Final bound-state formation effect on dark matter annihilation

TL;DR

This paper investigates how bound state formation and Sommerfeld effects influence dark matter annihilation, especially in near-degenerate mass scenarios, revealing significant modifications to relic abundance calculations.

Contribution

It provides the first detailed analysis of bound state formation effects on dark matter relic abundance, including partial wave contributions and vector boson emission processes.

Findings

FBS significantly alters relic abundance in certain mass split regions.

FBS effect is comparable to FSS effect in degenerate scenarios.

First calculation of p-wave FSS effect in a specific model.

Abstract

If the annihilation products of dark matter (DM) are non-relativistic and couples directly to a light force mediator, the non-perturbation effect like final state bound state (FBS) formation and final state Sommerfeld (FSS) effect must be considered. Non-relativistic region of final particles will appear when there is small mass split between DM and products, so we study those effects in the degenerate region of mass (including kinematics forbidden case) using two specific models. We demonstrate that FBS effect will significantly modify the DM relic abundance comparing to the standard perturbation calculation in some mass split region. We emphasize that FBS effect is comparable to the FSS effect in those mass split. The conservation angular momentum are subtle considering FBS formation, in some cases there may be not $s$-wave, so we use two models exhibit the different partial wave FBS effect contribution. We also show that the FBS formation with vector boson emission process also contributes in DM relic abundance, and first calculate the $p$-wave FSS effect in the specific model.