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Measurements of the Cosmic-Ray Positron Fraction From 1 to 50 GeV

HEAT Collaboration, S. W. Barwick, E. Schneider, J. J. Beatty, G. A. de Nolfo, A. Bhattacharyya, C. R. Bower, J. A. Musser, C. J. Chaput, S. Coutu, S. McKee, G. Tarle, A. D. Tomasch, J. Knapp, D. M. Lowder, D. Muller, S. P. Swordy, E. Torbet, S. L. Nutter

TL;DR

The paper measures the cosmic-ray positron fraction in the 1–50 GeV range using the HEAT balloon-borne instrument across two flights. It employs a magnetic spectrometer plus TRD, EC, and ToF to identify $e^{ m ±}$ and control backgrounds, with atmospheric corrections validated by depth data. The combined HEAT-94 and HEAT-95 results show that the positron fraction does not rise above ~10 GeV, though a modest excess over purely secondary production cannot be ruled out, and low-energy behavior may reflect solar modulation of charge-sign. These measurements constrain propagation and source models of cosmic rays and motivate further high-precision observations around solar cycle transitions.

Abstract

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Ft. Sumner, New Mexico in 1994, and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight from Lynn Lake, Manitoba in 1995 permitted measurements over a larger energy interval, from 1 GeV to 50 GeV. In this letter we present results on the positron fraction based on data from the Lynn Lake flight, and compare these with the previously published results from the Ft. Sumner flight. The results confirm that the positron fraction does not increase with energy above ~10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960's. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

Measurements of the Cosmic-Ray Positron Fraction From 1 to 50 GeV

TL;DR

The paper measures the cosmic-ray positron fraction in the 1–50 GeV range using the HEAT balloon-borne instrument across two flights. It employs a magnetic spectrometer plus TRD, EC, and ToF to identify and control backgrounds, with atmospheric corrections validated by depth data. The combined HEAT-94 and HEAT-95 results show that the positron fraction does not rise above ~10 GeV, though a modest excess over purely secondary production cannot be ruled out, and low-energy behavior may reflect solar modulation of charge-sign. These measurements constrain propagation and source models of cosmic rays and motivate further high-precision observations around solar cycle transitions.

Abstract

Two measurements of the cosmic-ray positron fraction as a function of energy have been made using the High Energy Antimatter Telescope (HEAT) balloon-borne instrument. The first flight took place from Ft. Sumner, New Mexico in 1994, and yielded results above the geomagnetic cutoff energy of 4.5 GeV. The second flight from Lynn Lake, Manitoba in 1995 permitted measurements over a larger energy interval, from 1 GeV to 50 GeV. In this letter we present results on the positron fraction based on data from the Lynn Lake flight, and compare these with the previously published results from the Ft. Sumner flight. The results confirm that the positron fraction does not increase with energy above ~10 GeV, although a small excess above purely secondary production cannot be ruled out. At low energies the positron fraction is slightly larger than that reported from measurements made in the 1960's. This effect could possibly be a consequence of charge dependence in the level of solar modulation.

Paper Structure

This paper contains 5 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Detector Description
  3. Flight Summaries
  4. Data analysis
  5. Results

Figures (2)

  • Figure 1: E/p distributions for $e^\pm$ selection. The dashed curve is a Monte Carlo calculation of the expected distributions. The cross-hatched region indicates the E/p interval used in the final data sample. Note that the vertical scale is logarithmic.
  • Figure 2: The positron fraction as a function of energy obtained from the two data sets. The error bars applied to the HEAT data points represent statistical errors only.