Constraining the phase of Bs-(B)over-bars mixing

New physics contributions to B-s-(B) over bar (s) mixing can be parametrized by the size (r(s)(2)) and the phase (2 theta(s)) of the total mixing amplitude relative to the standard model amplitude. The phase has so far been unconstrained. We first use the D0 measurement of the semileptonic CP asymmetry A(SL) to obtain the first constraint on the semileptonic CP asymmetry in B-s decays, A(SL)(s)=-0.008 +/- 0.011. Then we combine recent measurements by the CDF and D0 Collaborations-the mass difference (Delta M-s), the width difference (Delta Gamma(s)), and A(SL)(s)-to constrain 2 theta(s). The errors on Delta Gamma(s) and A(SL)(s) should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2 theta(s)similar to 3 pi/2 and, in particular, 2 theta(s)similar to pi/2, are disfavored.