Eur. Phys. J. Special Topics 229, 3229–3241 (2020)
https://doi.org/10.1140/epjst/e2020-000028-x

Review

No axion solution to strong CP using parity and supersymmetry

Maryland Center for Fundamental Physics, Department of Physics, University of Maryland, College Park, MD 20742, USA

^a e-mail: rmohapat@umd.edu

Received: 20 February 2020
Accepted: 30 September 2020
Published online: 14 December 2020

Abstract

A major theoretical problem of the otherwise successful standard model (SM) is the presence of an arbitrary amount of CP violation induced by the periodic vacuum structure of Quantum Chromodynamics (known as the strong CP problem). While the most popular solution to this problem is the Peccei-Quinn mechanism, it predicts a new superllight particle, the axion, which has not been found despite extensive experimental searches. An alternative solution to this problem that does not predict an axion is one based on a parity symmetric extension of SM, which also provides a framework for understanding the neutrino masses via the seesaw mechanism. In this mini-review, I describe how minimal versions of the parity solution to strong CP require supersymmetry and how a class of SO(10) theories provide a natural grand unified (GUT) embedding of these models. These approaches have the advantage that the observed CKM CP violation emerges in a simple way in contrast to some other non-axion approaches. We discuss the importance of a search for electric dipole moment of the neutron as a way to probe these solutions.