Abnormal threshold behaviors of photo-pion production off the proton in the GZK region

TL;DR

This paper investigates how Lorentz invariance violation could cause abnormal threshold behaviors in photo-pion production off protons near the GZK region, offering new ways to test fundamental physics with cosmic-ray data.

Contribution

It introduces a phenomenological framework to analyze threshold anomalies in pion production due to Planck-scale Lorentz violation, expanding the methods to constrain new physics.

Findings

Current observations tightly constrain some LV parameter regions.

Predicted threshold anomalies vary with different LV modifications.

Further experiments are needed to explore remaining LV parameter space.

Abstract

The confirmation of the existence of GZK cut-off was tortuous, leading to activities to explore new physics, such as the cosmic-ray new components, unidentified cosmic-ray origins, unknown propagation mechanism, and the modification of fundamental physics concepts like the tiny Lorentz invariance violation (LV). The confirmation of the GZK cut-off provides an opportunity to constrain the LV effect. We use a phenomenological framework to restudy the GZK mechanism under the Planck scale deformation of the proton and pion dispersion relations. Restudying the photon induced pion production of the proton $\mathrm{p}+\gamma\to\mathrm{p}+\pi^0$, we predict abnormal threshold behaviors of this reaction under different LV modifications. Therefore, we can study the LV effects not only from the conventional GZK cut-off, but also from potentially threshold anomalies of the pion production process. We divide the LV parameter space into three regions, and analyze the constraints from current observations in each region. The current observations have set strict constraints on a certain LV region. However, for others LV regions, further experimental observations and theoretical researches are still needed, and we also find survival space for some theoretical explorations that permit specific LV effects.