Cosmic-Ray Positrons: Are There Primary Sources?

TL;DR

Addressing whether primary positron sources exist beyond secondary production, the paper analyzes HEAT measurements of the positron fraction in the 1–50 GeV range against secondary-production models and several primary-source scenarios. It evaluates WIMP-annihilation (KT and MSSM-based BE) and other astrophysical mechanisms, providing best-fit amplitudes and confidence levels for each (e.g., $m_{\tilde{\chi}} = 380$ GeV/$c^2$ with amplitude $1.8$; CL $74\%$; $k=0.15$ for pulsar-related production; $p^* \sim 10$ GeV/$c$ in giant molecular clouds with amplitude $0.097 \pm 0.029$; CL $80\%$). Some models improve fits relative to pure secondaries, but no single scenario is decisively favored; distinguishing among exotic and astrophysical origins requires higher-statistics data and measurements beyond $50$ GeV. The work highlights potential connections to dark matter and local accelerators as explanations for the observed spectral feature in the positron fraction.

Abstract

Cosmic rays at the Earth include a secondary component originating in collisions of primary particles with the diffuse interstellar gas. The secondary cosmic rays are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.

Figures (2)

• Figure 1: a Compilation of measurements [buf:posfan:posagr:pos -bar:apjl] of the positron fraction between 0.05 and 50 GeV. The solid curve is a model calculation [pro:pos] assuming that all positrons are from secondary sources, and propagate according to a simple Galactic leaky-box model. "HEAT-combined" refers to the combination [bar:apjl] of the data sets from the two HEAT flights. b The positron fraction measured with the HEAT instrument, shown on a vertical linear scale. The solid curve is a leaky-box secondary model prediction [pro:pos], surrounded by an estimated band of uncertainty shown as the cross-hatching. The dashed curve is a secondary model prediction using Galactic diffusion [str:pos].
• Figure 2: a The HEAT positron fraction compared with best-fit model predictions with an additional positron component arising from annihilating dark matter neutralinos. The dashed curve is the baseline solar-modulated leaky-box secondary-production prediction [cle:pos], renormalized by a factor of 0.85. The solid curve shows an increased positron content due to annihilating 380 GeV/c$^2$ neutralinos in the model of Kamionkowski and Turner [tur:ind]. The dotted and dot-dash curves show an increased positron content due to annihilating 336 or 130 GeV/c$^2$ neutralinos, respectively, in the model of Baltz and Edsjö [bal:ind]. b The HEAT positron fraction compared with best-fit model predictions from astrophysical sources of positrons that are in addition to secondary production mechanisms. The dashed curve is the positron enhancement resulting from high-energy $\gamma$ rays converting to e$^+$e$^-$ pairs near the magnetic poles of pulsars [har:pos]. The dotted curve represents a positron enhancement due to high-energy $\gamma$ rays interacting with low-energy optical or UV photon fields [aha:pos]. The solid curve shows the enhancement from cosmic-ray interactions within giant molecular clouds [dog:pos].