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

We measure the time-dependent $CP$ asymmetry in $B^0\to K_S^0{\pi }^0\gamma$ decays for two regions of $K_S^0\mathrm{\text{−}}{\pi }^0$ invariant mass, $m(K_S^0{\pi }^0)$, using the final BABAR data set of $467\times {10}^6$ $B\overline{B}$ pairs collected at the PEP-II $e^{+}{e}^{-}$ collider at SLAC. We find $339\pm 24$ $B^0\to K^{*0}\gamma$ candidates and measure $S_{K^{*}\gamma }=-0.03\pm 0.29\pm 0.03$ and $C_{K^{*}\gamma }=-0.14\pm 0.16\pm 0.03$. In the range $1.1<m(K_S^0{\pi }^0)<1.8\mathrm{GeV}/c^2$ we find $133\pm 20$ $B^0\to K_S^0{\pi }^0\gamma$ candidates and measure $S_{K_S^0{\pi }^0\gamma }=-0.78\pm 0.59\pm 0.09$ and $C_{K_S^0{\pi }^0\gamma }=-0.36\pm 0.33\pm 0.04$. The uncertainties are statistical and systematic, respectively.

Figure 1

Signal-enhanced distributions for $m_{\mathrm{ES}}$ (top) and $\Delta E$ (bottom) for the $K^{*}$ region (left) and the non-$K^{*}$ region (right). We show the fit result (solid line) and PDFs for signal (long dashed line), continuum (short dashed line), and $B\overline{B}$ (dotted line).

Figure 2

sPlot (see text) of $\Delta t$ in the $K^{*}$ region (left) and the non-$K^{*}$ region (right), with $B_{\mathrm{tag}}$ tagged as $B^0$ (top) or ${\overline{B}}^0$ (center), and the asymmetry (bottom). The curves are the signal PDFs.

Figure 3

sPlot (see text) of $m(K_S^0{\pi }^0)$.