Observation of an anomalous positron abundance in the cosmic radiation

TL;DR

This study reports PAMELA's measurement of the cosmic-ray positron fraction from 1.5 to 100 GeV, combining a magnetic spectrometer with an imaging calorimeter to identify e+ and e− with high precision while controlling backgrounds. The results show a low-energy solar-modulation effect that depends on charge sign and a pronounced high-energy rise in the positron fraction that cannot be explained solely by secondary production, hinting at primary sources such as dark matter annihilation or nearby pulsars. The findings challenge standard models, motivate deeper solar-modulation modeling, and motivate ongoing data collection to extend the energy reach and investigate potential anisotropies. Overall, the work provides significant constraints on cosmic-ray propagation and potential new physics in the Galactic environment.

Abstract

Positrons are known to be produced in interactions between cosmic-ray nuclei and interstellar matter ("secondary production"). Positrons may, however, also be created by dark matter particle annihilations in the galactic halo or in the magnetospheres of near-by pulsars. The nature of dark matter is one of the most prominent open questions in science today. An observation of positrons from pulsars would open a new observation window on these sources. Here we present results from the PAMELA satellite experiment on the positron abundance in the cosmic radiation for the energy range 1.5 - 100 GeV. Our high energy data deviate significantly from predictions of secondary production models, and may constitute the first indirect evidence of dark matter particle annihilations, or the first observation of positron production from near-by pulsars. We also present evidence that solar activity significantly affects the abundance of positrons at low energies.

Figures (5)

• Figure 1: Calorimeter energy fraction $\cal{F}$. The fraction of calorimeter energy deposited inside a cylinder of radius 0.3 Molière radii, as a function of deflection. The axis of the cylinder is defined by extrapolating the particle track reconstructed by the spectrometer.
• Figure 2: Calorimeter energy fraction $\cal{F}$: 28--42 GV. Panel a shows the distribution of the energy fraction for negatively charged particles, selected as electrons in the upper part of the calorimeter. Panel b shows the same distribution for positively charged particles selected as protons in the bottom part of the calorimeter. Panel c shows positively charged particles, selected in the upper part of the calorimeter, i.e. protons and positrons.
• Figure 3: PAMELA positron fraction with other experimental data. The positron fraction measured by the PAMELA experiment compared with other recent experimental datamul87gol94bar97boe00alc00bea04gas06cle06. One standard deviation error bars are shown. If not visible, they lie inside the data points.
• Figure 4: PAMELA positron fraction with theoretical models. The PAMELA positron fraction compared with theoretical model. The solid line shows a calculation by Moskalenko & Strongmos98 for pure secondary production of positrons during the propagation of cosmic-rays in the galaxy. One standard deviation error bars are shown. If not visible, they lie inside the data points.
• Figure 5: Positron Event display. A 68 GeV positively-charged particle selected as positron. The bending (x) view is shown. The signals as detected by PAMELA detectors are shown along with the particle trajectory (solid line) reconstructed by the fitting procedure of the tracking system. The calorimeter shows the typical signature of an electromagnetic shower (plane 19 of the calorimeter x-view was malfunctioning).