Can Supersymmetry Naturally Explain the Positron Excess?

TL;DR

The paper investigates whether annihilating neutralino dark matter, amplified by local dark-matter substructure, can explain the HEAT cosmic positron excess. It combines neutralino annihilation spectra for several benchmark masses, a diffusion-loss propagation model for positrons, and a semi-analytic subhalo framework to estimate the necessary clump properties and their likelihood. The analysis finds that best fits to HEAT require a local clump at ~0.2–2 kpc with a large boost factor, but the probability of such a clump existing is of order 10^-4 or smaller under optimistic cross-section assumptions, making the SUSY explanation highly improbable. Consequently, while not strictly excluded, the results place strong constraints on DM annihilation scenarios and highlight the need for alternative sources or mechanisms for the HEAT positron excess.

Abstract

It has often been suggested that the cosmic positron excess observed by the HEAT experiment could be the consequence of supersymmetric dark matter annihilating in the galactic halo. Although it is well known that evenly distributed dark matter cannot account for the observed excess, if substantial amounts of local dark matter substructure are present, the positron flux would be enhanced, perhaps to the observed magnitude. In this paper, we attempt to identify the nature of the substructure required to match the HEAT data, including the location, size and density of any local dark matter clump(s). Additionally, we attempt to assess the probability of such substructure being present. We find that if the current density of neutralino dark matter is the result of thermal production, very unlikely ($\sim 10^{-4}$ or less) conditions must be present in local substructure to account for the observed excess.

Figures (7)

• Figure 1: The positron spectrum from neutralino annihilations for the most important annihilation modes. Solid lines represent the positron spectrum, per annihilation, for $\chi^0 \chi^0 \rightarrow b \bar{b}$, for LSPs with masses of 50, 150 and 600 GeV. The dotted lines are the same, but from the process $\chi^0 \chi^0 \rightarrow \tau^{+} \tau^{-}$. Dashed lines represent positrons from the process $\chi \chi \rightarrow W^{+}W^{-}$ for LSPs with masses of 150 and 600 GeV. The spectrum from $\chi \chi \rightarrow ZZ$ is very similar. The positron spectrum from WIMP annihilations will be the weighted sum of the spectra over the annihilation modes, such as those shown here. See text for more details.
• Figure 2: The number density of dark matter clumps between 7 and 10 kpc from the galactic center, as a function of the minimum value of $f \times M^2_{\rm{clump}}/V_{\rm{clump}}$ considered. The thick solid line shows the results for merger trees complete down to $\sim 3\times 10^6 M_{\odot}$, while the thick dashed line shows the results for higher-resolution trees complete down to $\sim 6\times 10^5 M_{\odot}$. The thick dotted line shows results for a different disruption efficiency (see text). The thin lines show the $\pm 1$-$\sigma$ halo-to-halo variation.
• Figure 3: The positron fraction, as a function of positron energy (in GeV), for a 50 GeV neutralino which annihilates 96% to $b \bar{b}$ and 4% to $\tau^+ \tau^-$. The solid line represents the distance to the dark matter clump at which the predicted spectrum best fits the data. Dotted and dashed lines represent the spectra for a source at (1 and 2$\sigma$) distances less than and greater than found for the best fit, respectively. The normalization was considered to be a free parameter. The error bars shown are for the 94-95 and 2000 HEAT flights.
• Figure 4: The predicted positron fraction, as a function of positron energy (in GeV), for a 150 GeV neutralino which annihilates 96% to $b \bar{b}$ and 4% to $\tau^+ \tau^-$. Otherwise, the same as in figure \ref{['fit1']}.
• Figure 5: The predicted positron fraction, as a function of positron energy (in GeV), for a 150 GeV neutralino which annihilates 58% to $W^+ W^-$ and 42% to $ZZ$. Otherwise, the same as in figure \ref{['fit1']}.