Recent Progress in the Physics of Axions and Axion-Like Particles

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. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Axion emission from supernova with axion-pion-nucleon contact interaction

“…space integration, and confirm that it is still negligible for the environmental parameters of SN 1987A. 7 Among the three possible nucleon-nucleon bremsstrahlung processes, n+n → n+n+a, n + p → n + p + a, and p + p → p + p + a, at leading order in pion-nucleon couplings only the second process is affected by C aπN through the first two diagrams of figure 4. The axion emissivity of the three bremsstrahlung processes is given by…”

“…The axion which was initially introduced as a solution to the strong CP problem [1][2][3][4] has turned out to have many interesting phenomenological consequences [5][6][7]. After recognizing that axions provide a compelling candidate for the dark matter in the Universe [8][9][10], a lot of efforts have been made to search for axions over the parameter space of the representative axion models [11][12][13][14].…”

Cold Particle Dark Matter

“…They are produced non-thermally in the early universe via a misaligment mechanism [146,147]. However, being ultralight and outside the mass window of CDM, which is the focus of this review, we do not pursue these details further; see, e.g., [148].…”

Simulations of axion-like particles in the post-inflationary scenario

“…These ALPs are a priori unconnected to QCD, but may nonetheless provide a window on high-energy physics far beyond the reach of colliders [12][13][14][15][16][17][18]. For this reason, and their potential role as DM, ALPs are now a subject of growing theoretical [19] and experimental [20] interest.…”