Current and future gamma-ray searches for dark-matter annihilation beyond the unitarity limit

TL;DR

This paper investigates the potential for gamma-ray observatories to detect ultra-heavy dark matter particles with masses between 30 TeV and 30 PeV, extending beyond traditional unitarity limits, and assesses the feasibility of such detections.

Contribution

It introduces the concept of indirect gamma-ray detection of ultra-heavy dark matter beyond the unitarity limit and evaluates the detection prospects with current and future gamma-ray observatories.

Findings

Gamma-ray observatories can detect certain ultra-heavy dark matter parameters.

Expected upper limits on dark matter cross sections are computed for different instruments.

Instruments can probe a wide mass range, including point-like and geometric cross sections.

Abstract

For decades, searches for electroweak-scale dark matter (DM) have been performed without a definitive detection. This lack of success may hint that DM searches have focused on the wrong mass range. A proposed candidate beyond the canonical parameter space is ultra-heavy DM (UHDM). In this work, we consider indirect UHDM annihilation searches for masses between 30 TeV and 30 PeV, extending well beyond the unitarity limit at $\sim$100 TeV, and discuss the basic requirements for DM models in this regime. We explore the feasibility of detecting the annihilation signature, and the expected reach for UHDM with current and future Very-High-Energy (VHE; $>$ 100 GeV) $\gamma$-ray observatories. Specifically, we focus on three reference instruments: two Imaging Atmospheric Cherenkov Telescope arrays, modeled on VERITAS and CTA-North, and one Extended Air Shower array, motivated by HAWC. With reasonable assumptions on the instrument response functions and background rate, we find a set of UHDM parameters (mass and cross section) for which a $\gamma$-ray signature can be detected by the aforementioned observatories. We further compute the expected upper limits for each experiment. With realistic exposure times, the three instruments can probe DM across a wide mass range. At the lower end, it can still have a point-like cross section, while at higher masses the DM could have a geometric cross section, indicative of compositeness.