There are two ways to access the value of cos(2β), which allows to remove the ambiguity in between β and π/2 - β that comes from the measurement of sin(2β)

• From the time dependent analysis of the decay B → J/ψ K^*0_S, it is possible to extract the values of both sin(2β) and cos(2β), using the angular analysis of the Vector-Vector final state.
• Using the same Dalitz approach of the DK measurement of γ, it is possible to measure β from B⁰ → D⁰π⁰ decays (up to a β → β+π ambiguity) from the interference of the B⁰ and B⁰ decay amplitudes, as suggested in hep-ph/0503174.

Both BaBar and Belle performed these measurements, obtaining the results shown below. from the combination of the result, we found that cos(2β) < 0 is exluded with a probability of 99.95%.

(EPS), [JPG] cos2β > 0.48 @ 95% prob. from the average of all the measurements cos2β > 0 @ 99.95% prob.

(EPS) [JPG] bound from cos(2β). The allowed end excluded regions for sin(2β) are shown as a reference