Indications against dynamical CPT symmetry restoration in quantum gravity

TL;DR

This paper investigates whether CPT symmetry can be emergent or must be fundamental in quantum gravity, concluding that CPT cannot be emergent and must be preserved at all energy scales.

Contribution

It provides a Renormalization Group analysis showing CPT symmetry cannot emerge from broken high-energy interactions in quantum gravity.

Findings

CPT symmetry is not emergent from quantum gravity.

Imposing CPT as fundamental preserves it in asymptotically safe gravity.

Tiny CPT violations at high energies lead to significant low-energy breaking.

Abstract

CPT symmetry is at the heart of the Standard Model of particle physics and experimentally very well tested, but expected to be broken in some approaches to quantum gravity. It thus becomes pertinent to explore which of the two alternatives is realized: (i) CPT symmetry is emergent, so that it is restored in the low-energy theory, even if it is broken beyond the Planck scale, (ii) CPT symmetry cannot be emergent and must be fundamental, so that any approach to quantum gravity, in which CPT is broken, is ruled out. We explore this by calculating the Renormalization Group flow of CPT violating interactions under the impact of quantum fluctuations of the metric. We find that CPT symmetry cannot be emergent and conclude that quantum-gravity approaches must avoid the breaking of CPT symmetry. As a specific example, we discover that in asymptotically safe quantum gravity CPT symmetry remains intact, if it is imposed as a fundamental symmetry, but it is badly broken at low energies if a tiny amount of CPT violation is present in the transplanckian regime.