GeV-Scale Thermal WIMPs: Not Even Slightly Dead (1805.10305v2)

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.

• The paper introduces a model-independent approach to constrain WIMP annihilation cross-sections by aggregating data across all relevant Standard Model final states.
• It integrates results from Planck, Fermi-LAT, and AMS-02 to define a 'WIMP window' spanning roughly 20 GeV to 100 TeV.
• The study addresses astrophysical and cosmological uncertainties, guiding future experiments to explore the remaining viable parameter space for thermal WIMPs.

Insights on GeV-Scale Thermal WIMPs: A Professional Overview

This paper explores the current viability of Weakly Interacting Massive Particles (WIMPs) as dark matter candidates by assessing their annihilation cross-sections. The authors focus on GeV-scale thermal WIMPs, which, according to well-established theoretical models, should have emerged through the freezeout process in the early universe. This framework predicts specific values for the annihilation cross-section, vital for experiments attempting to detect or exclude such particles.

Main Contributions

1. Model-Independent Limit Construction: The paper presents a pioneering approach to derive model-independent constraints on the WIMP annihilation cross-section. Unlike previous studies that often focused on specific interaction channels, this work aggregates data across all possible Standard Model (SM) final states, excluding neutrinos, to evaluate a general limit.
2. Comprehensive Data Integration: The authors integrate results from Planck's CMB measurements, Fermi-LAT's gamma-ray observations of dwarf spheroidal galaxies, and AMS-02's positron data. This data synthesis offers a robust model-independent range of permissible dark matter masses and cross-sections.
3. Establishing the WIMP Window: The analysis identifies a constrained mass range for thermal WIMPs, dubbed the "WIMP window," bracketed between an observational lower limit of approximately 20 GeV and an upper limit set by unitarity at around 100 TeV. This window delineates the parameter space that needs further exploration for these WIMP candidates.
4. Discussion of Astrophysical and Cosmological Constraints: Significant effort is devoted to ensuring that the results are robust against uncertainties in cosmic-ray propagation, the local dark matter density, and statistical significance. By considering the least-constrained combinations of decay channels, the authors stress that prior exclusions might have been overly optimistic.

Implications and Future Directions

The implications of these findings are twofold. Practically, they suggest that while certain high-mass WIMPs might be under exclusion pressure, there is still substantial parameter space at lower masses that has not been conclusively probed. Theoretically, the work underscores the resilience of the WIMP paradigm under standardized cosmological histories and annihilation processes.

For future research, the paper outlines an integration of additional datasets from ongoing and future experiments like DES, CTA, and potentially neutrino observatories to close the WIMP window robustly. Any significant discovery within this parameter space could substantially impact particle physics and cosmology, or alternatively, existing models of WIMPs may need reevaluation if the parameter space is exhaustively searched without success.

Thus, this paper represents a meticulous re-evaluation of the WIMP hypothesis, emphasizing the need for precise, broad-spectrum inquiry into potential SM interactions of these dark matter candidates.