GeV-Scale Thermal WIMPs: Not Even Slightly Dead

TL;DR

This paper establishes the first largely model-independent lower mass limit for thermal WIMPs at around 20 GeV, defining a WIMP mass window between this and the unitarity bound, and discusses future probing methods.

Contribution

It provides the first model-independent limit on the total annihilation cross section of thermal WIMPs, refining the mass constraints and defining the WIMP window.

Findings

Lower mass limit for thermal WIMPs is approximately 20 GeV.

Allowed annihilation channel branching ratios weaken detection sensitivity.

The WIMP mass window is between 20 GeV and 100 TeV.

Abstract

Weakly Interacting Massive Particles (WIMPs) have long reigned as one of the leading classes of dark matter candidates. The observed dark matter abundance can be naturally obtained by freezeout of weak-scale dark matter annihilations in the early universe. This "thermal WIMP" scenario makes direct predictions for the total annihilation cross section that can be tested in present-day experiments. While the dark matter mass constraint can be as high as $m_\chi\gtrsim100$ GeV for particular annihilation channels, the constraint on the total cross section has not been determined. We construct the first model-independent limit on the WIMP total annihilation cross section, showing that allowed combinations of the annihilation-channel branching ratios considerably weaken the sensitivity. For thermal WIMPs with s-wave $2\rightarrow2$ annihilation to visible final states, we find the dark matter mass is only known to be $m_\chi\gtrsim20$ GeV. This is the strongest largely model-independent lower limit on the mass of thermal-relic WIMPs, together with the upper limit on the mass from the unitarity bound ($m_\chi\lesssim 100$ TeV), it defines what we call the "WIMP window". To probe the remaining mass range, we outline ways forward.