Fine structure constant and the CMB damping scale

The recent measurements of the Cosmic Microwave Background anisotropies at arc minute angular scales performed by the ACT and SPT experiments are probing the damping regime of CMB fluctuations. The analysis of these data sets unexpectedly suggests that the effective number of relativistic degrees of freedom is larger than the standard value of N-eff = 3.04, and inconsistent with it at more than two standard deviations. In this paper we study the role of a mechanism that could affect the shape of the CMB angular fluctuations at those scales, namely, a change in the recombination process through variations in the fine structure constant. We show that the new CMB data significantly improve the previous constraints on variations of alpha, with alpha/alpha(0) = 0.984 +/- 0.005, i.e. hinting also at a more than two standard deviation from the current, local, value alpha(0). A significant degeneracy is present between alpha and N-eff, and when variations in the latter are allowed the constraints on alpha are relaxed and again consistent with the standard value. Deviations of either parameter from their standard values would imply the presence of new, currently unknown physics.