Determination of the sign of the decay width difference in the bs0 system

The interference between the K ⁺K ^- S-wave and P-wave amplitudes in Bs0→J/ψK ⁺K ^- decays with the K ⁺K ^- pairs in the region around the (1020) resonance is used to determine the variation of the difference of the strong phase between these amplitudes as a function of K ⁺K ^- invariant mass. Combined with the results from our CP asymmetry measurement in Bs0→J/ψ decays, we conclude that the Bs0 mass eigenstate that is almost CP=+1 is lighter and decays faster than the mass eigenstate that is almost CP=-1. This determines the sign of the decay width difference ΔΓ _sΓ _L-Γ _H to be positive. Our result also resolves the ambiguity in the past measurements of the CP violating phase _s to be close to zero rather than π. These conclusions are in agreement with the standard model expectations.