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

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 $d({\bar{\theta }}^2)/dt\le 6\times 10^{-15}{\text{yr}}^{-1}$, 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.

6 pages, 1 figure, v2: matches published version in PRL

References in corpus (15)

• Ultralight scalars as cosmological dark matter
• An Improved Experimental Limit on the Electric Dipole Moment of the Neutron
• Measurement of the permanent electric dipole moment of the neutron
• The Diffuse Supernova Neutrino Background is detectable in Super-Kamiokande
• Improved limits for violations of local position invariance from atomic clock comparisons
• The status of varying constants: a review of the physics, searches and implications
• First constraint on cosmological variation of the proton-to-electron mass ratio from two independent telescopes
• The design of the n2EDM experiment
• 3D template-based $Fermi$-LAT constraints on the diffuse supernova axion-like particle background
• A constraint on a varying proton--electron mass ratio 1.5 billion years after the Big Bang
• The PanEDM Neutron Electric Dipole Moment Experiment at the ILL
• $\theta$-dependence of light nuclei and nucleosynthesis
• Statistical coupling constants from hidden sector entanglement
• Altered Axion Abundance from a Dynamical Peccei-Quinn Scale
• Constraint on a varying proton-to-electron mass ratio from H2 and HD absorption at z = 2.34