[Paper Review] Neutralino Dark Matter from Indirect Detection Revisited

Phill Grajek, Gordon Kane+3 more|ArXiv.org|Jul 9, 2008

Dark Matter and Cosmic Phenomena42 references20 citations

TL;DR

This paper revisits indirect detection of wino-like neutralino dark matter in non-thermal production scenarios, showing that large annihilation cross sections naturally produce detectable signals in positrons, antiprotons, and gamma rays without requiring astrophysical boost factors. It finds that a 200–500 GeV wino LSP could be observed by GLAST if the dark matter profile is not too cuspy, with tension between positron excesses and gamma-ray bounds under NFW profiles.

ABSTRACT

We revisit indirect detection possibilities for neutralino dark matter, emphasizing the complementary roles of different approaches. While thermally produced dark matter often requires large astrophysical "boost factors" to observe antimatter signals, the physically motivated alternative of non-thermal dark matter can naturally provide interesting signals, for example from light wino or Higgsino dark matter. After a brief review of cosmic ray propagation, we discuss signals for positrons, antiprotons, synchrotron radiation and gamma rays from wino annihilation in the galactic halo, and examine their phenomenology. For pure wino dark matter relevant to the LHC, PAMELA and GLAST should report signals.

Motivation & Objective

• To re-evaluate indirect detection prospects for neutralino dark matter, focusing on non-thermal production mechanisms that avoid reliance on large astrophysical boost factors.
• To assess the compatibility of observed positron excesses (e.g., from HEAT/AMS-01/PAMELA) with constraints from gamma-ray and synchrotron radiation data.
• To determine the detectability of light wino dark matter (200–500 GeV) in future experiments like GLAST, under different dark matter density profiles.
• To explore the interplay between indirect detection, direct detection, and LHC signatures for wino-like LSPs with non-thermal origins.
• To examine how Higgsino admixtures affect direct detection and neutrino-based indirect detection signals.

Proposed method

• Uses cosmic ray propagation models to compute fluxes of positrons, antiprotons, and gamma rays from wino annihilation in the galactic halo.
• Applies the standard dark matter annihilation source term with cross sections scaled by the local dark matter density squared, using the J-factor defined as $ J = ho^2 / m_ ext{χ}^2 $ integrated along the line of sight.
• Compares predicted signals to observational data from EGRET, PAMELA, and GLAST, using NFW and isothermal dark matter profiles.
• Evaluates the impact of different astrophysical J-factors on detectability, particularly the suppression factor needed to reconcile positron excesses with gamma-ray bounds.
• Assesses the sensitivity of GLAST to pure wino dark matter over 5 years of observation, assuming an NFW profile.
• Considers constraints from synchrotron radiation and the potential for indirect detection via neutrinos from solar capture.

Experimental results

Research questions

• RQ1 Can non-thermal wino dark matter explain the observed positron excesses from HEAT/AMS-01/PAMELA without requiring large boost factors?
• RQ2 What are the constraints on wino dark matter from gamma-ray observations by EGRET and GLAST, particularly under different dark matter density profiles?
• RQ3 To what extent can the tension between positron excesses and gamma-ray bounds be resolved by softening the dark matter profile below the NFW cusp?
• RQ4 How detectable is a 200–500 GeV wino LSP in GLAST, and what J-factor suppression is required to avoid conflict with current gamma-ray limits?
• RQ5 What role do Higgsino admixtures play in enhancing direct detection and neutrino-based indirect detection signals for wino-like LSPs?

Key findings

• A non-thermally produced wino LSP with a mass of 200–500 GeV can naturally produce large annihilation cross sections, enabling detectable signals in indirect detection without requiring astrophysical boost factors.
• For an NFW dark matter profile, wino masses below ~300 GeV should have already been detected by EGRET via gamma rays, indicating tension with the observed positron excess if the signal is from dark matter.
• A suppression of the J-factor by a factor of ~6 below the NFW value is required to reconcile a 200 GeV wino with EGRET gamma-ray bounds while still explaining the positron excess.
• GLAST is expected to detect a pure wino LSP up to 500 GeV at 3σ significance after 5 years of observation, assuming an NFW profile.
• The absence of a gamma-ray signal in EGRET data suggests that the dark matter profile must be less cuspy than NFW to accommodate a light wino, reducing the tension between positron and gamma-ray data.
• Higgsino admixtures in the wino LSP can enhance direct detection cross sections and increase capture rates on the Sun, enabling potential indirect detection via neutrinos.