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First observation of $D^0- \bar D^0$ oscillations in $D^0 \to K^+\pi^-\pi^+\pi^-$ decays and measurement of the associated coherence parameters

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Abstract

Charm meson oscillations are observed in a time-dependent analysis of the ratio of $D^0\to K^+\pi^-\pi^+\pi^-$ to $D^0\to K^-\pi^+\pi^-\pi^+$ decay rates, using data corresponding to an integrated luminosity of $3.0\,{\rm fb}^{-1}$ recorded by the LHCb experiment. The measurements presented are sensitive to the phase-space averaged ratio of doubly Cabibbo-suppressed to Cabibbo-favoured amplitudes $r_{D}^{K3\pi}$ and the product of the coherence factor $R_{D}^{K3\pi}$ and a charm mixing parameter $y^{'}_{K3\pi}$. The constraints measured are $r_{D}^{K3\pi}=(5.67 \pm 0.12)\times10^{-2}$, which is the most precise determination to date, and $R_{D}^{K3\pi} \cdot y^{'}_{K3\pi} = (0.3 \pm 1.8)\times 10^{-3}$, which provides useful input for determinations of the CP-violating phase $\gamma$ in $B^\pm \to D K^\pm, D \to K^\mp\pi^\pm\pi^\mp\pi^\pm$ decays. The analysis also gives the most precise measurement of the $D^0\to K^+\pi^-\pi^+\pi^-$ branching fraction, and the first observation of $D^0-\bar D^0$ oscillations in this decay mode, with a significance of 8.2 standard deviations.

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Decay-time integrated $\Delta m$ distributions for RS (left) and WS (right) candidates with the fit result superimposed.

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Decay-time evolution of the background-subtracted and efficiency corrected WS/RS ratio (points) with the results of the unconstrained (solid line), mixing-constrained (dashed/dotted line), and no-mixing (dashed line) fits superimposed. The bin centres are set to the decay-time where $R(t)$ is equal to the bin integrated ratio $\tilde{R}$ from the unconstrained fit.

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Confidence-level (CL) regions in the $ R^{K3\pi}_{D} - \delta_{D}^{K3\pi} $ plane taken from the mixing-constrained fit.

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Tables and captions

Results of the decay-time dependent fits to the WS/RS ratio for the unconstrained and mixing-constrained fit configurations. The results include all systematic uncertainties.

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Supplementary Material [file]

Supplementary material full pdf

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This archive contains extra figures related to LHCb-PAPER-2015-057 supplementary.pdf - A PDF file with all figures and captions Fig*.pdf - Figures in PDF format Fig*.eps - Figures in EPS format Fig*.png - Figures in PNG format

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.