Domestic Axion

TL;DR

This paper proposes a novel axion model composed entirely of Standard Model fermions, addressing the strong CP problem and predicting unique experimental signatures like isotope-dependent forces at microscopic scales.

Contribution

It introduces a new axion scenario where the axion is mainly the $ ext{η'}$ meson with a neutrino component, linking fermion condensates to solving the strong CP problem.

Findings

The axion is mainly the $ ext{η'}$ meson with a neutrino admixture.

The model naturally generates up-quark and neutrino masses from fermion condensates.

Predicts isotope-dependent attractive forces at submicron distances.

Abstract

We attempt to identify a phenomenologically viable solution to the strong $CP$ problem in which the axion is composed entirely out of Standard Model fermion species. The axion consists predominantly of the $\eta'$ meson with a minuscule admixture of a pseudoscalar bilinear composite of neutrinos, $\eta_{\nu}$. The Peccei-Quinn symmetry is an axial symmetry that acts on the up quark and the neutrino species and is spontaneously broken by the QCD condensate of quarks as well as the condensate of neutrinos triggered by chiral gravitational anomaly. The up-quark mass is spontaneously generated by the neutrino condensate which plays the role of an additional composite Higgs doublet with the compositeness scale of the order of the neutrino masses. Such a scenario is highly economical: it solves the strong $CP$ problem, generates the up-quark and neutrino masses from fermion condensates and simultaneously protects the axion shift symmetry against gravitational anomaly. The phenomenology is different from the standard hidden axion case. One of the experimental signatures is the existence of a gravity-competing isotope-dependent attractive force among nucleons at (sub)micron distances.