AMS analysis group
Physikalisches Institut Ib, RWTH Aachen



Measurement of the positron fraction e+/( e++e-) from 1 - 50 GeV with AMS-01 using converted bremsstrahlung photons

ams-01 on shuttle As a prototype for the
AMS-02 experiment, the AMS-01 detector was flown on board the space shuttle Discovery in a near earth orbit during the STS-91 mission from June 2nd to 12th, 1998. AMS-01 has recorded a large amount of data for the determination of particle fluxes in the earth's vicinity, leading to precise proton and helium flux spectra, measurements of the characteristics of leptons in near earth orbit, and to a significant improvement of the upper limit on the relative flux of antihelium to helium in cosmic rays (see publications of the AMS collaboration).

The flux of cosmic positrons is of special interest. As antiparticles, the origin of positrons in the galaxy is almost exclusively of secondary origin, meaning that they are produced in reactions of freely propagating particles rather than directly in supernovae. This results in a smooth flux spectrum which follows a power law [1]. However, the present data from several experiments unveil that there are hints of an excess in the spectrum at energies above 7 GeV [2]. It has been shown that the probability of global fits to the data can be significantly improved when primary positron production from processes involving supersymmetric particles are taken into account [3]. The most promising candidate for such a process is the annihilation of the neutralino, the presumably lightest supersymmetric particle.

conversion principleThe results concerning the flux of cosmic positrons which have been published by the AMS collaboration yet are limitited to a momentum range of up to 3 GeV, since the discrimination of the vast proton background is constrained for single track events due to the characteristics of the AMS-01 subdetectors. A new analysis is now dedicated to identify high energetic positrons and electrons by reconstructing the conversion pairs from photons produced by bremsstrahlung - a process which is suppressed for protons by a factor of more than 6 orders of magnitude with respect to positrons. The signal events distinguish themselves by the presence of three tracks in the silicon tracker. The sensitivity of AMS-01 for these events reaches up to approximately 50 GeV.

[1] I.V. Moskalenko, A.W.Strong. Production and propagation of cosmic-ray positrons and electrons. ApJ 496 (1998) p. 694
[2] M.A. Du Vernois et al. Cosmic-ray electrons and positrons from 1 to 100 GeV: Measurements with HEAT and their interpretation. ApJ 559 (2001) p. 296
[3] W. de Boer et al. Indirect Evidence for the Supersymmetric Nature of Dark Matter from the Combined Data on Galactic Positrons, Antiprotons and Gamma Rays hep-ph/0309029, 2003


Dark matter