Capture and Decay of Electroweak WIMPonium

TL;DR

This paper investigates the formation, decay, and potential observability of WIMPonium bound states in electroweak dark matter, especially focusing on winos, and provides computational tools and insights applicable to broader dark sector models.

Contribution

It introduces analytic and numerical methods for calculating WIMPonium formation and decay rates, and assesses their significance for dark matter annihilation and detection.

Findings

Wino WIMPonium formation rate is suppressed compared to direct annihilation.

Soft photon emission from bound states could help identify dark matter quantum numbers.

Methods developed are applicable to nonabelian dark sectors with massless and massive force carriers.

Abstract

The spectrum of Weakly-Interacting-Massive-Particle (WIMP) dark matter generically possesses bound states when the WIMP mass becomes sufficiently large relative to the mass of the electroweak gauge bosons. The presence of these bound states enhances the annihilation rate via resonances in the Sommerfeld enhancement, but they can also be produced directly with the emission of a low-energy photon. In this work we compute the rate for SU(2) triplet dark matter (the wino) to bind into WIMPonium -- which is possible via single-photon emission for wino masses above 5 TeV for relative velocity v < O(10^{-2}) -- and study the subsequent decays of these bound states. We present results with applications beyond the wino case, e.g. for dark matter inhabiting a nonabelian dark sector; these include analytic capture and transition rates for general dark sectors in the limit of vanishing force carrier mass, efficient numerical routines for calculating positive and negative-energy eigenstates of a Hamiltonian containing interactions with both massive and massless force carriers, and a study of the scaling of bound state formation in the short-range Hulthen potential. In the specific case of the wino, we find that the rate for bound state formation is suppressed relative to direct annihilation, and so provides only a small correction to the overall annihilation rate. The soft photons radiated by the capture process and by bound state transitions could permit measurement of the dark matter's quantum numbers; for wino-like dark matter, such photons are rare, but might be observable by a future ground-based gamma-ray telescope combining large effective area and a low energy threshold.