The Amaterasu particle: constraining the superheavy dark matter origin of UHECRs

TL;DR

This paper investigates whether the ultra-high-energy cosmic ray event Amaterasu could originate from decaying superheavy dark matter in the Milky Way, using multi-messenger constraints from gamma-ray and neutrino observations to limit SHDM properties.

Contribution

The study provides new constraints on superheavy dark matter parameters based on the Amaterasu event and multi-messenger data, and assesses future telescope sensitivities.

Findings

Current gamma-ray non-detections severely limit SHDM explanations.

Future neutrino telescopes will provide competitive bounds on SHDM.

Upcoming gamma-ray detectors will significantly improve constraints on SHDM lifetime.

Abstract

Amaterasu, the second most energetic ($244$ EeV) cosmic ray particle has been recently detected by the Telescope Array (TA) surface detector. The origin of the TA Amaterasu event is puzzling, as its arrival direction points back to a void in the local Universe, lacking conventional astrophysical ultra-high-energy (UHE) cosmic ray sources. Hence, we explore the possibility if this TA Amaterasu event could have originated from the decay of superheavy dark matter (SHDM) in the Milky Way. Such an origin also opens up multi-messenger detection channels in both UHE gamma-rays and UHE neutrinos. In this present work, using the TA Amaterasu event and the multi-messenger limits/sensitivities from various UHE telescopes, we place stringent constraints on the lifetime and mass of the SHDM. We find that the non-detection of the corresponding gamma-rays at the Pierre Auger Observatory (PAO) and the TA is in severe tension with the SHDM parameter space required to explain the TA Amaterasu event. Additionally, we extend the multi-messenger analysis to the future UHE gamma-ray and UHE neutrino telescopes such as PAO upgrade, GRAND 200k and IceCube-Gen2. We find that the bounds from the future neutrino telescopes will be able to compete with the present UHECR bounds. However, compared to the existing UHE gamma-ray bounds, the future PAO upgrade and the GRAND 200k gamma-ray detectors will improve the bounds on SHDM lifetime by at least one order of magnitude.