Measurement of cos 2β in B^0 → D(*)h^0 with D → K^0_Sπ^+π^− decays by a combined time-dependent Dalitz plot analysis of BABAR and Belle data

We report measurements of sin 2β and cos 2β using a time-dependent Dalitz plot analysis of B^0 → D(*)h^0 with D → K^0_Sπ^+π^− decays, where the light unflavored and neutral hadron h^0 is a π^0, η, or ω meson. The analysis uses a combination of the final data sets of the BABAR and Belle experiments containing 471×10^6 and 772×10^6  BB pairs collected at the Υ(4S) resonance at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. We measure sin 2β = 0.80±0.14(stat)±0.06(syst)±0.03(model) and cos 2β = 0.91±0.22(stat)±0.09(syst)±0.07(model). The result for the direct measurement of the angle is β = (22.5±4.4(stat)±1.2(syst)±0.6(model))°. The last quoted uncertainties are due to the composition of the D^0 → K^0_Sπ^+π^− decay amplitude model, which is newly established by a Dalitz plot amplitude analysis of a high-statistics e^+e^− → cc data sample as part of this analysis. We find the first evidence for cos 2β > 0 at the level of 3.7 standard deviations. The measurement excludes the trigonometric multifold solution π/2−β = (68.1±0.7)° at the level of 7.3 standard deviations and therefore resolves an ambiguity in the determination of the apex of the CKM Unitarity Triangle. The hypothesis of β = 0° is ruled out at the level of 5.1 standard deviations, and thus CP violation is observed in B^0 → D(*)h^0 decays. The measurement assumes no direct CP violation in B^0 → D(*)h^0 decays.