Discrepant hardening observed in cosmic-ray elemental spectra

TL;DR

This study reports the observation of spectral hardening in cosmic-ray elemental spectra above 200 GeV/nucleon, challenging the simple power-law model and suggesting complex origins or interactions.

Contribution

First direct measurements showing spectral hardening and differences between protons and helium nuclei in cosmic rays at high energies.

Findings

Spectral hardening observed above ~200 GeV/nucleon.

Differences found between proton and helium spectra.

Results challenge the simple power-law model of cosmic-ray spectra.

Abstract

The balloon-borne Cosmic Ray Energetics And Mass (CREAM) experiment launched five times from Antarctica has achieved a cumulative flight duration of about 156 days above 99.5% of the atmosphere. The instrument is configured with complementary and redundant particle detectors designed to extend direct measurements of cosmic-ray composition to the highest energies practical with balloon flights. All elements from protons to iron nuclei are separated with excellent charge resolution. Here we report results from the first two flights of ~70 days, which indicate hardening of the elemental spectra above ~200 GeV/nucleon and a spectral difference between the two most abundant species, protons and helium nuclei. These results challenge the view that cosmic-ray spectra are simple power laws below the so-called knee at ~1015 eV. This discrepant hardening may result from a relatively nearby source, or it could represent spectral concavity caused by interactions of cosmic rays with the accelerating shock. Other possible explanations should also be investigated.