Probing Decoupling in Dark Sectors with the Cosmic Microwave Background

TL;DR

This paper investigates how decoupling relativistic particles in dark sectors affect the CMB's acoustic peaks, enabling constraints on their properties and decoupling epoch with future experiments.

Contribution

It introduces a method to identify scale-dependent signatures of decoupling species in the CMB, extending beyond constant shifts and allowing determination of decoupling redshift.

Findings

Stage IV CMB can measure N_eff and decoupling redshift with high precision.

Decoupling species cause scale-dependent amplitude and phase shifts in acoustic peaks.

Constraints weaken for smaller N_eff but remain significant for certain redshift ranges.

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 $\Delta N_{eff}$. 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 $\Delta N_{eff}$ but the redshift $z_{dec,X}$ at which the new species decoupled. For $\Delta N_{eff} >0.334$, a Stage IV CMB experiment could determine $N_{eff}$ at the percent level and $z_{dec,X}$ at the $\sim 10\%$ level. For smaller values, $\Delta N_{eff}\sim 0.1$, constraints on $z_{dec,X}$ weaken but remain $\sim 20-50\%$ for $z_{dec,X} \sim \mathcal{O}(10^3-10^4)$. As an application, we study the contributions to $\Delta N_{eff}$ and determine the $z_{dec,X}$ values for simple implementations of the so-called $N$naturalness model.