Antiprotons at high energy
Antiproton/Proton ratio reported by PAMELA with other measurements, figure taken from arXiv:0810.4994
The recent measurements of PAMELA of the antiproton to proton ratio at high energy, shown in the figure, with a compilation of previous data, indicate levels which are consistent with secondary production from cosmic ray interactions during propagation. There is no need to invoke an exotic source of antiprotons to explain these data. When PEBS is upgraded to a version with a superconducting magnet (PEBS-2), we could make a high-statistics measurement of antiprotons into the 100GeV region by introducing a gas RICH detector into the instrument. This instrument provides excellent Lorentz factor resolution in the range γ~18-100 resulting in a mass resolution that can adequately separate antiprotons from negative kaons/pions/muons up to 100 GeV. PEBS-2 should be able to see the effect of energy-dependence in the escape of protons from the galaxy by observing the decrease in the antiproton/proton ratio at high energy. For the first time this observation would directly tie the propagation history of protons to those of heavy nuclei, where this effect is seen for instance in the B/C or Sub-Fe/Fe ratios. Addressing another important scientific goal we would also like to investigate, for the first time, the identification and measurement of 3He/4He at high energy (~100GeV/n) using PEBS-2. This ratio provides another high energy secondary from light nuclei which can also be used to investigate the propagation of helium nuclei in the galaxy. Again this can be compared with the energy dependent escape of heavy nuclei to directly measure if there is a common galactic history across the elemental spectrum. These measurements are not possible with AMS-2 because there is no gas RICH included in this instrument. Part of this proposal, is to design and construct a gas RICH for PEBS-2 using new photon detection technology of silicon photomultipliers which has recently become available. This effort would begin in 2012.