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Is the θ[over ¯] Parameter of QCD Constant?
Hooman Davoudiasl1, Julia Gehrlein1, Robert Szafron1
1Physics Department, Brookhaven National Laboratory, High Energy Theory Group, Upton, New York 11973, USA.
Physical Review Letters
|October 28, 2022
Summary
This study investigates the time-dependence of the theta parameter in Quantum Chromodynamics (QCD). Researchers found no evidence for variations that would alter the early universe, setting new constraints on its cosmological evolution.
Area of Science:
- Cosmology
- Fundamental Physics
- Particle Physics
Background:
- Testing variations in fundamental constants offers insights into new physics.
- The theta parameter of Quantum Chromodynamics (QCD) has been less studied for cosmological variation compared to other constants.
- Previous laboratory searches primarily constrain the current value of the theta parameter.
Purpose of the Study:
- To explore methods for investigating the time dependence of the QCD theta parameter.
- To constrain the cosmological variation of the theta parameter using CP-conserving effects.
- To analyze potential implications of a varying theta parameter for the early universe.
Main Methods:
- Utilizing CP-conserving effects to constrain the variation of the theta parameter over cosmological timescales.
- Converting atomic clock constraints on the variation of fundamental constants.
- Developing theoretical models linking varying theta parameters to observable phenomena.
Main Results:
- No evidence found for theta parameter variation that would suggest an 'iron-deficient' early Universe.
- Inferred an upper limit on the rate of change: d(θ[over ¯]²)/dt ≤ 6×10⁻¹⁵ yr⁻¹ (at 1σ).
- Proposed an axion model where a varying theta parameter could explain excess diffuse gamma-ray background from supernova axion decays.
Conclusions:
- CP-conserving effects provide viable constraints on the cosmological variation of the QCD theta parameter.
- Current data does not support significant theta parameter evolution in the early Universe.
- A varying theta parameter, potentially linked to axion physics, could have observable cosmological consequences.
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