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 fb$^{-1}$. The $y_{CP}$ parameter is measured to be $(0.57 pm 0.13(rm{stat.}) pm 0.09(rm{syst.}))%$, in agreement with, and as precise as, the current world-average value.