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.

arXiv comment6 pages, 1 figure, v2: matches published version in PRL

References in corpus (54)

• Ultralight scalars as cosmological dark matter
• Search for New Physics with Atoms and Molecules
• An Improved Experimental Limit on the Electric Dipole Moment of the Neutron
• Chiral Dynamics in Nucleons and Nuclei
• Electric dipole moments as probes of new physics
• Measurement of the permanent electric dipole moment of the neutron
• Violations of fundamental symmetries in atoms and tests of unification theories of elementary particles
• Planck-Scale Physics and the Peccei-Quinn Mechanism
• GRAVITY AND GLOBAL SYMMETRIES
• Electric Dipole Moments of Nucleons, Nuclei, and Atoms: The Standard Model and Beyond
• Solutions to the strong CP problem in a world with gravity
• Reduced Limit on the Permanent Electric Dipole Moment of ${}^{199}$Hg
• The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems
• Roy-Steiner-equation analysis of pion-nucleon scattering
• The Diffuse Supernova Neutrino Background is detectable in Super-Kamiokande
• Matching pion-nucleon Roy-Steiner equations to chiral perturbation theory
• 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
• Probing axions with neutron star inspirals and other stellar processes
• Astrophysical Limits on the Evolution of Dimensionless Physical Constants over Cosmological Time
• The nature of the Diffuse Gamma-Ray Background
• QCD at Fixed Topology
• Stringent null constraint on cosmological evolution of the proton-to-electron mass ratio
• The UVES Large Program for Testing Fundamental Physics II: Constraints on a Change in μ Towards Quasar HE 0027-1836
• Keck telescope constraint on cosmological variation of the proton-to-electron mass ratio
• A new constraint on the time dependence of the proton-to-electron mass ratio. Analysis of the Q 0347-383 and Q 0405-443 spectra
• Bounds on axion-like particles from the diffuse supernova flux
• Performance of the upgraded ultracold neutron source at Los Alamos National Laboratory and its implication for a possible neutron electric dipole moment experiment
• Effects of theta on the deuteron binding energy and the triple-alpha process
• First constraint on cosmological variation of the proton-to-electron mass ratio from two independent telescopes
• New constraint on cosmological variation of the proton-to-electron mass ratio from Q0528-250
• The meV mass frontier of axion physics
• An Observational Determination of the Proton to Electron Mass Ratio in the Early Universe
• Search for a drifting proton--electron mass ratio from H${}_2$
• A New Cryogenic Apparatus to Search for the Neutron Electric Dipole Moment
• Molecular hydrogen in the z = 2.66 damped Lyman-alpha absorber toward Q J0643-5041
• The design of the n2EDM experiment
• 3D template-based $Fermi$-LAT constraints on the diffuse supernova axion-like particle background
• QSO 0347-383 and the invariance of m_p/m_e in the course of cosmic time
• 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
• Constraint on a varying proton-to-electron mass ratio from molecular hydrogen absorption toward quasar SDSS J123714.60+064759.5
• $\theta$-dependence of light nuclei and nucleosynthesis
• Comprehensive Symmetric-Hybrid ring design for pEDM experiment at below $10^{-29}e\cdot$cm
• Variation of $\alpha$ from a Dark Matter Force
• Constraint on a variation of the proton-to-electron mass ratio from H2 absorption towards quasar Q2348-011
• 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
• Fine Structure Constant Variation from a Late Phase Transition
• H${}_2$/HD molecular data for analysis of quasar spectra in search of varying constants
• How the minuscule can contribute to the big picture: the neutron electric dipole moment project at TRIUMF
• The Pulsed Neutron Beam EDM Experiment
• An Emergent Solution to the Strong CP Problem