b→sγ constraints on the minimal supergravity model with large tanβ

In the minimal supergravity model (MSUGRA), as the parameter tanβ increases, the charged Higgs boson and light bottom squark masses decrease, which can potentially increase contributions from tH±, gbj and Zibj loops in the decay b→sγ. We update a previous QCD improved b→sγ decay calculation to include in addition the effects of gluino and neutralino loops. We find that, in the MSUGRA model, loops involving charginos also increase, and dominate over tW, tH±, gq and Ziq contributions for tanβ approx-gt 5-10. We find for large values of tanβ∼35 that most of the parameter space of the MSUGRA model for μ<0 is ruled out due to too large a value of branching ratio B(b→sγ). For μ>0 and large tanβ, most of parameter space is allowed, although the regions with the least fine-tuning (low m0 and m1/2) are ruled out due to too low a value of B(b→sγ). We compare the constraints from b→sγ to constraints from the neutralino relic density, and to expectations for sparticle discovery at CERN LEP2 and the Fermilab Tevatron p bar p colliders. Finally, we show that non-universal GUT scale soft breaking squark mass terms can enhance gluino loop contributions to b→sγ decay rate even if these are diagonal. copyright 1998 The American Physical Society