Finding eV-scale Light Relics with Cosmological Observables

Abstract

Cosmological data provide a powerful tool in the search for physics beyond the Standard Model (SM). An interesting target are light relics, new degrees of freedom which decoupled from the SM while relativistic. Nearly massless relics contribute to the radiation energy budget, and are commonly searched through variations in the effective number N eff of neutrino species. Additionally, relics with masses on the eV scale (meV-10 eV) become non-relativistic before today, and thus behave as matter instead of radiation. This leaves an imprint in the clustering of the large-scale structure of the universe, as light relics have important streaming motions, mirroring the case of massive neutrinos. Here we forecast how well current and upcoming cosmological surveys can probe light massive relics (LiMRs). We consider minimal extensions to the SM by both fermionic and bosonic relic degrees of freedom. By combining current and upcoming cosmic-microwave-background and large-scale-structure surveys, we forecast the significance at which each LiMR, with different masses and temperatures, can be detected. We find that a very large coverage of parameter space will be attainable by upcoming experiments, opening the possibility of exploring uncharted territory for new physics beyond the SM.

Unraveling the Dirac Neutrino with Cosmological and Terrestrial Detectors

“…However, there are many compelling beyond the SM scenarios in which the sterile states acquire a thermal distribution in the early universe and survive today as cosmological relics. We demonstrate how this leads to deviations in the cosmological neutrino observables (namely, N eff and Σm ν ) that are correlated with one another, as expected in general for eV-scale relics [13], and also correlated with terrestrial neutrino observables (namely, m νe and Σ i m i ), as also pointed out recently in Ref. [14].…”

“…How will this data inform our understanding of neutrinos and their role in cosmology? If these experiments discover that N eff 3.044 and Σm ν Σ i m i , this would provide strong evidence for the presence of a new cosmological relic with mass m = O(0.1) eV [13]. What are the candidates for this eV-scale relic and how can we distinguish them with laboratory tests?…”

“…See also Ref. [13] for a discussion of general eV-scale relics and their impact on cosmological observables, and see Refs. [14,25] for a discussion of Dirac neutrinos in cosmology.…”

Imprint of relativistic particles on the anisotropies of the stochastic gravitational-wave background

“…The fractional energy density of decoupled relativistic particles varies since η i down to temperatures around 0.1 MeV, when it reaches a constant value which depends on the chosen N eff ; for instance for 3 light neutrino species it corresponds to f dec (η T <0.1 MeV ) = 0.4 (different evolutions of f dec (η) for different particle candidates are shown for instance in [43]). In this interval Φ and Ψ evolve following eqs.…”

Non-thermal Hot Dark Matter from Inflaton/Moduli Decay: The Momentum Distribution and Relaxing the Cosmological Mass Bound

“…Finally, there is lot of optimism that near future experiments will be able to distinguish or detect hot dark matter candidates with different particle physics origin (see e.g [98]). For this, understanding the subtle effects which different hot DM particle imprint on CMB power spectra (for our case low phase shift, high suppression) that could be measured by CMB-S4 [99] experiments is very important.…”