WIMPonium and Boost Factors for Indirect Dark Matter Detection

TL;DR

This work demonstrates that WIMP dark matter can annihilate via near-threshold WIMPonium bound states, producing velocity-dependent boost factors in the annihilation rate that need not rely on density enhancements. The analysis combines a simple Yukawa toy model with threshold-resonance physics (BP form) to show how elastic and inelastic near-threshold processes amplify annihilation and open radiative-capture channels, yielding a rich spectrum of discrete gamma-ray lines from decays and bound-state transitions. The key contributions are (i) the identification of a dynamical, velocity-dependent boost mechanism decoupled from $ ho^2$, (ii) the derivation of scaling relations for radiative capture and bound-state decays/transitions, and (iii) the implications for indirect detection strategies and collider searches, including potential Higgs-portal connections. Collectively, the results highlight the observational potential of gamma-ray lines as direct probes of DM self-interactions and bound-state dynamics, and they point to dwarf galaxies as promising targets due to their low velocity dispersions.

Abstract

We argue that WIMP dark matter can annihilate via long-lived "WIMPonium" bound states in reasonable particle physics models of dark matter (DM). WIMPonium bound states can occur at or near threshold leading to substantial enhancements in the DM annihilation rate, closely related to the Sommerfeld effect. Large "boost factor" amplifications in the annihilation rate can thus occur without large density enhancements, possibly preferring colder less dense objects such as dwarf galaxies as locations for indirect DM searches. The radiative capture to and transitions among the WIMPonium states generically lead to a rich energy spectrum of annihilation products, with many distinct lines possible in the case of 2-body decays to $γγ$ or $γZ$ final states. The existence of multiple radiative capture modes further enhances the total annihilation rate, and the detection of the lines would give direct over-determined information on the nature and self-interactions of the DM particles.

Figures (5)

• Figure 1: Contour plot of the enhancement factor $R$. Shown is a typical "path" in parameter space as the velocity of the $s$ states decreases from the value at freeze-out to that relevant for indirect detection. The path is not exactly horizontal due to thermal corrections to the masses and couplings.
• Figure 2: A 3d version of Figure \ref{['2d']}.
• Figure 3: The $\beta$-dependence of $R$ at a fixed value of $\alpha/\epsilon$ as derived from the numerical solution of Eq.(\ref{['schro']}).
• Figure 4: Diagram illustrating, for various bound states, the rates of annihilation (double line arrows), and transitions emitting a $\phi_n$ or $a_n$ (single line arrows). We have omitted the $\phi_sa_s$$l=0$ and $l=2$ bound state energy levels and the associated transitions and decays for clarity.
• Figure 5: Schematic diagrams showing annihilation of, or transition between, different bound states leading to discrete $\gamma\gamma$ or $\gamma Z$ lines. The states in the triangle can be any charged scalar, $W^\pm$ gauge boson, or fermion that couples to $\phi_n, a_n$, in general via mixing with Higgs states.