Systematic Issues in Electron and Positron MeasurementsWhile much progress has been made recently in the observations of electrons and positrons in cosmic rays, there remain some nagging doubts on the veracity of some aspects of the newly published results. The overall issue is the very difficult elimination of those hadronic background events which masquerade as electrons or positrons. The problem is exacerbated at high energies, where the ratio of the numbers of protons to electrons to positrons increases. All of the new measurements from PAMELA, ATIC, and HESS (also Fermi/GLAST when these new data are published) suffer from the need to accurately quantify and subtract a level of hadronic background from their data. In the case of ground-based measurements like HESS the background problem is particularly severe. For all experimental efforts, an accuracy of better than 1:104 is needed relative to the raw proton flux intensity. If this is not achieved, changes in slope of the derived electron or positron spectra will arise because of the flatter overall spectrum of protons. This hadronic background arises in all calorimeter measurements due to hadronic interactions near the top of the calorimeter transferring a substantial fraction of its energy into a π0 leading to a cascade essentially indistinguishable from an electron. In the proposed PEBS program we plan to have positron and electron measurements which essentially remove this hadronic background by combining a transition radiation detector (TRD) with a calorimeter. The TRD is capable of separating high Lorentz factor (γ>1000) particles from those at lower γ with high rejection power independent of any calorimeter measurements. This provides an essentially hadron-free measurement of positrons and electrons up to potential energies of ~2TeV. The only other planned or flying experiment which has this capability is AMS-2 which only has a limt of ~1TeV, we shall discuss the complementarity of PEBS and AMS-2 below.
Single tube spectra measured during a testbeam of a small TRD prototype, together with a comparison to a Monte Carlo simulation. The shoulder in the electron spectrum above 8 keV is due to transition radiation x-ray photons.