We study the CMB constraint on non-Gaussianity in CDM isocurvature perturbations. Non-Gaussian isocurvature perturbations can be produced in various models at the very early stage of the Universe. Since the isocurvature perturbations little affect the structure formation at late times, CMB is the best probe of isocurvature non-Gaussianity at least in the near future. In this paper, we focus on uncorrelated isocurvature perturbations and constrain their non-Gaussianity. For this purpose, we

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... y several state-of-art techniques for the analysis of CMB data and simulation. We use the WMAP 7 year data of temperature anisotropy. When the adiabatic perturbations are assumed to be Gaussian, we obtained a constraint on the isocurvature non-Gaussianity alpha^2 f_NL^(ISO)=40+-66 for the scale invariant isocurvature power spectrum, where alpha is the ratio of the power spectrum of isocurvature perturbations to that of the adiabatic ones. When we assume that the adiabatic perturbations can also be non-Gaussian, we obtain f_NL=38+-24 and alpha^2 f_NL^(ISO)=-8+-72. We also discuss implications our results for the axion CDM isocurvature model.