Wino Dark Matter Under Siege

Abstract: A fermion triplet of SU(2)_L - a wino - is a well-motivated dark matter candidate. This work shows that present-day wino annihilations are constrained by indirect detection experiments, with the strongest limits coming from H.E.S.S. and Fermi. The bounds on wino dark matter are presented as a function of mass for two scenarios: thermal (winos constitute a subdominant component of the dark matter for masses less than 3.1 TeV) and non-thermal (winos comprise all the dark matter). Assuming the NFW halo model, the H.E.S.S. search for gamma-ray lines excludes the 3.1 TeV thermal wino; the combined H.E.S.S. and Fermi results completely exclude the non-thermal scenario. Uncertainties in the exclusions are explored. Indirect detection may provide the only probe for models of anomaly plus gravity mediation where the wino is the lightest superpartner and scalars reside at the 100 TeV scale.

• The paper demonstrates that indirect gamma-ray observations exclude thermal wino dark matter below 3.1 TeV under NFW model assumptions.
• It employs detailed calculations with Sommerfeld enhancements and one-loop corrections to refine wino annihilation cross-sections.
• The study also constrains non-thermal wino scenarios and projects that future CTA experiments could further tighten dark matter detection limits.

Analysis of "Wino Dark Matter Under Siege"

The study entitled "Wino Dark Matter Under Siege" investigates the prospect of wino-like dark matter as a credible candidate for the Universe's missing mass. The investigation primarily concentrates on the constraints placed on wino dark matter through indirect detection methods, chiefly using data from the High Energy Stereoscopic System (H.E.S.S.) and the Fermi Gamma-Ray Space Telescope (Fermi). The research also explores scenarios with both thermal and non-thermal origins of wino dark matter, examining their viability in light of current observations and theoretical frameworks such as the Navarro–Frenk–White (NFW) halo model.

Key Findings and Numerical Results

A wino, categorized under a TeV-scale weakly interacting massive particle (WIMP), is considered due to its alignment with the thermal relic hypothesis. For thermal winos, the team finds that winos below 3.1 TeV do not comprise the dominant component of dark matter. This is established by analyzing the exclusion limits as a function of mass, with H.E.S.S's search for gamma-ray lines ruling out the thermal wino scenario at precisely 3.1 TeV under the NFW model assumptions. The paper highlights a noteworthy exclusion of the non-thermal scenario by combined observations from H.E.S.S. and Fermi when using the same density profile.

To quantify these boundaries, the analysis utilizes the cross-sections of wino annihilations into two major channels: $W^+W^-$ and photon-producing channels such as $\gamma\gamma$ and $\gamma Z^0$. The perturbative cross-section for processes like $\chi^0\,\chi^0 \rightarrow W^+W^-$ is coupled with Sommerfeld enhancements, highlighting the non-negligible impact of these non-perturbative effects, especially at low velocities. One-loop corrections are found to reduce the photon-induced annihilation cross-sections significantly, by a factor of about 3 to 4.

Implications and Theoretical Considerations

The findings have profound implications for models featuring wino dark matter as the lightest supersymmetric particle, particularly in frameworks like anomaly-mediated supersymmetry breaking or split supersymmetry models. Here, the wino serves as the lightest superpartner with no renormalizable interactions with the Higgs boson, evidencing limitations in direct detection prospects. Due to these limitations, indirect detection remains a primary method for exploring such candidates.

While the results substantially constrain the parameter space for wino DM, significant uncertainties persist, especially regarding the dark matter density profile. Astrophysical models such as the Einasto and Burkert profiles, in comparison to the NFW, exhibit considerable variability in their exclusion limits, underscoring the necessity for improved empirical constraints on these profiles.

Future Outlook

Looking forward, experiments such as the Cherenkov Telescope Array (CTA) are projected to enhance the sensitivity for wino detection, excluding even broader regions of parameter space. The paper’s predictions suggest that if current model assumptions hold, CTA could exclude thermal winos down to masses significantly below the thresholds set by H.E.S.S. Moreover, advancements in cosmic ray experiments, such as AMS-02, could further refine constraints on WIMP annihilation channels, corroborating or refuting the existence of wino dark matter within the predicted mass window.

In summary, the meticulous evaluation and integration of Sommerfeld effects, together with the constraints from indirect detection experiments, offer a robust framework for assessing the viability of wino dark matter. The study effectively synthesizes observational data with theoretical constructs, highlighting the increasingly constrained landscape of supersymmetric dark matter candidates and laying a solid foundation for future detection strategies.