First Evidence for cos2β>0 and Resolution of the Cabibbo-Kobayashi-Maskawa Quark-Mixing Unitarity Triangle Ambiguity

Abstract

We present first evidence that the cosine of the C P -violating weak phase 2 β is positive, and hence exclude trigonometric multifold solutions of the Cabibbo-Kobayashi-Maskawa (CKM) Unitarity Triangle using a time-dependent Dalitz plot analysis of B 0 → D ( * ) h 0 with D → K 0 S π + π − decays, where h 0 ∈ { π 0 , η , ω } denotes a light unflavored and neutral hadron. The measurement is performed combining the final data sets of the BABAR and Belle experiments collected at the Υ ( 4 S ) resonance at the asymmetric-energy B factories PEP-II at SLAC and KEKB at KEK, respectively. The data samples contain ( 471 ± 3 ) × 10 6 B ¯ B pairs recorded by the BABAR detector and ( 772 ± 11 ) × 10 6 B ¯ B pairs recorded by the Belle detector. The results of the measurement are sin 2 β = 0.80 ± 0.14 ( stat ) ± 0.06 ( syst ) ± 0.03 ( model ) and cos 2 β = 0.91 ± 0.22 ( stat ) ± 0.09 ( syst ) ± 0.07 ( model ) . The result for the direct measurement of the angle β of the CKM Unitarity Triangle is β = [ 22.5 ± 4.4 ( stat ) ± 1.2 ( syst ) ± 0.6 ( model ) ] ° . The measurement assumes no direct C P violation in B 0 → D ( * ) h 0 decays. The quoted model uncertainties are due to the composition of the D 0 → K 0 S π + π − decay amplitude model, which is newly established by performing a Dalitz plot amplitude analysis using a high-statistics e + e − → c ¯ c data sample. C P violation is observed in B 0 → D ( * ) h 0 decays at the level of 5.1 standard deviations. The significance for cos 2 β > 0 is 3.7 standard deviations. The trigonometric multifold solution π / 2 − β = ( 68.1 ± 0.7 ) ° is excluded at the level of 7.3 standard deviations. The measurement resolves an ambiguity in the determination of the apex of the CKM Unitarity Triangle.