Tensor non-Gaussianity from axion-gauge-fields dynamics: parameter search

Abstract

We calculate the bispectrum of scale-invariant tensor modes sourced by spectator SU(2) gauge fields during inflation in a model containing a scalar inflaton, a pseudoscalar axion and SU(2) gauge fields. A large bispectrum is generated in this model at tree-level as the gauge fields contain a tensor degree of freedom, and its production is dominated by self-coupling of the gauge fields. This is a unique feature of non-Abelian gauge theory. The shape of the tensor bispectrum is approximately an equilateral shape for 3 m Q 4, where m Q is an effective dimensionless mass of the SU(2) field normalised by the Hubble expansion rate during inflation. The amplitude of non-Gaussianity of the tensor modes, characterised by the ratio B h /P 2 h , is inversely proportional to the energy density fraction of the gauge field. This ratio can be much greater than unity, whereas the ratio from the vacuum fluctuation of the metric is of order unity. The bispectrum is effective at constraining large m Q regions of the parameter space, whereas the power spectrum constrains small m Q regions.

Tensor Non-Gaussianity Search: Current Status and Future Prospects

“…The detailed spectral feature varies with the shape of the coupling f (φ) or the axion potential [7,51]. For example, adopting f (φ) ∝ φ and a nearly flat potential, one can obtain a scale-invariant shape as [26]…”

“…The strongest constraints are obtained through the GW power spectrum measurement, and the GW bispectrum provides complementary information. In the model including a SU(2)-gauge coupling, the GW bispectrum can dominate the scalar one, and more interestingly, in some regions of the parameter space, the GW bispectrum has high detectability compared to the GW power spectrum [8,51]. The GW bispectrum is nearly scale-invariant and therefore parametrized by f ttt,eq NL , which is related to the tensor-to-scalar ratio r and the energy density fraction of the SU(2) gauge field Ω A according to [8] f ttt,eq…”

Squeezed tensor non-Gaussianity in non-attractor inflation

“…Tensor non-Gaussianity can also be an important observable for characterizing the primordial stochastic gravitational wave background at CMB scales, and have been explored in other contexts, see e.g [20,23,107]…”

Production and backreaction of fermions from axion- SU(2) gauge fields during inflation

“…Despite being energetically subdominant, the axion-SU (2) gauge field can produce potentially detectable signals during inflation. It can generate stochastic backgrounds of chiral gravitational waves [10,12,[14][15][16], tensor non-Gaussianity [17][18][19] and the observed matter asymmetry in the Universe [20][21][22][23]. 1 The upcoming LiteBIRD [26,27] and CMB Stage-4 experiments are set to provide further constraints on the axion-gauge fields [28][29][30].…”

“…The bound from the fermion particle production, depicted by the orange solid line (for m = H and ξ ϕ = 1) and the area underneath, does not lead to additional bounds on the observationally relevant parameter space. The ξ A on the horizontal axis is the same as m Q in [12,17,18].…”