Measurement of $CP$ observables in $B^0\to D K^{*0}$ with $D\to K^+K^-$

The ratio of partial widths and the $CP$ asymmetry for $B^0$ decays to $D K^{*0}$ with $D\to K^+K^-$ are measured using $1.0\,{\rm fb}^{-1}$ of $pp$ collision data collected by the LHCb experiment in 2011. These observables are sensitive to the value of the angle $\gamma$ of the unitarity triangle. The partial width asymmetry is found equal to \begin{equation*} {\cal A}_d^{KK} = -0.47^{+0.24}_{-0.25} \pm 0.02, \end{equation*} where the first uncertainties are statistical and the second systematic. The ratios of the average partial widths in the $CP$ decay to the average partial width in the flavour specific decay $\overline{D}^0\to K^+\pi^-$ is also measured and equals to \begin{equation*} {\cal R}_d^{KK} = 1.42^{+0.41}_{-0.35} \pm 0.07. \end{equation*}