Axionic Wormholes with $R^2$ Correction in Metric and Palatini Formulations

Abstract

QCD axion models have been proposed as a solution to the strong CP problem. QCD instanton effects explicitly violate the global U(1) Peccei-Quinn (PQ) symmetry, and the axion potential is minimized at the CP conserving points. However, it is expected that the global U(1) PQ symmetry is explicitly violated by quantum gravity. This gravitational violation may potentially undermine the PQ solution to the strong CP problem, which is referred to as the axion quality problem. One source of explicit PQ violation is axionic wormholes, such as the Giddings-Strominger model. Generally speaking, the extent to which the U(1) PQ symmetry is violated depends on models. One straightforward extension of the Einstein gravity involves quadratic scalar curvature, which is motivated in cosmological contexts, such as inflation. The aim of this work is to investigate the effects of $R^2$ corrections on axionic wormholes. In this paper, we compute wormhole solutions and evaluate the size of explicit PQ violation in the models with the $R^2$ term. Furthermore, We illustrate the differences between the metric and Palatini formulations.