Constraints on secret neutrino interactions after Planck

Neutrino interactions beyond the standard model of particle physics may affect the cosmological evolution and can be constrained through observations. We consider the possibility that neutrinos possess secret scalar or pseudoscalar interactions mediated by the Nambu-Goldstone boson of a still unknown spontaneously broken global U(1) symmetry, as in, e.g., Majoron models. In such scenarios, neutrinos still decouple at T similar or equal to 1 MeV, but become tightly coupled again ("recouple") at later stages of the cosmological evolution. We use available observations of the cosmic microwave background (CMB) anisotropies, including Planck 2013 and the joint BICEP2/Planck 2015 data, to derive constraints on the quantity gamma(4)(nu nu) parameterizing the neutrino collision rate due to scalar or pseudoscalar interactions. We consider both a minimal extension of the standard Lambda CDM model, and more complicated scenarios with extra relativistic degrees of freedom or non-vanishing tensor amplitude. For a wide range of dataset and model combinations, we find a typical constraint gamma(4)(nu nu) less than or similar to 0.9 x 10(-27) (95% C.L.), implying an upper limit on the redshift z(nu rec) of neutrino recoupling less than or similar to 8500, leaving open the possibility that the latter occured well before hydrogen recombination. In the framework of Majoron models, the upper limit on gamma(nu nu) roughly translates on a constraint g less than or similar to 8.2 x 10(-7) on the Majoron-neutrino coupling constant g. In general, the data show a weak (similar to 1 sigma) but intriguing preference for non-zero values of gamma(4)(nu nu),with best fits in the range gamma(4)(nu nu) = (0.15-0.35) x 10(-27), depending on the particular dataset. This is more evident when either high-resolution CMB observations from the ACT and SPT experiments are included, or the possibility of non-vanishing tensor modes is considered. In particular, for the minimal model Lambda CDM+gamma(nu nu) and including the Planck 2013, ACT and SPT data, we report gamma(4)(nu nu) = (0.44(-0.36)(+0.17)) x 10(-27) (300 less than or similar to Z(vrec) less than or similar to 5500) at 68% confidence level.