The phenomenon of the axion kinetic misalignment with a generic PQ-breaking operator

Abstract

We investigate the phenomenology induced by generic PQ-breaking operators within the axion kinetic misalignment framework. We analyze their impact on the relic density of axion dark matter (DM), the PQ quality problem, axion-mediated fifth-force, as well as Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB) constraints. A nonzero initial axion velocity gives rise to brief periods of early matter domination and axion kinetic domination, leading to a nonstandard cosmological evolution. We compute the resulting gravitational wave (GW) signal from global cosmic strings and find that, because these nonstandard epochs are extremely short, the signal is highly suppressed and beyond the reach of existing experiments. Finally, we perform a parameter space scan, identify the regions and benchmark point that are consistent with all experimental constraints.

Figures (7)

• Figure 1: Axion relic density as a function of the initial field value. The red line is the saturated relic density. We employ $m=5$, $n=1$, $\lambda_n=10^{-10}$, $\lambda=10^{-20}$, $\delta_n=0$, $\epsilon=1$, and $f_a=10^{8}~\mathrm{GeV}$.
• Figure 2: Constraints on the PQ quality from nEDM measurements. The shaded region is excluded. We set $n=1, \delta_n=0.1, \lambda_n=10^{-20}, f_a=10^{8} \mathrm{GeV}$.
• Figure 3: The red line shows the nEDM bound. The solid blue line indicates the fifth-force constraint on $g_{aN}$ in the kinetic misalignment scenario, while the blue dashed line is excluded by the nEDM bound. Evidently, for QCD axions the nEDM bound is stronger than the fifth-force limit. we employe $\delta=\frac{\pi}{2}, m=4, n=1, \lambda_n=10^{-10}$. Figure from Ref. AxionLimit.
• Figure 4: Cosmological history including an axion-induced kination era.
• Figure 5: Peak frequency as a function of $S_i$ and $f_a$. We employ $m=4$, $n=1$, $\lambda_n=10^{-10}$, $\delta_n=0$, $\lambda=10^{-10}$, and $\alpha=0.1$. The solid and dashed lines correspond to $f_a=10^{9}~\mathrm{GeV}$ and $S_i=8\times 10^{16}~\mathrm{GeV}$, respectively.