Measurement of the Charm-Mixing Parameter $y_{CP}$

A measurement of the charm-mixing parameter $y_{CP}$ using $D^0\to K^{+}{K}^{-}$, $D^0\to {\pi }^{+}{\pi }^{-}$, and $D^0\to K^{-}{\pi }^{+}$ decays is reported. The $D^0$ mesons are required to originate from semimuonic decays of $B^{-}$ and ${\overline{B}}^0$ mesons. These decays are partially reconstructed in a data set of proton-proton collisions at center-of-mass energies of 7 and 8 TeV collected with the LHCb experiment and corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The $y_{CP}$ parameter is measured to be $(0.57\pm 0.13(\mathrm{stat})\pm 0.09(\mathrm{syst}))\%$, in agreement with, and as precise as, the current world-average value.

Figure 1

Distribution of $D^0$ mass for candidates passing the selection with fit projections overlaid: (top) $D^0\to K^{+}{K}^{-}$ decays, (center) $D^0\to {\pi }^{+}{\pi }^{-}$ decays, and (bottom) $D^0\to K^{-}{\pi }^{+}$ decays.

Figure 2

Ratio of decay-time acceptances from simulation for (top) $D^0\to K^{+}{K}^{-}$ over $D^0\to K^{-}{\pi }^{+}$ decays and (bottom) $D^0\to {\pi }^{+}{\pi }^{-}$ over $D^0\to K^{-}{\pi }^{+}$ decays.

Figure 3

Acceptance-corrected signal-yield ratio of (top) $D^0\to K^{+}{K}^{-}$ over $D^0\to K^{-}{\pi }^{+}$ decays and (bottom) $D^0\to {\pi }^{+}{\pi }^{-}$ over $D^0\to K^{-}{\pi }^{+}$ decays as a function of $D^0$ decay time, with fit projection overlaid.