Determination of the Sign of the Decay Width Difference in the B0s System

Abstract

The interference between the K + K − S -wave and P -wave amplitudes in B 0 s → J / ψ K + K − decays with the K + K − pairs in the region around the ϕ ( 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 C P asymmetry measurement in B 0 s → J / ψ ϕ decays, we conclude that the B 0 s mass eigenstate that is almost C P = + 1 is lighter and decays faster than the mass eigenstate that is almost C P = − 1 . This determines the sign of the decay width difference Δ Γ s ≡ Γ L − Γ H to be positive. Our result also resolves the ambiguity in the past measurements of the C P violating phase ϕ s to be close to zero rather than π . These conclusions are in agreement with the standard model expectations.