We discuss the energy scales of the explicit breaking terms of the global symmetries USW~ needed for the quinessential axion (QA) and the ultra-light axion (ULA). The appropriate scale of QA is about $10^{8}$ GeV.
The equations of electrodynamics are altered in the presence of a classical coherent axion dark matter background field, changing the dispersion relation for electromagnetic waves. Careful measurements of the frequency stability in sensitive atomic c
locks could in principle provide evidence for such a background for $f_a ge 10^7$ GeV. Turning on a background magnetic field might enhance these effects in a controllable way, and interferometric measurements might also be useful for probing the time-varying photon dispersion relation that results from a coherent cosmic axion background.
We study the impact of virtual axions on the polarization of photons inside a cavity during the interaction of high-power laser pulses. A novel detection scheme for measuring the axion-induced ellipticity signal during the Light-by-Light (LBL) scatte
ring process is investigated. We show that a momentum exchange between photons in a probe laser beam and a high-intensity target beam may lead to a resonance at the physical mass of the axion. Consequently, the resonant enhancement of vacuum birefringence gives rise to a large ellipticity signal. This signal enhancement can be applied in order to discriminate between the axion contribution to LBL scattering and the standard model contribution due to electron-positron pairs. The sensitivity of the scheme is studied for experimentally feasible probe light sources and ultrahigh intensity laser backgrounds. It is shown that this technique has the potential to probe the QCD axion in the mass range $10^{-2} textrm{eV} lesssim m_{a} lesssim 1 textrm{eV}$. In this region the axion induced signal surpasses the standard model background.
We exploit the complementarity among supersymmetry, inflation, axions, Big Bang Nucleosynthesis (BBN) and Cosmic Microwave Background Radiation (CMB) to constrain supersymmetric axion models in the light of the recent Planck and BICEP results. In par
ticular, we derive BBN bounds coming from altering the light element abundances by taking into account hadronic and electromagnetic energy injection, and CMB constraints from black-body spectrum distortion. Lastly, we outline the viable versus excluded region of these supersymetric models that might account for the mild dark radiation observed.
The QCD axion solving the strong CP problem may originate from antisymmetric tensor gauge fields in compactified string theory, with a decay constant around the GUT scale. Such possibility appears to be ruled out now by the detection of tensor modes
by BICEP2 and the PLANCK constraints on isocurvature density perturbations. A more interesting and still viable possibility is that the string theoretic QCD axion is charged under an anomalous U(1)_A gauge symmetry. In such case, the axion decay constant can be much lower than the GUT scale if moduli are stabilized near the point of vanishing Fayet-Illiopoulos term, and U(1)_A-charged matter fields get a vacuum value far below the GUT scale due to a tachyonic SUSY breaking scalar mass. We examine the symmetry breaking pattern of such models during the inflationary epoch with the Hubble expansion rate 10^{14} GeV, and identify the range of the QCD axion decay constant, as well as the corresponding relic axion abundance, consistent with known cosmological constraints. In addition to the case that the PQ symmetry is restored during inflation, there are other viable scenarios, including that the PQ symmetry is broken during inflation at high scales around 10^{16}-10^{17} GeV due to a large Hubble-induced tachyonic scalar mass from the U(1)_A D-term, while the present axion scale is in the range 10^{9}-5times 10^{13} GeV, where the present value larger than 10^{12} GeV requires a fine-tuning of the axion misalignment angle. We also discuss the implications of our results for the size of SUSY breaking soft masses.
The axion is a hypothetical, well-motivated dark-matter particle whose existence would explain the lack of charge-parity violation in the strong interaction. In addition to this original motivation, an `axiverse of ultra-light axions (ULAs) with mass
es $10^{-33},{rm eV}lesssim m_{rm a}lesssim 10^{-10},{rm eV}$ also emerges from string theory. Depending on the mass, such a ULA contributes to the dark-matter density, or alternatively, behaves like dark energy. At these masses, ULAs classical wave-like properties are astronomically manifested, potentially mitigating observational tensions within the $Lambda$CDM paradigm on local-group scales. ULAs also provide signatures on small scales such as suppression of structure, interference patterns and solitons to distinguish them from heavier dark matter candidates. Through their gravitational imprint, ULAs in the presently allowed parameter space furnish a host of observational tests to target in the next decade, altering standard predictions for microwave background anisotropies, galaxy clustering, Lyman-$alpha$ absorption by neutral hydrogen along quasar sightlines, pulsar timing, and the black-hole mass spectrum.