New physics as a possible explanation for the Amaterasu particle

TL;DR

This paper explores whether Lorentz Invariance Violation (LIV) could explain the origin of the ultra-high-energy cosmic ray event 'Amaterasu', expanding the possible source region and fitting observational data better.

Contribution

It investigates the impact of LIV on UHECR propagation and demonstrates that LIV can plausibly explain the Amaterasu event and improve data fits.

Findings

LIV increases the mean free path of particles at Amaterasu's energy to hundreds of Mpc.

Data fits are improved with LIV assumptions, both with and without Amaterasu.

Robustness of LIV explanation is supported by compatibility with Pierre Auger Observatory data.

Abstract

The Telescope Array experiment has recently reported the most energetic event detected in the hybrid technique era, with a reconstructed energy of 240 EeV, which has been named ``Amaterasu'' after the Shinto deity. Its origin is intriguing since no powerful enough candidate sources are located within the region consistent with its propagation horizon and arrival direction. In this work, we investigate the possibility of describing its origin in a scenario of new physics, specifically under a Lorentz Invariance Violation (LIV) assumption. The kinematics of UHECR propagation under a phenomenological LIV approach is investigated. The total mean free path for a particle with Amaterasu's energy increases from a few Mpc to hundreds of Mpc for $-\delta_{\rm{had},0} > 10^{-22}$, expanding significantly the region from which it could have originated. A combined fit of the spectrum and composition data of Telescope Array under different LIV assumptions was also performed. The data is best fitted with some level of LIV both with and without Amaterasu. Robustness with data from the Pierre Auger Observatory is investigated by exploring an intermediate composition scenario. Similar improvements in the description of the data with LIV are found for that. New physics in the form of LIV could, thus, provide a plausible and robust explanation for the Amaterasu particle.