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The axion solution to the strong CP problem is delicately sensitive to Peccei-Quinn breaking contributions that are misaligned with respect to QCD instantons. Heavy QCD axion models are appealing because they avoid this so-called quality problem. We show that generic realizations of this framework can be probed by the LIGO-Virgo-KAGRA interferometers, through the stochastic gravitational wave (GW) signal sourced by the long-lived axionic string-domain wall network, and by upcoming measurements of the neutron Electric Dipole Moment. Additionally, we provide predictions for searches at future GW observatories, which will further explore the parameter space of heavy QCD axion models.
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In this paper, we investigate the Axion-like Particle inflation by applying the multi-nature inflation model, where the end of inflation is achieved through the phase transition (PT). The events of PT should not be less than $200$, which results in the free parameter $ngeq404$. Under the latest CMB restrictions, we found that the inflation energy is fixed at $10^{15} rm{GeV}$. Then, we deeply discussed the corresponding stochastic background of the primordial gravitational wave (GW) during inflation. We study the two kinds of $n$ cases, i.e., $n=404, 2000$. We observe that the magnitude of $n$ is negligible for the physical observations, such as $n_s$, $r$, $Lambda$, and $Omega_{rm{GW}}h^2$. In the low-frequency regions, the GW is dominated by the quantum fluctuations, and this GW can be detected by Decigo at $10^{-1}~rm{Hz}$. However, GW generated by PT dominates the high-frequency regions, which is expected to be detected by future 3DSR detector.
Advanced LIGO may be the first experiment to detect gravitational waves. Through superradiance of stellar black holes, it may also be the first experiment to discover the QCD axion with decay constant above the GUT scale. When an axions Compton wavelength is comparable to the size of a black hole, the axion binds to the black hole, forming a gravitational atom. Through the superradiance process, the number of axions occupying the bound levels grows exponentially, extracting energy and angular momentum from the black hole. Axions transitioning between levels of the gravitational atom and axions annihilating to gravitons can produce observable gravitational wave signals. The signals are long-lasting, monochromatic, and can be distinguished from ordinary astrophysical sources. We estimate up to O(1) transition events at aLIGO for an axion between 10^-11 and 10^-10 eV and up to 10^4 annihilation events for an axion between 10^-13 and 10^-11 eV. In the event of a null search, aLIGO can constrain the axion mass for a range of rapidly spinning black hole formation rates. Axion annihilations are also promising for much lighter masses at future lower-frequency gravitational wave observatories; the rates have large uncertainties, dominated by supermassive black hole spin distributions. Our projections for aLIGO are robust against perturbations from the black hole environment and account for our updated exclusion on the QCD axion of 6*10^-13 eV < ma < 2*10^-11 eV suggested by stellar black hole spin measurements.
The strong CP problem is a compelling motivation for physics beyond the Standard Model. The most popular solutions invoke a global Peccei-Quinn symmetry, but are challenged by quantum gravitational corrections which are thought to be incompatible with global symmetries, arguing that realistic theories contain additional structure. We explore a construction in which the Peccei-Quinn symmetry is protected to arbitrary order by virtue of a supersymmetric, confining $SU(N)_L times SU(N) times SU(N)_R times U(1)_X$ product gauge group, achieving $bartheta < 10^{-11}$ for an $SU(5)$ model with $f_a lesssim 3 times 10^{11}$ GeV. This construction leads to low energy predictions such as a $U(1)_X$ gauge symmetry, and for $X = B-L$ engineers a naturally order ~TeV value for the $mu$ parameter of the MSSM.
Axion-like fields heavier than about $10^{-27}$eV are expected to oscillate in the radiation dominated epoch when the Hubble parameter drops below their mass. Considering the Chern-Simons coupling with a dark gauge boson, large amount of dark photons are produced during a short time interval through tachyonic resonance instability. The produced dark photons then source gravitational tensor modes leading to chiral gravitational waves. Through this process, one can indirectly probe a large parameter space of coupled axion-dark photon models. In this work we first find an analytic expression for the number density of the dark photons produced during the tachyonic resonance regime. Second, by using the saddle point approximation we find an analytic expression for the gravitational wave spectrum in terms of the mass, coupling and misalignment angle. Our analytic results can be used for the observational analysis of these types of scenarios.
The relic gravitational wave background due to tensor linear perturbations generated during Higgs inflation is computed. Both the Standard Model and a well-motivated phenomenological completion (that accounts for all the experimentally confirmed evidence of new physics) are considered. We focus on critical Higgs inflation, which improves on the non-critical version and features an amplification of the tensor fluctuations. The latter property allows us to establish that future space-borne interferometers, such as DECIGO, BBO and ALIA, may detect the corresponding primordial gravitational waves.
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