Supersymmetry and the positron excess in cosmic rays

Abstract

Recently the HEAT balloon experiment has confirmed an excess of high-energy positrons in cosmic rays. They could come from annihilation of dark matter in the galactic halo. We discuss expectations for the positron signal in cosmic rays from the lightest superpartner. The simplest interpretations are incompatible with the size and shape of the excess if the relic LSPs evolved from thermal equilbrium. Non-thermal histories can describe a sufficient positron rate. Reproducing the energy spectrum is more challenging, but perhaps possible. The resulting light superpartner spectrum is compatible with collider physics, the muon anomalous magnetic moment, Z-pole electroweak data, and other dark matter searches.

Figures (2)

• Figure 1: Positron fraction as a function of energy. The lower dashed line is the expected signal fraction with no LSP annihilations for a certain set of astrophysical assumptions described in the text. The solid lines also include the positrons and electrons from the annihilations in the galactic halo of LSPs with mass $m_\chi=83\, {\rm GeV}$ and $m_\chi=200\, {\rm GeV}$, with boost factors of 2.7 and 3.9 respectively. The 1994-1995 HEAT data is represented by the solid line cross-hairs, and the 2000 HEAT data by the dashed line cross-hairs.
• Figure 2: The solid line is the average positron energy distribution from cascade decays resulting from LSP annihilations into $W$-boson pairs. The mass of the LSP is 100 GeV in this example. The dashed line tracks the positrons from $W^+\to e^+$ direct decays, and the dotted line from $W^+\to \mu^+/\tau^+\to e^+$ direct decays.