Charged Higgs mass bound from the b → s γ process in the minimal supergravity model

We study the constraint on the mass of the charged Higgs boson in the minimal supergravity model based on the recent measurement of the inclusive b → s γ decay. It is shown that the lower bound for the charged Higgs mass crucially depends on the sign of the higgsino mass parameter (μ). For μ < 0, the bound exceeds 400 GeV when the ratio of two Higgs vacuum expectation values (tanβ) is larger than 10. No strong bound is obtained for μ > 0 due to cancellations between charged Higgs and supersymmetric particle contributions. For 3 < ∼ tanβ < ∼ 5, a charged Higgs lighter than 180 GeV is excluded by this process irrespective of the sign of μ. Flavor changing neutral current (FCNC) processes play a unique role in searching for physics beyond the standard model (SM) of elementary particles. These processes are sensitive to virtual effects of new particles, since the FCNC processes in SM do not occur at the tree level. These processes can thus be more powerful than direct particle searches in putting constraints on the parameter space of various new physics. In particular, the radiative decay of the b quark, b → s γ , deserves a special attention. It has been noticed that in a two Higgs doublet model (THDM) the charged Higgs boson can give a substantial contribution to the b → s γ rate [1, 2]. Recently, the CLEO group [3] has reported the first measurement of the inclusive b → s γ branching ratio Br( b → s γ ) = (2.32 ± 0.51 ± 0.29 ± 0.32) × 10 . In fact, this result constrains the mass of the charged Higgs boson in a certain type of THDM called Model II [1, 2] to be larger than 260 GeV [3]. The Higgs sector in the minimal supersymmetric (SUSY) extension of SM is a special case of Model II THDM. However, the above-mentioned limit cannot be directly applied, because SUSY particles can contribute to the b → s γ process in addition to the SM particles and the charged Higgs. It is a natural question to ask how the charged Higgs mass limit is modified in the SUSY extension of SM. Many authors have discussed the b → s γ process in SUSY [4]-[18]. Although the SM and the charged Higgs contribution to the b → s γ amplitude has the same sign, it is found that the SUSY loop can interfere either constructively or destructively with them. The limit for the charged Higgs mass from this process can be weakened by the effects of the SUSY particles. The minimal supergravity model provides an attractive framework for SUSY extension of SM. In this model, masses and mixing parameters for SUSY particles can be expressed by a few soft SUSY breaking parameters as well as gauge and Yukawa coupling constants. The b → s γ branching ratio thus depends on much smaller number of free parameters compared to that in general SUSY standard models. It has been noticed [8, 13, 16] that the sign of the SUSY loop contributions with respect to those of the SM and charged Higgs is strongly correlated with the sign of the higgsino mass parameter i.e. the μ parameter in the minimal supergravity model. In this paper, we compare the CLEO data with the prediction of the minimal supergravity model and determine the allowed region in the parameter space of the