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 T1 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 γ<inf>νν</inf>⁴, parameterizing the neutrino collision rate due to scalar or pseudoscalar interactions. We consider both a minimal extension of the standard ΛCDM 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 γ<inf>νν</inf>⁴ 0.9× 10^-27 (95% C.L.), implying an upper limit on the redshift z<inf>νrec</inf> of neutrino recoupling 0850, leaving open the possibility that the latter occured well before hydrogen recombination. In the framework of Majoron models, the upper limit on γ<inf>νν</inf> roughly translates on a constraint g 8.2× 10^-7 on the Majoron-neutrino coupling constant g. In general, the data show a weak (∼ 1σ) but intriguing preference for non-zero values of γ<inf>νν</inf>⁴, with best fits in the range γ<inf>νν</inf>⁴ = (0.15-0.35)× 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 ΛCDM+γ<inf>νν</inf> and including the Planck 2013, ACT and SPT data, we report γ<inf>νν</inf>⁴=(0.44^+0.17<inf>-0.36</inf>)×10^-27 (0300 z<inf>νrec</inf> 550) at 68% confidence level.