We present a measurement of the time-dependent $CP$ asymmetry in $B^0 \to K_{ \rm S}^0 \: \pi^{ +} \pi^{ -} \gamma$ 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 ${ \rm \Upsilon(4S)}$ 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 $m^2(K_{ \rm S}^0 \pi^{ +})-m^2(K_{ \rm S}^0 \pi^{ -})$ 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.
Categories: hep-ex