© 2016 CERN. 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-favored amplitudes rDK3π and the product of the coherence factor RDK3π and a charm mixing parameter yK3π′. The constraints measured are rDK3π=(5.67±0.12)×10-2, which is the most precise determination to date, and RDK3πyK3π′=(0.3±1.8)×10-3, which provides useful input for determinations of the CP-violating phase γ in B±→DK±, D→Kπ±ππ± decays. The analysis also gives the most precise measurement of the D0→K+π-π+π- branching fraction, and the first observation of D0-D-0 oscillations in this decay mode, with a significance of 8.2 standard deviations.
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