Measurement of the time-dependent $CP$-violating asymmetry in $B^0\to K_S^0{\pi }^0\gamma$ decays

We present a measurement of the time-dependent $CP$-violating asymmetry in $B^0\to K^{*0}\gamma$ decays with $K^{*0}\to K_S^0{\pi }^0$ based on $232\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays collected with the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC. In a sample containing $157\pm 16$ signal decays, we measure $S_{K^{*0}\gamma }=-0.21\pm 0.40\pm 0.05$ and $C_{K^{*0}\gamma }=-0.40\pm 0.23\pm 0.03$, where the first error is statistical and the second systematic. We also explore $B^0\to K_S^0{\pi }^0\gamma$ decays with $1.1<m_{K_S^0{\pi }^0}<1.8$ $\mathrm{GeV}/c^2$ and find $59\pm 13$ signal events with $S_{K_S^0{\pi }^0\gamma }=0.9\pm 1.0\pm 0.2$ and $C_{K_S^0{\pi }^0\gamma }=-1.0\pm 0.5\pm 0.2$.

Figure 1

Signal and background (inset) distributions for $m_{\mathrm{ES}}$ (left) and $\Delta E$ (right) for $0.8<m_{K_S^0{\pi }^0}<1.0$ $\mathrm{GeV}/c^2$ obtained with the weighting technique described in the text. The curves represent the PDFs used in the fit, normalized to the fitted yield.

Figure 2

Signal distribution for $\Delta t$, obtained with the weighting technique described in the text, with $B_{\mathrm{tag}}$ tagged as $B^0$ (top) or ${\overline{B}}^0$ (center), and the asymmetry (bottom). The curves represent the PDFs for signal decays in the likelihood fit, normalized to the final fit result.

Figure 3

Constant-likelihood contours in the $S$-$C$ plane for $B^0\to K^{*0}\gamma$ corresponding to $-2\Delta \log L=1$, $2$ and $3$. The dashed circle is the physical boundary.

Figure 4

Distribution for $m_{K_S^0{\pi }^0}$ obtained with the weighting technique described in the text. For events with $m_{K_S^0{\pi }^0}>1.1$ $\mathrm{GeV}/c^2$ the cut on the cosine of the helicity angle $\cos {\theta }_{K^{*}}$ is not applied.

Figure 5

Signal and background (inset) distributions for $m_{\mathrm{ES}}$ (left) and $\Delta E$ (right) for $1.1<m_{K_S^0{\pi }^0}<1.8$ $\mathrm{GeV}/c^2$ obtained with the weighting technique described in the text. The curves represent the PDFs used in the fit, normalized to the fitted yield.