- The paper demonstrates that axions with pure derivative coupling and strict shift symmetry fail to generate the potential required to resolve the strong CP problem.
- It reveals that breaking the shift symmetry within a Weyl-invariant Einstein-Cartan framework can induce necessary quantum corrections for viable axion–QCD interactions.
- The study challenges traditional Peccei-Quinn models by proposing a gravitational mechanism that mitigates issues from quantum gravity violations of global symmetries.
Gravitational Analysis of the QCD Axion
The paper "On the Gravitational Origin of the QCD Axion" by Georgios K. Karananas, Mikhail Shaposhnikov, and Sebastian Zell addresses the possibility of the axion, a candidate solution to the strong CP problem in quantum chromodynamics (QCD), being of gravitational origin. The strong CP problem arises due to a lack of observed CP violation in QCD despite theoretical predictions permitting it. A proposed solution involves the axion, a pseudoscalar particle that dynamically suppresses CP violation in QCD. This paper assesses the feasibility of a gravitationally-derived axion, examining its theoretical foundation and implications.
The authors begin by critiquing traditional models where axions arise from spontaneous symmetry breaking within certain extensions to the Standard Model (SM), such as global Peccei-Quinn symmetry models. While these models theoretically provide a mechanism for axion generation, they introduce a multitude of new degrees of freedom and face the theoretical challenge of quantum gravity effects potentially violating these global symmetries, thus undermining their efficacy in solving the strong CP problem.
The paper considers the alternative hypothesis that axions might have a gravitational origin, derived from Einstein-Cartan (EC) gravity, which incorporates torsion alongside curvature. The authors initially explore the possibility of such a gravitational axion, coupling derivatively to fermionic currents. However, their analysis demonstrates that this approach does not provide a satisfactory mechanism, as it preserves an exact shift symmetry, preventing the axion from developing a potential that could dynamically resolve the strong CP problem.
To address these shortcomings, the paper proposes scenarios within EC gravity where the axion-like field could explicitly break the shift symmetry, allowing interaction with quarks and gluons necessary to address CP violation. The authors point out that Weyl-invariant EC gravity offers a promising framework for generating quantum effects that might induce the necessary breaking of shift symmetry, overcoming challenges posed by loop-generated corrections.
Several important results are highlighted:
- Shift Symmetry Limitation: Axions that couple to fermionic currents purely through derivatives in models maintaining shift-symmetry cannot solve the strong CP problem.
- Quantum Corrections: In the absence of exact shift symmetry, quantum corrections might establish necessary interactions for axion-like particles to engage with QCD, though these require careful tuning to remain viable without introducing inconsistencies.
- Weyl-Invariant Framework: The authors identify the Weyl-invariant subset of EC gravity as a viable approach. This framework's quantum consistency could facilitate the axion's emergence, with its coupling to QCD induced through quantum effects.
Overall, this research contributes a detailed examination of gravitational axions, challenging traditional notions while exploring viable gravitational solutions within theoretical physics. Future studies might focus on the theoretical consistency and observable implications of Weyl-invariance in quantum gravity contexts, potentially advancing understanding of axion physics and addressing fundamental issues in cosmology and particle physics.
This paper advances the discussion on constructing models within theoretical physics that effectively incorporate gravitational phenomena in resolving established problems such as strong CP violation, thereby enriching the understanding of the universal interactions at fundamental levels.