Is the PAMELA Positron Excess Winos?

TL;DR

This paper investigates whether a 200 GeV wino-like neutralino, annihilating primarily to $W$ bosons with $\langle \sigma v \rangle \approx 2\times10^{-24}\ \text{cm}^3\ \text{s}^{-1}$, can explain the PAMELA positron excess. It shows that under standard cosmic-ray propagation and energy-loss assumptions, the wino alone struggles to fit the data, but a substantial relaxation of the positron energy-loss time $\tau$ (or modest density-boost factors) could reconcile the signal with PAMELA. The authors examine gamma-ray and antiproton constraints, finding tension with EGRET and antiproton data that is strongly regime-dependent on halo profiles and propagation uncertainties. They argue that upcoming FGST measurements and continued PAMELA data will be critical in testing this scenario, with a confirmed signal implying a non-thermal cosmological history and potential SUSY discoveries at the LHC.

Abstract

Recently the PAMELA satellite-based experiment reported an excess of galactic positrons that could be a signal of annihilating dark matter. The PAMELA data may admit an interpretation as a signal from a wino-like LSP of mass about 200 GeV, normalized to the local relic density, and annihilating mainly into W-bosons. This possibility requires the current conventional estimate for the energy loss rate of positrons be too large by roughly a factor of five. Data from anti-protons and gamma rays also provide tension with this interpretation, but there are significant astrophysical uncertainties associated with their propagation. It is not unreasonable to take this well-motivated candidate seriously, at present, in part because it can be tested in several ways soon. The forthcoming PAMELA data on higher energy positrons and the FGST (formerly GLAST) data, should provide important clues as to whether this scenario is correct. If correct, the wino interpretation implies a cosmological history in which the dark matter does not originate in thermal equilibrium.

Figures (4)

• Figure 1: Positron flux ratio for Wino-like Neutralino with a mass of $200$ GeV, normalized to the local relic density. We set the height of the propagation region at $4$ kpc and consider varying values for the energy loss rate ($\tau=1,2,5$) in units of $10^{16}$ s. The solid bottom line represents a conventional astrophysical background Baltz:1998xv.
• Figure 2: The anti-proton flux ratio for a $200$ GeV wino-like neutralino as a function of kinetic energy. The lowest curve represents the conventional astrophysical background, whereas the remaining curves are the signal plus background for the $200$ GeV candidate. These curves are the flux from dark matter annihilations given different choices for propagation model -- all of which have been parametrically fixed by matching to the well known spectrum of secondary/primary fluxes (e.g. B/C ratio) Donato:2003xg.
• Figure 3: Positron flux ratio for a wino-like neutralino with a mass of $200$ GeV. The lowest curve represents the astrophysical background, whereas the remaining curves are the flux ratio for (large) energy loss rate of $\tau=5 \times 10^{16}$ s and varying propagation model (as discussed in the text).
• Figure 4: The differential flux for the $200$ GeV wino-like neutralino and an NFW profile averaged over the minimum angular resolution of FGST (i.e. $\Delta \Omega = 10^{-5}$ sr) and integrated over a $0.5^{\circ} \times 0.5^{\circ}$ region around the galactic center. For the diffuse background we take the 'conventional' galprop model discussed in the text. The error bars represent the statistical uncertainty after one year of observations and do not account for systematical uncertainties.