Measurement of phi1 using B --> D[Ks pi+ pi-] h0

We present a preliminary measurement of the angle phi1 of the CKM Unitarity Triangle using time-dependent analysis of D -> Ks pi+ pi- decays produced in neutral B meson decay to a neutral D meson and a light meson (B -> D h). The method allows one to directly extract the value of 2phi1. The ambiguity between 2phi1 and pi-2phi1 in the measurement of sin(2phi1) can then be resolved. We obtain phi1=16 +- 21 +- 11. The 95% CL region including systematic error is -30<phi1<62, thus ruling out the second solution from sin(2phi1)=0.726 +- 0.037 at the 97% CL.