Observation of $CP$ Violation in $B\to \eta \prime K^0$ Decays

We present measurements of the time-dependent $CP$-violation parameters $S$ and $C$ in $B^0\to {\eta }^{\prime }{K}^0$ decays. The data sample corresponds to $384\times {10}^6$ $B\overline{B}$ pairs produced by $e^{+}{e}^{-}$ annihilation at the ${\rm Y}(4S)$. The results are $S=0.58\pm 0.10\pm 0.03$ and $C=-0.16\pm 0.07\pm 0.03$. We observe mixing-induced $CP$ violation with a significance of 5.5 standard deviations in this $b\to s$ penguin dominated mode.

Figure 1

Distributions projected (see text) onto (a) $m_{\mathrm{ES}}$ and (b) $\Delta E$ for ${\eta }^{\prime }{K}_S^0$ candidates, and (c) $\Delta E$ for ${\eta }^{\prime }{K}_L^0$ candidates. The solid lines shows the full fit result and the dashed lines show the background contributions.

Figure 2

Projections (see text) onto $\Delta t$ for (a) ${\eta }^{\prime }{K}_S^0$ and (c) ${\eta }^{\prime }{K}_L^0$ of the data (points with error bars for $B^0$ tags in solid circles and ${\overline{B}}^0$ tags in empty rectangles), fit function (solid and dashed lines for $B^0$ and $B^0$ tagged events, respectively), and background function (shaded regions). We show the asymmetry between $B^0$ and ${\overline{B}}^0$ tags for (b) ${\eta }^{\prime }{K}_S^0$ and (d) ${\eta }^{\prime }{K}_L^0$; the lines represent the fit functions.