Measurement of $CP$ Violation in $B^0\to K_S^0{\pi }^0$ Decays at Belle II

We report a measurement of the $CP$-violating parameters $C$ and $S$ in $B^0\to K_S^0{\pi }^0$ decays at Belle II using a sample of $387\times {10}^{6}B\overline{B}$ events recorded in $e^{+}{e}^{-}$ collisions at a center-of-mass energy corresponding to the $\mathrm{\Upsilon }(4S)$ resonance. These parameters are determined by fitting the proper decay-time distribution of a sample of 415 signal events. We obtain $C=-0.04_{-0.15}^{+0.14}\pm 0.05$ and $S=0.75_{-0.23}^{+0.20}\pm 0.04$, where the first uncertainties are statistical and the second are systematic.

Figure 1

Distributions of (a) $M_{\mathrm{bc}}^{\prime }$, (b) $\mathrm{\Delta }E$, and (c) $C_{\mathrm{BDT}}^{\prime }$ with fit projections overlaid for both $B^0$ and ${\overline{B}}^0$ candidates satisfying the criteria $5.27<M_{\mathrm{bc}}^{\prime }<5.29\mathrm{GeV}$, $-0.15<\mathrm{\Delta }E<0.10\mathrm{GeV}$, $|\mathrm{\Delta }t|<10.0\mathrm{ps}$, and $C_{\mathrm{BDT}}^{\prime }>0.0$ (except for the variable displayed). The solid curve shows the fit projection, while various fit components are explained in the legends. Distribution of (d) $\mathrm{\Delta }t$ for tagged $B^0$ and ${\overline{B}}^0$ candidates after subtracting background with the ${}_s\mathcal{P}\mathrm{lot}$ method [37]. The asymmetry, defined as $[N(B_{\mathrm{tag}}^0)-N({\overline{B}}_{\mathrm{tag}}^0)]/[N(B_{\mathrm{tag}}^0)+N({\overline{B}}_{\mathrm{tag}}^0)]$, is displayed underneath along with the fit projection.