An even lighter QCD axion
Abstract
We explore whether the axion which solves the strong CP problem can naturally be much lighter than the canonical QCD axion. The Z_{N} symmetry proposed by Hook, with N mirror and degenerate worlds coexisting in Nature and linked by the axion field, is considered in terms of generic effective axion couplings. We show that the total potential is safely approximated by a single cosine in the large N limit, and we determine the analytical formula for the exponentially suppressed axion mass. The resulting universal enhancement of all axion interactions relative to those of the canonical QCD axion has a strong impact on the prospects of axionlike particle experiments such as ALPS II, IAXO and many others: experiments searching for generic axionlike particles have in fact discovery potential to solve the strong CP problem. The finite density axion potential is also analyzed and we show that the Z_{N} asymmetric background of highdensity stellar environments sets already significant modelindependent constraints: 3 ≤ N ≲ 47 for an axion scale f_{a} ≲ 2.4 × 10^{15} GeV, with tantalizing discovery prospects for any value of f_{a} and down to N ∼ 9 with future neutron star and gravitational wave data, down to the ultralight mass region. In addition, two specific ultraviolet Z_{N} completions are developed: a composite axion one and a KSVZlike model with improved PecceiQuinn quality.
 Publication:

Journal of High Energy Physics
 Pub Date:
 May 2021
 DOI:
 10.1007/JHEP05(2021)184
 arXiv:
 arXiv:2102.00012
 Bibcode:
 2021JHEP...05..184D
 Keywords:

 Beyond Standard Model;
 CP violation;
 Discrete Symmetries;
 Chiral Lagrangians;
 High Energy Physics  Phenomenology;
 High Energy Physics  Experiment;
 High Energy Physics  Theory
 EPrint:
 35 pages and 11 figures