Probing decoupling in dark sectors with the cosmic microwave background

Abstract

The acoustic peaks in the angular power spectrum of cosmic microwave background (CMB) temperature and polarization anisotropies play an important role as a probe of the nature of new relativistic particles contributing to the radiation density in the early universe, parametrized by Δ Neff. The amplitude and phase of the acoustic oscillations provide information about whether the extra species are free-streaming particles, like neutrinos, or tightly-coupled, like the photons, during eras probed by the CMB. On the other hand, some extensions of the Standard Model produce new relativistic particles that decouple from their own non-gravitational interactions after neutrinos, but prior to photons. We study the signature of new relativistic species that decouple during this intermediate epoch. We argue that the decoupling species will cause a scale-dependent change in the amplitude and phase shift of the acoustic oscillations, different from the usual constant shifts on small scales. For intermediate decoupling times, the phase and amplitude shifts depend not only on Δ Neff but the redshift zdec,X at which the new species decoupled. For Δ Neff > 0.334, a Stage IV CMB experiment could determine Neff at the percent level and zdec,X at the ∼ 10% level. For smaller values, Δ Neff ∼ 0.1, constraints on zdec,X weaken but remain ∼ 20–50% for zdec,X ∼ \U0001d4aa(103–104). As an application, we study the contributions to Δ Neff and determine the zdec,X values for simple implementations of the so-called Nnaturalness model.