Is the $ \theta$ parameter of QCD constant?

Julia Gehrlein
Brookhaven National Lab [BNL]
Wednesday, May 11, 2022
11:00 am
Zoom Seminar


Testing the variation of fundamental constants of Nature can provide valuable insights into new physics scenarios. While many constraints have been derived for Standard Model coupling constants and masses, the $\theta$ parameter of QCD has often been ignored in these studies. This letter discusses potentially promising paths to investigate the time dependence of the $\theta$ parameter. While laboratory searches for CP-violating signals of $\theta$ yield the most robust bounds on today's value of $\theta$, we show that CP-conserving effects provide constraints on the variation of $\theta$ over cosmological time scales. We find no significant evidence for a variation of $\theta$ except for a mild hint around $z\sim 4$ which would imply an ``iron-deficient" Universe at high redshifts. Finally, we also sketch an axion model which results in a varying $\theta$ and could lead to excess diffuse gamma ray background, from decays of axions produced in high redshift supernova explosions.

Yuri Shirman