New Positron Spectral Features from Supersymmetric Dark Matter - a Way to Explain the PAMELA Data?

Abstract

The space-borne antimatter experiment PAMELA has recently reported a surprising rise in the positron to electron ratio at high energies. It has also recently been found that electromagnetic radiative corrections in some cases may boost the gamma-ray yield from supersymmetric dark matter annihilations in the galactic halo by up to three or four orders of magnitude, providing distinct spectral signatures for indirect dark matter searches to look for. Here, we investigate whether the same type of corrections can also lead to sizeable enhancements in the positron yield. We find that this is indeed the case, albeit for a smaller region of parameter space than for gamma rays; selecting models with a small mass difference between the neutralino and sleptons, like in the stau coannihilation region in mSUGRA, the effect becomes more pronounced. The resulting, rather hard positron spectrum with a relatively sharp cutoff may potentially fit the rising positron ratio measured by the PAMELA satellite. To do so, however, very large "boost factors" have to be invoked that are not expected in current models of halo structure. If the predicted cutoff would also be confirmed by later PAMELA data or upcoming experiments, one could either assume non-thermal production in the early universe or non-standard halo formation to explain such a spectral feature as an effect of dark matter annihilation. At the end of the paper, we briefly comment on the impact of radiative corrections on other annihilation channels, in particular antiprotons and neutrinos.

Figures (3)

• Figure 1: The solid line gives the total number of positrons per neutralino pair annihilation and positron energy for the benchmark model BM3 of ibsusy ($m_\chi=233$ GeV, $m_{\tilde{e}}=240$ GeV). Shown separately is the same quantity without radiative corrections (dotted line) and, on top of this, only the $e^+e^-\gamma$ final states (dashed line).
• Figure 2: Scan over mSUGRA (left) and MSSM-9 (right) models that shows the enhancement in the positron flux (at $E_{e^+}=0.9\,m\chi$) due to radiative corrections vs. the mass splitting between the lightest selectron and the neutralino, $\delta\equiv\left(m_{\tilde{e}}-m_\chi\right)/m_\chi$. Also indicated in this figure are the benchmark model BM3 from ibsusy and a further benchmark model BM5' as introduced in the text.
• Figure 3: The solid line is the expected flux ratio $e^+/(e^++e^-)$ as calculated following galprop. The data points are the combined HEAT heat and PAMELA data pamela_idm08. Furthermore, the expected flux ratio for our benchmark models is shown without (dotted lines) and after taking into account radiative corrections (dashed lines). See text for further details.