Constraining UV freeze-in of light relics with current and next-generation CMB observations

TL;DR

This paper explores how current and future CMB observations can constrain UV freeze-in production of light relics, which are weakly coupled particles generated in the early Universe, by analyzing their impact on the effective number of neutrino species, $ _{ m eff}$.

Contribution

It provides new calculations of $ _{ m eff}$ contributions from UV freeze-in models and assesses the sensitivity of upcoming CMB experiments compared to other constraints.

Findings

Next-generation CMB experiments can significantly constrain UV freeze-in models.

$ _{ m eff}$ from UV freeze-in depends on specific BSM physics and reheating temperature.

CMB constraints can complement or surpass astrophysical and collider bounds.

Abstract

Cosmological observations allow to measure the abundance of light relics produced in the early Universe. Most studies focus on the thermal freeze-out scenario, yet light relics produced by freeze-in are generic for models in which new light degrees of freedom do not couple strongly enough to the Standard Model (SM) plasma to allow for full thermalization in the early Universe. In ultraviolet (UV) freeze-in scenarios, rates for light relic production associated with non-renormalizable interactions typical of beyond the SM (BSM) models grow with temperature more quickly than the Hubble rate. Thus, relatively small couplings to the SM can be probed by current and next-generation cosmic microwave background (CMB) experiments. We investigate several representative benchmark BSM models, such as axion-like particles from Primakoff production, massless dark photons and light right-handed neutrinos. We calculate contributions to the effective number of neutrino species, $\Delta N_{\rm eff}$, in corners of parameter space not previously considered and discuss the sensitivity of CMB experiments compared to other probes. In contrast to freeze-out scenarios, $\Delta N_{\rm eff}$ from UV freeze-in is more dependent on both the specific BSM physics model and the reheating temperature. Depending on the details of the BSM scenario, we find that the sensitivity of next-generation CMB experiments can complement or surpass the current astrophysical, laboratory or collider constraints on the couplings of the SM to the light relic.