Abstract
It is well-known that the universe is opaque to the propagation of Ultra-High-Energy Cosmic Rays (UHECRs) since these particles dissipate energy during their propagation interacting with the background fields present in the universe, mainly with the Cosmic Microwave Background (CMB) in the so-called GZK cut-off phenomenon. Some experimental evidence seems to hint at the possibility of a dilation of the GZK predicted opacity sphere. It is well-known that kinematical perturbations caused by supposed quantum gravity (QG) effects can modify the foreseen GZK opacity horizon. The introduction of Lorentz Invariance Violation can indeed reduce, and in some cases making negligible, the CMB-UHECRs interaction probability. In this work, we explore the effects induced by modified kinematics in the UHECR lightest component phenomenology from the QG perspective. We explore the possibility of a geometrical description of the massive fermions interaction with the supposed quantum structure of spacetime in order to introduce a Lorentz covariance modification. The kinematics are amended, modifying the dispersion relations of free particles in the context of a covariance-preserving framework. This spacetime description requires a more general geometry than the usual Riemannian one, indicating, for instance, the Finsler construction and the related generalized Finsler spacetime as ideal candidates. Finally we investigate the correlation between the magnitude of Lorentz covariance modification and the attenuation length of the photopion production process related to the GZK cut-off, demonstrating that the predicted opacity horizon can be dilated even in the context of a theory that does not require any privileged reference frame.
Highlights
quantum gravity (QG) can perturb at sufficiently high energies the free particle kinematics reducing the interaction with the background fields, mainly the Cosmic Microwave Background (CMB), and the related energy dissipation caused by the GZK effect and photodissociation process, resulting in an enlargement of the volume of the universe accessible at Ultra High Energy Cosmic Rays (UHECRs)
We investigate the phenomenological effects introduced by QG in the UHECRs propagation, where the main effects are caused by the modification of the kinematics
We investigated the possibility of exploiting the UHECR physics to detect supposed signatures of the quantum structure of spacetime
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. QG can perturb at sufficiently high energies the free particle kinematics reducing the interaction with the background fields, mainly the CMB, and the related energy dissipation caused by the GZK effect and photodissociation process, resulting in an enlargement of the volume of the universe accessible at UHECRs. A natural way of looking for QG signals involves the determination of UHECR propagation length. The QG perturbations are introduced in the free particle kinematics modifying the dispersion relations (DRs) This idea is motivated by the hypothesis that the effects induced by the quantum structure of the background can be geometrized requiring a more general structure to set the theory than the usual Riemann geometry, that is the Finsler one. We analyze the results with particular emphasis on the possibility of obtaining detectable phenomenological effects in this sector in the context of a covariant framework, that retains a modified version of the kinematical symmetry group (the Lorentz/Poincaré one)
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