Year: 2026
Type: article
Abstract: A bstract We argue that the electromagnetic θ -term is a physical parameter of the Standard Model coupled to gravity. Specifically, in the context of 4-dimensional Einstein-Maxwell theory we show that there exist Euclidean field configurations that have finite action, are asymptotically flat, and feature non-zero electromagnetic second Chern number. These “gravitational Abelian instantons” correspond to a dyonic extension of a Euclidean wormhole. We argue that these configurations should be included in the gravitational path integral, and that doing so generates a non-perturbative contribution to the vacuum energy density that is θ -dependent. We provide a Lorentzian interpretation of these instantons as capturing the effect of quantum fluctuations corresponding to pair production and annihilation of charged black holes. When θ is the expectation value of a dynamical axion field, the instantons presented here generate a potential for the axion, thereby breaking the axion shift symmetry. This provides yet another example of how quantum gravity violates global symmetries through the existence of black holes.
Source: Journal of High Energy Physics
Authors: Isabel Garcia Garcia, Elliot Maderazo
Institution: University of Washington
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Funder: U.S. Department of Energy
OpenAlex ID: W4417464932
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