Time-dependent Dalitz Plot Analysis of B0 --> Ks pi+ pi-

We perform a time-dependent Dalitz plot analysis of $B^0 \to K^0_{\scriptscriptstyle S}\pi^+\pi^-$ in order to extract the ${CP}$ violation parameters of $f_0(980) K^0_{\scriptscriptstyle S}$ and $\rho^0(770) K^0_{\scriptscriptstyle S}$ and direct ${CP}$-asymmetries of $K^{*+}(892) \pi^-$. The results are obtained from a data sample of $(383\pm3)\times10^{6}$ $B \bar{B}$ decays, collected with the BaBar detector at the PEP2 asymmetric--energy $B$ factory at SLAC. The measured values of $2\beta_{\rm eff}$ in $B^0$ decays to $f_0(980)K^0_{\scriptscriptstyle S}$ and $\rho^0(770) K^0_{\scriptscriptstyle S}$ are $(89^{+22}_{-20} \pm 5 \pm 8)^\circ$ and $(37 ^{+19}_{-17} \pm 5 \pm 6)^\circ$, respectively, where the first quoted uncertainty is statistical, the second is systematic and the third is Dalitz plot signal model uncertainty. We measure the significance of $2\beta_{\rm eff}(f_0(980)K^0_{\scriptscriptstyle S})\neq 0$ to be $4.3 \sigma$. In decays to $K^*(892) \pi$ we find $A_{{CP}} = -0.18 \pm 0.10 \pm 0.03 \pm 0.03$. The measured phase difference between the decay amplitudes of $B^0 \to K^{*+}(892) \pi^-$ and $\bar{B}^0 \to K^{*-}(892) \pi^+$ is $(-164 \pm 24 \pm 12 \pm 15)^\circ$. All results are preliminary.