Constraining the Phase of B_s - \bar{B}_s Mixing

New physics contributions to B_s-\bar{B}_s mixing can be parametrized by the size (r_s^2) and the phase (2θ_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 \pm 0.011. Then we combine recent measurements by the CDF and D0 collaborations - the mass difference (ΔM_s), the width difference (ΔΓ_s) and A_SL^s - to constrain 2θ_s. The errors on ΔΓ_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 2θ_s ~ 3π/2 and, in particular, 2θ_s ~ π/2, are disfavored.

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