Probing the ATIC peak in the cosmic-ray electron spectrum with H.E.S.S

TL;DR

The study tests the ATIC-reported peak in the cosmic-ray electron spectrum by extending H.E.S.S. measurements to lower energies (down to ~340 GeV) and employing robust background rejection. It finds a smooth, broken-power-law electron spectrum with a break near ~0.9 TeV and no evident ATIC-like peak, aligning with FERMI up to ~1 TeV and constraining exotic explanations. A Kaluza-Klein dark-matter scenario around 620 GeV is not supported by H.E.S.S. at the 99% CL. Overall, the work reinforces conventional astrophysical origins for high-energy electrons and narrows the parameter space for dark-matter interpretations.

Abstract

The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has - together with the PAMELA detection of a rise in the positron fraction up to 100 GeV - motivated many interpretations in terms of dark matter scenarios; alternative explanations assume a nearby electron source like a pulsar or supernova remnant. Here we present a measurement of the cosmic-ray electron spectrum with H.E.S.S. starting at 340 GeV. While the overall electron flux measured by H.E.S.S. is consistent with the ATIC data within statistical and systematic errors, the H.E.S.S. data exclude a pronounced peak in the electron spectrum as suggested for interpretation by ATIC. The H.E.S.S. data follow a power-law spectrum with spectral index of 3.0 +- 0.1 (stat.) +- 0.3 (syst.), which steepens at about 1 TeV.

Figures (3)

• Figure 1: The measured distribution of the parameter $\zeta$, compared with distributions for simulated protons and electrons, for showers with reconstructed energy between 0.34 and 0.7 TeV (the energy range of the extension towards lower energies compared to the analysis presented in paper1). The best fit model combination of electrons and protons is shown as a shaded band. The proton simulations use the SIBYLL hadronic interaction model. Distributions differ from the ones presented in Fig. 1 of paper1 because of the energy dependence of the $\zeta$ parameter.
• Figure 2: The energy spectrum E$^3$ dN/dE of cosmic-ray electrons as measured by ATIC (atic2), PPB-BETS (PPB_BETS), emulsion chamber experiments (Kobayashi), FERMI (fermi) (the gray band shows the FERMI systematic uncertainty, the double arrow labeled with $+5 \% \atop -10 \%$ the uncertainty of the FERMI energy scale), and H.E.S.S. Previous H.E.S.S. data (paper1) are shown as blue points, the result of the low-energy analysis presented here as red points. The shaded bands indicate the approximate systematic error arising from uncertainties in the modeling of hadronic interactions and in the atmospheric model in the two analyses. The double arrow indicates the effect of an energy scale shift of 15%, the approximate systematic uncertainty on the H.E.S.S. energy scale. The fit function is described in the text.
• Figure 3: The energy spectrum E$^3$ dN/dE of cosmic-ray electrons measured by H.E.S.S. and balloon experiments. Also shown are calculations for a Kaluza-Klein signature in the H.E.S.S. data with a mass of 620 GeV and a flux as determined from the ATIC data (dashed-dotted line), the background model fitted to low-energy ATIC and high-energy H.E.S.S. data (dashed line) and the sum of the two contributions (solid line). The shaded regions represent the approximate systematic error as in Fig. \ref{['fig2']}.