High Quality QCD Axion at Gravitational Wave Observatories.

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 and proton electric dipole moments. Additionally, we provide predictions for searches at future GW observatories, which will further explore the parameter space of heavy QCD axion models.

Do Direct Detection Experiments Constrain Axionlike Particles Coupled to Electrons?

Several laboratory experiments have published limits on axionlike particles (ALPs) with feeble couplings to electrons and masses in the kilo-electron-volt to mega-electron-volt range, under the assumption that such ALPs comprise the dark matter. We note that ALPs decay radiatively into photons, and show that for a large subset of the parameter space ostensibly probed by these experiments, the lifetime of the ALPs is shorter than the age of the Universe. Such ALPs cannot consistently make up the dark matter, which significantly affects the interpretation of published limits from GERDA, Edelweiss-III, SuperCDMS, and Majorana. Moreover, constraints from x-ray and γ-ray astronomy exclude a wide range of the ALP-electron coupling, and supersede all current laboratory limits on dark matter ALPs in the 6 keV to 1 MeV mass range. These conclusions are rather model independent, and can only be avoided at the expense of significant fine-tuning in theories where the ALP has additional couplings to other particles.

Observable Windows for the QCD Axion Through the Number of Relativistic Species.

We show that when the QCD axion is directly coupled to quarks with c_{i}/f∂_{µ}aq[over ¯]_{i}γ^{µ}γ^{5}q_{i}, such as in Dine-Fischler-Srednicki-Zhitnitsky models, the dominant production mechanism in the early Universe at temperatures 1 GeV≲T≲100 GeV is obtained via q_{i}q[over ¯]_{i}↔ga and q_{i}g↔q_{i}a, where g are gluons. The production of axions through such processes is maximal around T≈m_{i}, where m_{i} are the different heavy quark masses. This leads to a relic axion background that decouples at such temperatures, leaving a contribution to the effective number of relativistic degrees of freedom, which can be larger than the case of decoupling happens the electroweak phase transition, ΔN_{eff}≲0.027. Our prediction for the t quark is 0.027≤ΔN_{eff}≤0.036 for 10^{6} GeV≲f/c_{t}≲4×10^{8} GeV and for the b quark is 0.027≤ΔN_{eff}≤0.047 for 10^{7} GeV≲f/c_{b}≲3×10^{8} GeV. For the c quark the window can only be roughly estimated as 0.027<ΔN_{eff}≲O(0.1), for f/c_{c}≲(2-3)×10^{8} GeV, since axions can still be partially produced in a regime of strong coupling, when α_{s}≳1. These contributions are comparable to the sensitivity of future CMB S4 experiments, thus opening an alternative window to detect the axion and to test the early Universe at such temperatures.