Robust Gamma Ray Signature of WIMP Dark Matter

TL;DR

This work derives a largely model-independent prediction for the gamma-ray flux from WIMP dark matter annihilation, focusing on final state radiation (FSR) from charged annihilation products. By exploiting collinear factorization, it provides analytic expressions for the FSR spectrum, including a robust sharp edge at $E_\gamma \approx m_\chi$ when annihilation yields charged fermions, and shows how the endpoint remains distinctive even when fragmentation photons are present. The authors express the observable flux in terms of the total annihilation cross section, a set of charge-weighted channel fractions, and the astrophysical $J$-factor, enabling predictions across scenarios (e.g., UED, democratic) and telescope types. They evaluate detection prospects for GLAST and ground-based ACTs, discussing background models and discovery criteria, and conclude that the edge feature offers a strong, practically detectable signature under favorable halo densities and instrument capabilities. This work thus links particle physics predictions to concrete astrophysical observables, providing a clear target for indirect dark matter searches.

Abstract

If dark matter consists of weakly interacting massive particles (WIMPs), annihilation of WIMPs in the galactic center may lead to an observable enhancement of high energy gamma ray fluxes. We predict the shape and normalization of the component of the flux due to final state radiation by charged particles produced in WIMP annihilation events. The prediction is made without any assumptions about the microscopic theory responsible for WIMPs, and depends only mildly on the unknown distribution of the total WIMP annihilation cross section among the possible final states. In particular, if the WIMPs annihilate into a pair of charged fermions (leptons or quarks), the photon spectrum possesses a sharp edge feature, dropping abruptly at a photon energy equal to the WIMP mass. If such a feature is observed, it would provide strong evidence for the WIMP-related nature of the flux enhancement, as well as a measurement of the WIMP mass. We discuss the prospects for observational discovery of this feature at ground-based and space-based gamma ray telescopes.

Figures (8)

• Figure 1: Values of the quantity $\sigma_{\rm an}$ allowed at 2$\sigma$ level as a function of WIMP mass. The lower and upper bands correspond to models where the WIMP is an $s$- and $p$-annihilator, respectively. Reproduced from Ref. us.
• Figure 2: Comparison of the photon spectrum obtained by a direct calculation in the UED model with the radius of the extra dimension $R=(499.07$ GeV$)^{-1}$ (red histogram) and the spectrum predicted by Eq. (\ref{['wwapprox']}) (blue line) for the case of $B_1B_1\rightarrow e^+e^-\gamma$ annihilation at $\sqrt{s}=1001$ GeV. The mass of the lightest Kaluza-Klein particle (the first excited mode $B_1$ of the hypercharge gauge boson) is 500 GeV.
• Figure 3: Photon spectrum produced by final state radiation and fragmentation of a primary $u$ quark with an energy of 250 GeV. The histograms represent PYTHIA predictions for the total photon flux (blue) and the final state radiation flux alone (red). The black dashed line represents the prediction of Eq. (\ref{['wwapprox']}).
• Figure 4: The quantity $g$, defined in Eq. (\ref{['gdef']}), as a function of the WIMP mass $m_\chi$, in the UED scenario (blue line) and the "democratic" scenario (red line). In the UED scenario, the annihilation fractions for two-body final states are taken to scale as $Y^4N_c$, where $Y$ is the hypercharge of the final state particles, and $N_c=3$ for quarks and $1$ for other states. In the second scenario, the annihilation fractions for all kinematically accessible two-fermion final states are equal (up to a factor of $N_c$).
• Figure 5: The number of signal (blue) and background (red) events at a representative atmospheric Cerenkov telescope with a collection area given in Eq. (\ref{['effarea']}), an exposure time $T=50$ hrs, and a field of view $\Delta\Omega=4\times 10^{-3}$ sr. The signal is computed assuming the UED scenario with an 800 GeV WIMP and a galactic model with $\bar{J}(\Delta\Omega)=10^5$.