2008/07/31 by Jihn E. Kim, Gianpaolo Carosi · 1 citation
Physics and Astronomy · #hep-ph #astro-ph #hep-ex #hep-th
paper · pdf · doi:10.1103/revmodphys.82.557,10.1103/revmodphys.91.049902
published as Rev.Mod.Phys.82:557-602,2010, Rev.Mod.Phys.91:049902,2019(E) · 47 pages with 32 figures
arxiv created 2009/03/03 · arxiv updated 2020/01/08
Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle |θ| \lesssim 10-11. Since QCD explains vast experimental data from the 100 MeV scale to the TeV scale, it is better to explain this smallness of |θ| in the QCD framework, which is the strong \Ca\Pa problem. Now, there exist two plausible solutions to this problem, one of which leads to the existence of the very light axion. The axion decay constant window, 109 \gev\lesssim Fa\lesssim 1012 \gev for a \cal O(1) initial misalignment angle θ1, has been obtained by astrophysical and cosmological data. For Fa\gtrsim 1012 GeV with θ1<\cal O(1), axions may constitute a significant fraction of dark matter of the universe. The supersymmetrized axion solution of the strong \Ca\Pa problem introduces its superpartner the axino which might have affected the universe evolution significantly. Here, we review the very light axion (theory, supersymmetrization, and models) with the most recent particle, astrophysical and cosmological data, and present prospects for its discovery.