Rate measurement of D0 → K+ π-π0 and constraints on D0 - D0̄ mixing

We present an observation and time-integrated rate measurement of the decay D0 → K+ π− π0 produced in 9fb−1 of e+e− collisions near the r(4S) resonance. The signal is inconsistent with an upward fluctuation of the background by 4.9 standard deviations. We measured the time-integrated rate of D0 → K+ π− π0 normalized to the rate of D¯0 → K+ π− π0 to be 0.00430.0011−0.0011 (stat) ± 0.0007(syst) This decay can be produced by doubly Cabibbo-suppressed decays or by the D0 evolving into a D¯0 through mixing, followed by a Cabibbo-favored decay to K+ π− π0. We also found the CP asymmetry A = (9+25−22)% be consistent with zero.