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

TL;DR

This paper determines the sign of the decay width difference in the B_s meson system by analyzing interference effects and CP asymmetries, confirming it is positive and aligning with Standard Model predictions.

Contribution

It provides the first direct determination of the sign of DeltaGamma_s using interference and CP asymmetry measurements, resolving previous ambiguities.

Findings

The lighter B_s mass eigenstate is almost CP=+1.

The decay width difference DeltaGamma_s is positive.

The CP violating phase phi_s is close to zero.

Abstract

The interference between the K+K- S-wave and P-wave amplitudes in B_s -> J/psi K+K- decays with the K+K- pairs in the region around the phi(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 measurements in B_s -> J/psi phi decays, we conclude that the B_s 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 DeltaGamma_s == Gamma_L -Gamma_H to be positive. Our result also resolves the ambiguity in the past measurements of the CP violating phase phi_s to be close to zero rather than pi. These conclusions are in agreement with the Standard Model expectations.