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Dark Radiation or Warm Dark Matter from long lived particle decays in the light of Planck

Pasquale Di Bari, Stephen F. King, Alexander Merle

TL;DR

The paper confronts Planck-era hints of dark radiation with a minimal particle-physics scenario in which a thermally produced χ2 decays after neutrino decoupling into a lighter stable χ1 and SM fermions. Depending on the mass ratio M1/M2, the decay products either augment DR via non-thermal neutrinos and a relativistic χ1 or yield a non-thermally produced warm dark matter candidate χ1, with the two outcomes being largely exclusive. A key analytic result shows ΔN_eff at recombination can reach ∼0.5 in the DR case for plausible M2, τ, and relic abundance, while a WDM regime can satisfy small-scale structure constraints with M1/M2 ≈ 10^-2–10^-1. In both branches, suitable decays can address the lithium abundance problem without grossly perturbing D/H or Yp, making this a flexible and testable framework linking DR, WDM, and BBN/CMB observables.

Abstract

Although Planck data supports the standard ΛCDM model, it still allows for the presence of Dark Radiation corresponding up to about half an extra standard neutrino species. We propose a scenario for obtaining a fractional "effective neutrino species" from a thermally produced particle which decays into a much lighter stable relic plus standard fermions. At lifetimes much longer than 1 sec, both the relic particles and the non-thermal neutrino component contribute to Dark Radiation. By increasing the stable-to-unstable particle mass ratio, the relic particle no longer acts as Dark Radiation but instead becomes a candidate for Warm Dark Matter with mass O(1keV - 100GeV). In both cases it is possible to address the lithium problem.

Dark Radiation or Warm Dark Matter from long lived particle decays in the light of Planck

TL;DR

The paper confronts Planck-era hints of dark radiation with a minimal particle-physics scenario in which a thermally produced χ2 decays after neutrino decoupling into a lighter stable χ1 and SM fermions. Depending on the mass ratio M1/M2, the decay products either augment DR via non-thermal neutrinos and a relativistic χ1 or yield a non-thermally produced warm dark matter candidate χ1, with the two outcomes being largely exclusive. A key analytic result shows ΔN_eff at recombination can reach ∼0.5 in the DR case for plausible M2, τ, and relic abundance, while a WDM regime can satisfy small-scale structure constraints with M1/M2 ≈ 10^-2–10^-1. In both branches, suitable decays can address the lithium abundance problem without grossly perturbing D/H or Yp, making this a flexible and testable framework linking DR, WDM, and BBN/CMB observables.

Abstract

Although Planck data supports the standard ΛCDM model, it still allows for the presence of Dark Radiation corresponding up to about half an extra standard neutrino species. We propose a scenario for obtaining a fractional "effective neutrino species" from a thermally produced particle which decays into a much lighter stable relic plus standard fermions. At lifetimes much longer than 1 sec, both the relic particles and the non-thermal neutrino component contribute to Dark Radiation. By increasing the stable-to-unstable particle mass ratio, the relic particle no longer acts as Dark Radiation but instead becomes a candidate for Warm Dark Matter with mass O(1keV - 100GeV). In both cases it is possible to address the lithium problem.

Paper Structure

This paper contains 6 sections, 25 equations, 2 figures.

Table of Contents

  1. Introduction
  2. The model
  3. The Dark Radiation scenario
  4. Stable relic as Warm Dark Matter
  5. Addressing the lithium problem
  6. Conclusions

Figures (2)

  • Figure 1: Dark Radiation from $\chi_2 \to \chi_1 + f + \bar{f}$ at recombination time $t_{\rm rec}$ for a lifetime $\tau = 10$ sec.
  • Figure 2: The masses $M_1$ required to obtain the correct WDM abundance after the decay of all $\chi_2$'s (with $\tau = 10$ sec), displayed for different masses $M_2$ as functions of the suppression parameter $\epsilon_2$. The invisible $Z$-decay width requires $\epsilon_2 \lesssim 0.23$ for $M_2 < M_Z/2$. Note that we get warm DM in the marked region, while CDM is only marginally possible due to $\epsilon_2 \leq 1$.