Is the $\bar \theta$ parameter of QCD constant?

TL;DR

This paper explores the potential variation of the QCD $ar heta$ parameter over cosmological timescales, using laboratory and astrophysical constraints, and discusses implications for new physics and axion models.

Contribution

It introduces methods to constrain the time variation of $ar heta$ using CP-conserving effects and atomic clock data, and proposes an axion model linking $ar heta$ variation to gamma-ray backgrounds.

Findings

No evidence for $ar heta$ variation over cosmological timescales.

Atomic clock data constrains $d({ar heta}^2)/dt \

Sketches an axion model predicting gamma-ray backgrounds from high-redshift supernovae.

Abstract

Testing the cosmological variation of fundamental constants of Nature can provide valuable insights into new physics scenarios. While many such constraints have been derived for Standard Model coupling constants and masses, the $\bar\theta$ parameter of QCD has not been as extensively examined. This letter discusses potentially promising paths to investigate the time dependence of the $\bar\theta$ parameter. While laboratory searches for CP-violating signals of $\bar\theta$ yield the most robust bounds on today's value of $\bar\theta$, we show that CP-conserving effects provide constraints on the variation of $\bar\theta$ over cosmological timescales. We find no evidence for a variation of $\bar\theta$ that could have implied an "iron-deficient" Universe at higher redshifts. By converting recent atomic clock constraints on a variation of constants, we infer $ d({\bar\theta}^2)/dt \leq 6\times 10^{-15}\text{yr}^{-1}$, at 1-$\sigma$. Finally, we also sketch an axion model that results in a varying $\bar\theta$ and could lead to excess diffuse gamma ray background, from decays of axions produced in high redshift supernova explosions.