Measurement of the $CP$ Asymmetry in $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$ Decays

A measurement of the $CP$ asymmetry in $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$ decays is presented, based on $1.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011. The measurement is performed in six bins of invariant mass squared of the ${\mu }^{+}{\mu }^{-}$ pair, excluding the $J/\psi$ and $\psi (2S)$ resonance regions. Production and detection asymmetries are removed using the $B^0\to J/\psi K^{*0}$ decay as a control mode. The integrated $CP$ asymmetry is found to be $-0.072\pm 0.040(\mathrm{stat})\pm 0.005(\mathrm{syst})$, consistent with the standard model.

Figure 1

Mass fits for $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$ decays used to extract the integrated $CP$ asymmetry. The curves displayed are the full mass fit (blue, solid line), the signal peak (red, short-dashed line), and the background (grey, long-dashed line). The mass fits on the top row correspond to the (a) $B^0$ and (b) ${\overline{B}}^0$ decays for one magnet polarity, while the bottom row shows the mass fits for (c) $B^0$ and (d) ${\overline{B}}^0$ for the reverse polarity.

Figure 2

Fitted value of ${\mathcal{A}}_{CP}$ in $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$ decays in bins of the ${\mu }^{+}{\mu }^{-}$ invariant mass squared ($q^2$). The red vertical lines mark the charmonium vetoes. The points are plotted at the mean value of $q^2$ in each bin. The uncertainties on each ${\mathcal{A}}_{CP}$ value are the statistical and systematic uncertainties added in quadrature. The dashed line corresponds to the $q^2$ integrated value, and the grey band is the $1\sigma$ uncertainty on this value.