Axions and the Strong CP Problem

Abstract

Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle | θ| 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 CP 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, 10 9 GeV Fa 10 12 GeV for a O(1) initial misalignment angle θ1, has been obtained by astrophysical and cosmological data. For Fa 10 12 GeV with θ1 < O(1), axions may constitute a significant fraction of dark matter of the universe. The supersymmetrized axion solution of the strong CP 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.

Axinos as dark matter particles

“…In discussing axino models, one should refer to the corresponding axion models. So, let us start with the axion shift symmetry and the reparametrization invariance as discussed in [12].…”

“…So we see immediately from this transformation for a single quark, that if it is massless, the corresponding c 1 , c 2 parameters disappear and we can shift away the anomaly term completely with no physical effect. This is in fact one alternative solution to the strong CP problem, and see [12] for a detailed discussion.…”

“…For determining the axion mass, all c 1 , c 2 and c 3 terms may be relevant, but only the combination c 2 + c 3 actually appears [12]. Usually, in the field theoretic axion models, we start with c 1 = 0.…”

“…The scale F a is constrained by astrophysical and cosmological bounds to lie in the narrow axion window 10 10 GeV < F a < 10 12 GeV [12].…”

“…The modelindependent axion in superstring models give instead c 2 = 0 and c 3 = 1. These values enable us to write down the axion-nucleon-nucleon couplings unambiguously for each model [12]. But in general axion models may contain both c 2 and c 3 with c 2 + c 3 = 0 [12] and may have the family dependencies of the variant axion [14] or invisible axion [15].…”