We consider Cosmic Microwave Background constraints on inflation models for which the primordial power spectrum is a mixture of perturbations generated by inflaton fluctuations and fluctuations in a curvaton field .
If future experiments do not detect isocurvature modes or large non-Gaussianity , it will not be possible to directly distinguish inflaton and curvaton contributions .
We investigate whether current and future data can instead constrain the relative contributions of the two sources .
We model the spectrum with a bimodal form consisting of a sum of two independent power laws , with different spectral indices .
We quantify the ability of current and upcoming data sets to constrain the difference \Delta n in spectral indices , and relative fraction f of the subdominant power spectrum at a pivot scale of k _ { 0 } = 0.017 { { Mpc ^ { -1 } } h } .
Data sets selected are the WMAP 7-year data , alone and in conjunction with South Pole Telescope data , and a synthetic data set comparable to the upcoming Planck data set .
We find that current data show no increase in quality of fit for a mixed inflaton/curvaton power spectrum , and a pure power-law spectrum is favored .
The ability to constrain independent parameters such as the tensor/scalar ratio is not substantially affected by the additional parameters in the fit .
Planck will be capable of placing significant constraints on the parameter space for a bimodal spectrum .