Abstract:

We take a fresh look at high energy radioactive nuclei data reported in the 90's and at the positron data recently reported by PAMELA. Our aim is to study the model independent implications of these data for the propagation time scales of cosmic rays in the Galaxy. Considering radioactive nuclei, using decaying charge to decayed charge ratios -- the only directly relevant data available at relativistic energies -- we show that a rigidity independent residence time is consistent with observations. The data for all nuclei can be described by f_{s,i}=(t_i/100 Myr)^{0.7}, where f_{s,i} is the suppression of the flux due to decay and t_i is the observer frame lifetime for nucleus specie i. Considering positron measurements, we argue that the positron flux is consistent with a secondary origin. Comparing the positron data with radioactive nuclei at the same energy range, we derive an upper bound on the mean electromagnetic energy density traversed by the positrons, \bar U_T<1.25 eV/cm^3 at a rigidity of R=40 GV. Charge ratio measurements within easy reach of the AMS-02 experiment, most notably a determination of the Cl/Ar ratio extending up to R\sim100 GV, will constrain the energy dependence of the positron cooling time. Such constraints can be used to distinguish between different propagation scenarios, as well as to test the secondary origin hypothesis for the positrons in detail.

Abstract: Does Nature have additional, massive U(1) gauge bosons? String theories, among others, predict such particles. General considerations lead us to believe they would kinetically mix with our photon, which would induce small couplings to electric charge. Recent astrophysical evidence for high energy electrons and positrons in the cosmic rays might be evidence for dark matter annihilations into heavy photons which subsequently decayed into high energy lepton pairs. This talk will discuss a proposal to search for massive photons in a new experiment at Jefferson Laboratory. Heavy photons would be produced in electron bremstrahlung on heavy targets, and decay to electron-positron pairs. They can be distinguished from the abundant QED trident background by virtue of their invariant mass and finite decay lengths. The talk will review motives for the search, the proposed apparatus, simulated performance, and how well we hope to do.