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We consider Cosmic Microwave Background constraints on inflation models forwhich the primordial power spectrum is a mixture of perturbations generated byinflaton fluctuations and fluctuations in a curvaton field. If futureexperiments do not detect isocurvature modes or large non-Gaussianity, it willnot be possible to directly distinguish inflaton and curvaton contributions. Weinvestigate whether current and future data can instead constrain the relativecontributions of the two sources. We model the spectrum with a bimodal formconsisting of a sum of two independent power laws, with different spectralindices. We quantify the ability of current and upcoming data sets to constrainthe difference $\Delta n$ in spectral indices, and relative fraction $f$ of thesubdominant power spectrum at a pivot scale of $k = 0.017\ {{\rm Mpc^{-1}}\h}$. Data sets selected are the WMAP 7-year data, alone and in conjunction withSouth Pole Telescope data, and a synthetic data set comparable to the upcomingPlanck data set. We find that current data show no increase in quality of fitfor a mixed inflaton/curvaton power spectrum, and a pure power-law spectrum isfavored. The ability to constrain independent parameters such as thetensor/scalar ratio is not substantially affected by the additional parametersin the fit. Planck will be capable of placing significant constraints on theparameter space for a bimodal spectrum.

Inflaton or Curvaton? Constraints on Bimodal Primordial Spectra from Mixed Perturbations