (low m(0) and m(1/2)) are ruled out due to too low a value of B(b-->s gamma). We compare the constrain ts from b-->s gamma to constraints from the neutralino relic density, and to expectations for sparticle discovery at CERN LEP2 and the Fermi lab Tevatron p (p) over bar colliders. Finally, we show that non-unive rsal GUT scale soft breaking squark mass terms can enhance gluino loop contributions to b -->s gamma decay rate even if these are diagonal. [S0556-2821(98)06313-9].