Discrete \texorpdfstring{$\theta$}{theta} Projection: A Gauge-Protected Solution to the Strong CP Problem Without Axions

TL;DR

This paper introduces a gauge-protected, axionless solution to the strong CP problem by gauging a discrete subgroup of the theta shift symmetry, which dynamically enforces a small CP-violating angle without axions.

Contribution

It proposes Discrete theta Projection, a novel gauge-based mechanism that stabilizes the CP-symmetric vacuum and avoids axion-related issues, with detailed theoretical and lattice diagnostics.

Findings

Enforces |θ̄| ≤ π/N without assuming small initial θ̄

Predicts neutron EDM suppression by 1/N

Provides lattice signatures like cusps at fractional periods

Abstract

We address the strong CP problem: why the physical QCD angle theta-bar must be extraordinarily small given the stringent bounds on the neutron electric dipole moment. Peccei-Quinn axion models can relax theta-bar dynamically, but rely on an approximate global symmetry expected to be violated by quantum gravity and face severe astrophysical and cosmological constraints. We propose Discrete theta Projection, an axionless, gauge-protected resolution obtained by gauging a finite cyclic subgroup $Z_N $of the $2\pi$ shift symmetry of theta. Coupling QCD to a compact, local and gapped topological sector orbifolds the path integral, identifying theta values that differ by $2\pi/N$ and admitting only instanton sectors whose topological charge lies in $Z_N$. In the large four-volume limit the vacuum energy becomes the lower envelope of the orbifold images, so the theory dynamically selects the branch closest to the CP-symmetric point, enforcing $|\bar{\theta}| \le \pi/N$ without assuming any prior smallness. Because the discrete shift is gauged, continuous renormalization of theta is forbidden; the construction can be formulated via higher-form/two-group structure with integer-quantized couplings fixed by anomaly inflow, ensuring radiative and gravitational stability and satisfying mixed gauge-gravity consistency conditions. The framework predicts a neutron EDM suppressed by $1/N$, no axion signatures, no domain-wall/isocurvature issues, and lattice diagnostics: piecewise-analytic theta dependence with cusps at odd fractions of the reduced period and a global curvature scaling as $1/N^2$. We provide the EFT construction, a nonperturbative proof of vacuum projection, a full anomaly analysis, and UV embeddings (including discrete clockwork chains) that generate large effective N while preserving integrality and consistency throughout.