Extra radiation injection after neutrino decoupling in the early Universe contributes to the effective number of neutrino species that can be constrained by the cosmic microwave background (CMB). However, any effective neutrino number itself cannot uniquely determine the underlying source. We argue that the degeneracy can be relaxed by CMB spectral distortions, which are caused by energy exchange between the extra radiation and photons. We consider neutrinogenic CMB spectral distortions, where extra energy is released in the form of neutrinos but still creates the CMB spectral distortions via electroweak interactions. Focusing on the CMB $\mu$ distortion for illustration purposes, we show that the synergy between the effective neutrino number and the CMB $\mu$ distortion provides a complementary probe of hidden sectors that dominantly couple to neutrinos, opening up parameter space that can be targeted by joint CMB anisotropy and spectral distortion experiments.

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• Cosmic microwave background
• Particle decays
• Phenomenology
• Neutrinos