Dark Radiation from Particle Decays during Big Bang Nucleosynthesis

TL;DR

This paper investigates how decaying particles during Big Bang Nucleosynthesis influence dark radiation signals in the CMB and BBN, providing a detailed analysis of their effects based on particle lifetime and energy density.

Contribution

It offers a quantitative framework for understanding how particle decays during BBN affect effective neutrino numbers in CMB and BBN, exploring intermediate decay regimes.

Findings

Short lifetime decays ( au_X < 0.1 sec) produce equal ff in CMB and BBN.

Long lifetime decays ( au_X > 1000 sec) result in a nonzero ff in BBN independent of lifetime.

Any combination of ff in CMB and BBN is achievable within observational constraints.

Abstract

Cosmic microwave background (CMB) observations suggest the possibility of an extra dark radiation component, while the current evidence from big bang nucleosynthesis (BBN) is more ambiguous. Dark radiation from a decaying particle can affect these two processes differently. Early decays add an additional radiation component to both the CMB and BBN, while late decays can alter the radiation content seen in the CMB while having a negligible effect on BBN. Here we quantify this difference and explore the intermediate regime by examining particles decaying during BBN, i.e., particle lifetimes \tau_X satisfying 0.1 sec < \tau_X < 1000 sec. We calculate the change in the effective number of neutrino species, N_{eff}, as measured by the CMB, \Delta N_{CMB}, and the change in the effective number of neutrino species as measured by BBN, \Delta N_{BBN}, as a function of the decaying particle initial energy density and lifetime, where \Delta N_{BBN} is defined in terms of the number of additional two-component neutrinos needed to produce the same change in the primordial helium-4 abundance as our decaying particle. As expected, for short lifetimes (\tau_X < 0.1 sec), the particles decay before the onset of BBN, and \Delta N_{CMB} = \Delta N_{BBN}, while for long lifetimes (\tau_X > 1000 sec), \Delta N_{BBN} is dominated by the energy density of the nonrelativistic particles before they decay, so that \Delta N_{BBN} remains nonzero and becomes independent of the particle lifetime. By varying both the particle energy density and lifetime, one can obtain any desired combination of \Delta N_{BBN} and \Delta N_{CMB}, subject to the constraint that \Delta N_{CMB} >= \Delta N_{BBN}. We present limits on the decaying particle parameters derived from observational constraints on \Delta N_{CMB} and \Delta N_{BBN}.