Study of B ->K pi pi gamma Decays

Using $471 \times 10^6 ~B \bar B$ decays recorded with the \babar\ detector at the PEP-II $e^+ e^-$ storage ring, we present the time-dependent \CP\ asymmetry measurement in the radiative penguin decay mode $B^0 \rightarrow K^0_S \rho(770)^0 \gamma$, yielding $S_{K^0_S \rho^0 \gamma} =-0.17\pm 0.32^{+0.07}_{-0.06}$. Since the result is extracted from the time-dependent \CP~asymmetry parameters $S_{K^0_S \pi^+ \pi^- \gamma} =0.14\pm 0.25^{+0.04}_{-0.03}$ and $C _{K^0_S \pi^+ \pi^- \gamma} =-0.39\pm$ measured in the neutral decay $B^0 \rightarrow K^0_S \pi^+ \pi^- \gamma$, we need to correct for the dilution of $K^*(892) \pi \gamma$ in $K \rho \gamma$. The dilution factor $D_{K^0_S \rho \gamma} =-0.79^{+0.18}_{-0.17}$ is determined from a study of the charged mode $B^+ \rightarrow K^+ \pi^+\pi^-\gamma$, which produces more signal events and is related to the neutral mode by isospin. We need detailed knowledge of the resonance structure in the $K^+ \pi^+ \pi^-$ mass spectrum and measure branching fractions of different resonances to $K \pi$ and $\pi \pi$ final states. We also measure the branching fractions ${\cal B} (B^+ \rightarrow K^+ \pi^+ \pi^- \gamma)= (27.2\pm 1.0 \pm 1.2) \times 10^{-6}$ and ${\cal B} (B^0 \rightarrow K^0_S \pi^+ \pi^- \gamma)= (24.0\pm 2.4^{+1.7}_{-1.8}) \times 10^{-6}$.