First Observation of D⁰-D⁰bar Oscillations in D⁰→K⁺π^-π⁺π^- Decays and Measurement of the Associated Coherence Parameters.

Charm meson oscillations are observed in a time-dependent analysis of the ratio of D⁰→K⁺π^-π⁺π^- to D⁰→K^-π⁺π^-π⁺ decay rates, using data corresponding to an integrated luminosity of 3.0 fb^-1 recorded by the LHCb experiment. The measurements presented are sensitive to the phase-space averaged ratio of doubly Cabibbo-suppressed to Cabibbo-favoured amplitudes r_D^K3π and the product of the coherence factor R_D^K3π and a charm mixing parameter y'_K3π. The constraints measured are r_D^K3π=(5.67 ± 0.12) × 10^-2, which is the most precise determination to date, and R_D^K3π · y'_K3π = (0.3 ± 1.8) × 10^-3, which provides useful input for determinations of the CP-violating phase γ in B^±→DK^±, D⁰→K⁺π^-π⁺π^- decays.  The analysis also gives the most precise measurement of the D⁰→K⁺π^-π⁺π^- branching fraction, and the first observation of D⁰-D⁰bar oscillations in this decay mode, with a significance of 8.2 standard deviations.