Abstract
The axion is a light pseudoscalar particle postulated to solve issues with the Standard Model, including the strong CP problem and the origin of dark matter. In recent years, there has been remarkable progress in the physics of axions in several directions. An unusual type of axion-like particle termed the relaxion was proposed as a new solution to the weak scale hierarchy problem. There are also new ideas for laboratory, astrophysical, or cosmological searches for axions; such searches can probe a wide range of model parameters that were previously inaccessible. On the formal theory side, the weak gravity conjecture indicates a tension between quantum gravity and a trans-Planckian axion field excursion. Many of these developments involve axions with hierarchical couplings. In this article, we review recent progress in axion physics, with particular attention paid to hierarchies between axion couplings. We emphasize that the parameter regions of hierarchical axion couplings are the most accessible experimentally. Moreover, such regions are often where important theoretical questions in the field are addressed, and they can result from simple model-building mechanisms.
Highlights
They are often where important theoretical questions in the field are addressed
For the plot in panel b, fa/NDW is identified as the field excursion a for ALP DM or dark energy, while fa/NDW is associated with the axion self-interaction strength for BHSR. (For a given mass, the lower fa/NDW, the stronger is the axion self-interaction.) We show the axion coupling hierarchy implied by the weak gravity conjecture (WGC)
It has been widely discussed that such a trans-Planckian field range obtained by the KNP mechanism may conflict with the axion WGC. In those discussions applied to the model of Equation 66, one often identifies gaH = q/ f0 and gaL = p/ f0 as the axion-instanton couplings spanning the convex hull constrained by the WGC, and we find that the corresponding convex hull cannot satisfy Equation 31 while giving aL = f0|n|/|n · p| > MP [27,28,29, 65,66,67]
Summary
Axions are periodic scalar fields and characterized by a scale fa termed the axion decay constant, which defines the full field range of the canonically normalized axion:. Σ = ρeia/ fa ( fa = 2 ρ ), or the zero mode of a p-form gauge field A(p) in models with extra dimension such as compactified string theories1 [4, 43,44,45]: A[(mp)1m2..mp] (x, y) = a(x)ω[(mp)1m2..mp] (y), where xμ and ym are the coordinates of four-dimensional Minkowski spacetime and the compact internal space Y, respectively, and ω(p) is a harmonic p-form in Y In the latter case, axion periodicity is assured by Dirac quantization of the axionic string, which is a low-energy remnant of the brane that couples (electrically or magnetically) to A(p) in the underlying UV theory. The gauge equivalence of A(p) on a p-cycle in Y, which determines the value of fa, is fixed by the couplings and scales involved in compactification [4, 45, 46] Axions, regardless of their UV origin, can be naturally light if the theory admits an approximate.
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