Rate measurement of D⁰ → K⁺ π⁻ π⁰ and constraints on D⁰ − D¯⁰ 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 0.0043^0.0011_−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.