HIgh-energy astroparticle physics as a window into quantum gravity

Abstract

The search for quantum gravity signatures in astrophysical observations is one of the central challenges of modern physics. In this thesis, we investigate the phenomenological consequences of scenarios that violate or deform Lorentz symmetries, focusing on extremely high-energy particles produced by cosmic rays and extragalactic sources. We employ a kinematical framework based on Finsler geometry, which enables the description of modified dispersion relations and momentum conservation laws compatible with κ-Poincaré–type symmetries. This approach leads to deformed Lorentz transformations containing terms that are amplified by the particle’s energy, opening the possibility of enhancing Planck-scale effects. The thesis emphasizes two main directions. The first concerns modifications to the lifetimes of relativistic particles, such as muons and pions. Along this line, we show that time dilation receives specific corrections in deformed relativity scenarios, making particle lifetimes a promising observable for experimental tests, whether in muon beam experiments or atmospheric showers. The second direction examines the kinematics of two-body decays, especially π →μ +ν, considering different momentum composition laws compatible with Deformed Special Relativity (DSR). Although the corrections are small, they can affect the relative production of muons and neutrinos in showers initiated by ultra-high-energy cosmic rays, contributing to the ongoing discussion of the muon puzzle. We also investigate the extragalactic and atmospheric propagation of ultra-high-energy photons, whose behavior is sensitive to alterations in the dispersion relation. In Lorentz Invariance Violation (LIV)/DSR scenarios, processes such as pair production may have their thresholds shifted, modifying the transparency horizon of the Universe and potentially altering the development of gamma-ray induced atmospheric showers. The results indicate that extremely high-energy astrophysical messengers—atmospheric muons, neutrinos, and ultra-high-energy photons—provide a privileged laboratory for probing quantum spacetime effects. Thus, this thesis establishes a coherent phenomenological framework linking models of modified relativity with observations in particle astrophysics, highlighting realistic observational avenues for testing quantum gravity hypotheses.