We show that with a good fit to the muon constraint, the CP asymmetry can be as large as 25% (15%) for ($B^0_d \to \mu^+ \mu^-$) in SUGRA models with nonuniversal gaugino masses and MSSM. If tau events indentified at B factories the CP asymmetry in can be a powerful probe on physics beyond SM. An interesting case is that new physics gives a prediction on the branching ratio which is still in the uncertain region of the SM prediction. The CP asymmetry for in this case can still reach 20%. So it is powerful to shed light on physics beyond SM while the CP asymmetry of in this case can only reach 2% at most which is too small to draw a definite conclusion on new physics effects at B factories.

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