New Target for Cosmic Axion Searches

Abstract

Future cosmic microwave background experiments have the potential to probe the density of relativistic species at the subpercent level. This sensitivity allows light thermal relics to be detected up to arbitrarily high decoupling temperatures. Conversely, the absence of a detection would require extra light species never to have been in equilibrium with the Standard Model. In this paper, we exploit this feature to demonstrate the sensitivity of future cosmological observations to the couplings of axions to all of the Standard Model degrees of freedom. In many cases, the constraints achievable from cosmology will surpass existing bounds from laboratory experiments and astrophysical observations by orders of magnitude.

Status of Neutrino Properties and Future Prospects—Cosmological and Astrophysical Constraints

“…Those effects are not necessarily related to neutrino physics, as the definition of N eff in terms of the number of relativistic degrees of freedom suggests. For example, the existence of a Goldstone boson that decouples well before the QCD phase transition would appear as an increased number of degrees of freedom, with N eff ≡ N eff − 3.046 = 0.027 [182]. Speaking however about changes in N eff that are somehow related to neutrino physics, the most notable example is probably the existence of one (or more) additional, sterile light eigenstate, produced through some mechanism in the early Universe.…”

Observing Dirac neutrinos in the cosmic microwave background

“…At temperatures above the electroweak scale, g saturates to 106.75, the maximum amount of entropy available from SM particles. Reference [8] has recently studied the impact of CMB-S4 on axions and axion-like particles (g s = 1), which are reasonably well motivated but could easily lead to an entropy-suppressed contribution ∆N eff 0.027 that is below the CMB-S4 reach. * kevork@uci.edu † Julian.Heeck@uci.edu…”

Impact of theoretical assumptions in the determination of the neutrino effective number from future CMB measurements

“…If, however, the coupling happens after the QCD transition, then any relativistic particle will contribute with a minimum value of ΔN eff ¼ 0.027. More precisely, the minimum contribution for a single real scalar particle is ΔN eff ¼ 0.027, for a Weyl fermion it is ΔN eff ¼ 0.047, and for a light vector boson it is ΔN eff ¼ 0.054 [11]. It is therefore clear that future CMB experiments reaching an experimental sensitivity of σðN eff Þ ¼ 0.013 will have the potential to rule out the existence of any relativistic particle beyond those predicted in the standard model at more than 95% C.L.…”

“…As discussed in [11], any particle that decouples from the primordial thermal plasma before the QCD transition will contribute with ΔN eff ¼ N eff − 3.046 ≥ 0.3. This number has already been tested with Planck, and near future data can fully falsify this hypothesis.…”