Measurement of time-dependent $CP$ asymmetries in $B^0 \to K_{\rm S}^0 \: π^{+} π^{-} γ$ decays at Belle and Belle II

Abstract

We present a measurement of the time-dependent $CP$ asymmetry in $B^0 \to K_{\rm S}^0 \: π^{+} π^{-} γ$ decays using a data set of 365 fb$^{-1}$ recorded by the Belle II experiment and the final data set of 711 fb$^{-1}$ recorded by the Belle experiment at the ${\rm Υ(4S)}$ resonance. The direct and mixing-induced time-dependent $CP$ violation parameters $C$ and $S$ are determined along with two additional quantities, $S^{+}$ and $S^{-}$, defined in the two halves of the $m^2(K_{\rm S}^0 π^{+})-m^2(K_{\rm S}^0 π^{-})$ plane. The measured values are $C = -0.17 \pm 0.09 \pm 0.04$, $S = -0.29 \pm 0.11 \pm 0.05$, $S^{+} = -0.57 \pm 0.23 \pm 0.10$ and $S^{-} = 0.31 \pm 0.24 \pm 0.05$, where the first uncertainty is statistical and the second systematic.

Figures (2)

• Figure 1: Unbinned maximum likelihood projections on $M_\mathrm{bc}$ (left) and $\Delta E$ (right) using the Belle (top) and Belle II (bottom) datasets.
• Figure 2: Unbinned maximum likelihood projection on $\Delta t$ (left) and unbinned maximum likelihood projection on $\Delta t$ split by tagged $B\xspace\xspace^0$ and $\overline{ B\xspace}{}\xspace^0$ candidates in a signal enhanced window defined by: $M_\mathrm{bc}\xspace>5.27$${\mathrm{\,Ge V\!/}c^2}$, $|\Delta E\xspace|< 0.1$$\mathrm{\,Ge V}$ and $r \in [0.875,1]$ (right), using the Belle (top) and Belle II (bottom) dataset.