Tickling the CMB damping tail: Scrutinizing the tension between the Atacama Cosmology Telescope and South Pole Telescope experiments

The Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) have recently provided new, very precise measurements of the cosmic microwave background (CMB) anisotropy damping tail. The values of the cosmological parameters inferred from these measurements, while broadly consistent with the expectations of the standard cosmological model, are providing interesting possible indications for new physics that are definitely worthy of investigation. The ACT results, while compatible with the standard expectation of three-neutrino families, indicate a level of CMB lensing, parametrized by the lensing amplitude parameter A_L, that is about 70% higher than expected. If not caused by an experimental systematic, an anomalous lensing amplitude could be produced by modifications of general relativity or coupled dark energy. Vice versa, the SPT experiment, while compatible with a standard level of CMB lensing, prefers an excess of dark radiation, parametrized by the effective number of relativistic degrees of freedom N_eff. Here we perform a new analysis of these experiments allowing simultaneous variations in both of these nonstandard parameters. We also combine these experiments, for the first time in the literature, with the recent WMAP9 data, one at a time. Including the Hubble Space Telescope prior on the Hubble constant and information from baryon acoustic oscillations surveys provides the following constraints from ACT: N_eff=3.54±0.41, A_L=1.64±0.32 at 68% C.L., while for SPT we have N_eff=3.78±0.33, A_L=0.79±0.11 at 68% C.L. In particular, the A_L estimates from the two experiments, even when a variation in N_eff is allowed, are in tension at more than 95% C.L.