Theoretical study of the Xi(1620) and Xi(1690) resonances in Xi_c -> pi^+ MB decays

Nonleptonic weak decays of Xi_c into pi^+ and a meson (M)-baryon (B) final state, MB, are analyzed from the viewpoint of probing S=-2 baryon resonances, i.e. Xi(1620) and Xi(1690), of which spin-parity and other properties are not well known. We argue that the weak decay of Xi_c is dominated by a single quark-line diagram, preferred by the Cabibbo-Kobayashi-Maskawa coefficient, color recombination factor, the diquark correlation, and the kinematical condition. The decay process has an advantage of being free from meson resonances in the pi^+M invariant mass distribution. The invariant mass distribution of the meson-baryon final state is calculated with three different chiral unitary approaches, assuming that the Xi(1620) and Xi(1690) resonances have J^P=1/2^-. It is found that a clear peak for the Xi(1690) is seen in the piXi and KbarLambda spectra. We also suggest that the ratios of the piXi, KbarLambda and KbarSigma final states are useful to distinguish whether the peak is originated from the Xi(1690) resonance or it is a KbarSigma threshold effect.