  Using an effective-theory approach, we analyze the impact of BR(Bs to mu+
mu-) in constraining new-physics models that predict modifications of the
Z-boson couplings to down-type quarks. Under motivated assumptions about the
flavor structure of the effective theory, we show that the bounds presently
derived from BR(Bs to mu+ mu-) on the effective Z-boson couplings are
comparable (in the case of minimal flavor violation) or significantly more
stringent (in the case of generic partial compositeness) with respect to those
derived from observables at the Z peak.
