Constraints to Lorentz violation and ultrahigh-energy electrons in D-foamy space-times

TL;DR

This paper investigates how constraints from ultrahigh-energy electron observations limit Lorentz violation models, but finds that certain string-inspired space-time foam models can naturally evade these constraints, affecting the expected vacuum Cherenkov radiation.

Contribution

It demonstrates that Lorentz violation constraints from vacuum Cherenkov radiation can be evaded in specific string-inspired space-time foam models involving D-branes.

Findings

Cherenkov constraints are evaded in D-brane foam models.

Electrons may not radiate despite exceeding photon speed.

Sub-luminous, energy-dependent light refractive indices are possible.

Abstract

We entertain the constraints that the absence of vacuum Cherenkov radiation of ultrahigh-energy electrons inferred from LHAASO observations of the Crab Nebula can impose on generic models in which Lorentz symmetry of the particle vacuum is violated, as established by some recent studies in \href{https://doi.org/10.1016/j.physletb.2022.137034}{\emph{Phys. Lett. B} {\bf 829} (2022) 137034}; \href{https://doi.org/10.1016/j.physletb.2022.137536}{{\bf 835} (2022) 137536}; \href{https://doi.org/10.1103/PhysRevD.108.063006}{\emph{Phys. Rev. D} {\bf108} (2023) 063006}. We demonstrate in the present paper, that implementing a phenomenological approach to the Lorentz violation, the rates of this vacuum process are substantial such that one is justified in deriving bounds on the violation scales from simple threshold analysis just as these works did. Albeit such results are likely effective then, they do not apply in the same form among scenarios. Specifically, we show that these Cherenkov constraints are naturally evaded in models of space-time foam inspired from~(supercritical) string theory, involving D-branes as space-time defects in a brane-world scenario, in which subluminous energy-dependent refractive indices of light have been suggested. We examine here two specific foam situations and find for both cases~(though, for different reasons) the potentiality that charged quanta such as electrons do \emph{not} radiate as they pass through the gravitational vacuum `medium' despite moving faster than photons.