New Measurement of the Cosmic-Ray Positron Fraction from 5 to 15 GeV

TL;DR

The measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV, and the combined data from the three HEAT flights indicate a small positron flux of nonstandard origin above 5GeV.

Abstract

We present a new measurement of the cosmic-ray positron fraction at energies between 5 and 15 GeV with the balloon-borne HEAT-pbar instrument in the spring of 2000. The data presented here are compatible with our previous measurements, obtained with a different instrument. The combined data from the three HEAT flights indicate a small positron flux of non-standard origin above 5 GeV. We compare the new measurement with earlier data obtained with the HEAT-e+- instrument, during the opposite epoch of the solar cycle, and conclude that our measurements do not support predictions of charge sign dependent solar modulation of the positron abundance at 5 GeV.

Figures (4)

• Figure 1: Distribution of the average restricted energy loss for particles in the rigidity range 4.5 -- 6.0 GV identified as negatively charged. The Gaussian functions were fitted prior to the final event selection in order to obtain a well-defined association between average restricted energy loss and particle species. After the final event selection is applied, non-electron events are significantly suppressed (lower histogram). All particles within the shaded area of the histogram, determined as described in the text, are selected as electrons (and as positrons in the corresponding positive rigidity bin).
• Figure 2: Distributions of average restricted energy loss for events after all selection criteria have been applied. The three rigidity bands, 4.5 -- 6.0 GV, 6.0 -- 8.9 GV, and 8.9 -- 14.8 GV, are shown from top to bottom, respectively. We accept events under the shaded area as positrons. The strong relativistic rise in the energy loss for hadrons and mesons compared to electrons (which are already heavily relativistic at GeV energies) results in the mass peak of the particle species moving closer together with increasing rigidity and ultimately limiting the particle identification.
• Figure 3: The positron fraction measured by HEAT-pbar compared to measurements with a different instrument (HEAT-e$^\pm$barwick97apj); The curve shows the expectation from the model calculation in ms98.
• Figure 4: The positron fraction as a function of energy for the combined HEAT-$e^\pm$ and HEAT-pbar data , compared to model predictions and other recent measurements ( CAPRICE boezio2000, Golden golden96, AMS ams_posi). Dates in parentheses give the year of the measurement and not the publication. The solid curve is the positron fraction based on a purely secondary production of positrons given by ms98. The dashed and dot dashed curves are the ratios including contributions from Higgsino LSP decay kane2002b and gamma-ray pulsars zhang, respectively.