Axions and the strong CP problem
Abstract
Current upper bounds on the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle θ¯≲10^{11} . Since QCD explains a great deal of experimental data from the 100MeV to the TeV scale, it is desirable to explain this smallness of θ¯ in the QCD framework; this is the strong CP problem. There now exist two plausible solutions to this problem, one of which leads to the existence of a very light axion. The axion decay constant window, 10^{9}≲F_{a}≲10^{12}GeV for an O(1) initial misalignment angle θ_{1} , has been obtained from astrophysical and cosmological data. For F_{a}≳10^{12}GeV with θ_{1}<O(1) , axions may constitute a significant fraction of the dark matter of the universe. The supersymmetrized axion solution of the strong CP problem introduces its superpartner the axino, which might have affected the evolution of the Universe significantly. The very light axion (theory, supersymmetrization, and models) using recent particle, astrophysical, and cosmological data, and present prospects for its discovery is reviewed here.
 Publication:

Reviews of Modern Physics
 Pub Date:
 January 2010
 DOI:
 10.1103/RevModPhys.82.557
 arXiv:
 arXiv:0807.3125
 Bibcode:
 2010RvMP...82..557K
 Keywords:

 14.80.Va;
 12.38.Aw;
 95.35.+d;
 11.30.j;
 General properties of QCD;
 Dark matter;
 Symmetry and conservation laws;
 High Energy Physics  Phenomenology;
 Astrophysics;
 High Energy Physics  Experiment;
 High Energy Physics  Theory
 EPrint:
 47 pages with 32 figures