Measurement of $C\!P$ violation in $B^{0}\rightarrow K_{S}^{0}π^{0}$ decays at Belle II

Abstract

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

Figures (1)

• Figure 1: Distributions of (a) $M^{\prime}_{\rm bc}$, (b) $\Delta E$, and (c) $C^{\prime}_{\rm BDT}$ with fit projections overlaid for both $B\xspace\xspace^0\xspace$ and $\overline{ B\xspace}{}\xspace^0\xspace$ candidates satisfying the criteria $5.27<M^{\prime}_{\rm bc}< 5.29\mathrm{\,Ge V}\xspace$, $-0.15<\Delta E<0.10\mathrm{\,Ge V}\xspace$, $|\Delta t|<$ 10.0 ps, and $C^{\prime}_{\rm BDT}>0.0$ (except for the variable displayed). The solid curve shows the fit projection, while various fit components are explained in the legends. Distribution of (d) $\Delta t$ for tagged $B\xspace\xspace^0\xspace$ and $\overline{ B\xspace}{}\xspace^0\xspace$ candidates after subtracting background with the $_s{\mathcal{P}}$lot method sPlot. The asymmetry, defined as $[N(B\xspace\xspace^0\xspace_{\rm tag})-N(\overline{ B\xspace}{}\xspace^0\xspace_{\rm tag})]/[N(B\xspace\xspace^0\xspace_{\rm tag})+N(\overline{ B\xspace}{}\xspace^0\xspace_{\rm tag})]$, is displayed underneath along with the fit projection.