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.

Highlights

  • A leading candidate for dark matter (DM) is a weakly interacting massive particle (WIMP) that is a thermal relic of the early Universe [1,2]

  • Comparison are the standard 100% scenarios commonly considered in the literature, τ final states probed by Fermi, and e final states probed by Alpha Magnetic Spectrometer (AMS)

  • It is clear that the general approach of comparing favorable single-channel limits with the thermal relic cross section badly overstates the degree to which thermal WIMPs have been probed

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Summary

Introduction

A leading candidate for dark matter (DM) is a weakly interacting massive particle (WIMP) that is a thermal relic of the early Universe [1,2]. For masses above ∼1 keV, such a particle behaves as cold dark matter (CDM) [3], with dynamics governed by purely gravitational interactions. CDM is in excellent agreement with all large-scale observations of the Universe, there are some persistent discrepancies on smaller scales, where baryonic physics is important. The defining feature of the thermal WIMP is that its relic abundance is naturally explained by the freezeout process [4] with a weak-scale cross section, Ωχh2 ∼ 0.1 pb × c=hσvi, where Ωχh2 ≈ 0.12 [5] is the DM density and hσvi is the thermally averaged annihilation cross section.

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