Constraining the phase of Bs-B̄s mixing

New physics contributions to Bs-B̄s mixing can be parametrized by the size (rs2) and the phase (2θs) 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 ASL to obtain the first constraint on the semileptonic CP asymmetry in Bs decays, ASLs=-0.008±0. 011. Then we combine recent measurements by the CDF and D0 Collaborations-the mass difference (ΔMs), the width difference (ΔΓs), and ASLs-to constrain 2θs. The errors on ΔΓs and ASLs should still be reduced to have a sensitive probe of the phase, yet the central values are such that the regions around 2θs∼3π/2 and, in particular, 2θs∼π/2, are disfavored.