Small Second Acoustic Peak from Interacting Cold Dark Matter?

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We consider a possibility to explain the observed suppression of the second acoustic peak in the anisotropy spectrum of the Cosmic Microwave Background (CMB) by interaction between a fraction of non-baryonic Cold Dark Matter (CDM) and normal baryonic matter. This scenario does not require any modifications in the standard Big Bang Nucleosynthesis (BBN). We estimate the required values of the cross-section-to-mass ratio for elastic scattering of CDM particles off baryons. In case of velocity-independent elastic scattering (in the velocity interval 10⁻⁵--10⁻³) we find that such particles do not contradict observational limits if they are heavier than 10⁵ GeV or lighter than 0.5 GeV. Another candidate, which may appear in the models with infinite extra dimensions, is a quasistable charged particle decaying through tunneling into extra dimensions. Finally a millicharged particle with the electric charge ranging from 10⁻⁴ to 10⁻¹ and with mass M 0.1 GeV--1 TeV also may be responsible for the suppression of the second acoustic peak. As a byproduct we point out that CMB measurements set new limits on the allowed parameter space for the millicharged particles.