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Gravitational Origin of the QCD Axion
Georgios K Karananas1,2, Mikhail Shaposhnikov3, Sebastian Zell1,2
1Ludwig-Maximilians-Universität, Arnold Sommerfeld Center, Theresienstraße 37, 80333 München, Germany.
Physical Review Letters
|January 2, 2026
Summary
Gravitational axions, proposed to solve the strong CP problem, face challenges in derivative-coupled models. Weyl-invariant Einstein-Cartan gravity offers a promising theoretical framework for generating these particles via quantum effects.
Area of Science:
- Theoretical physics
- Quantum gravity
- Particle physics
Background:
- Gravity can generate (pseudo)scalar fields, particularly through spacetime torsion.
- Axions of gravitational origin are a compelling candidate for resolving the strong CP problem.
- The feasibility of these gravitational axions requires critical examination.
Purpose of the Study:
- To critically assess the viability of gravitational axions as a solution to the strong CP problem.
- To identify the conditions necessary for generating gravitational axions through quantum effects.
Main Methods:
- Analysis of theoretical models where scalar fields couple to fermionic currents via derivatives.
- Investigation of quantum effects in modified gravitational theories.
Main Results:
- Models with derivative-only coupling of scalar fields to fermionic currents do not produce satisfactory axions.
- Specific conditions for generating gravitational axions via quantum effects have been identified.
Conclusions:
- The proposed axion solution to the strong CP problem is not universally feasible with simple derivative couplings.
- Weyl-invariant Einstein-Cartan gravity presents a promising theoretical avenue for realizing gravitational axions.
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