Cosmic Ray Helium Energy Spectrum from Dark Matter Mission

Galactic cosmic rays (GCRs) are energetic particles traveling across the Galaxy as high-energy beams, and are a unique probe to explore the astrophysical particle accelerators and the interstellar medium of the Galaxy. The energy spectrum of GCRs is expected to be a power-law form for energies below the “knee” (at 3 − 4 PeV) according to the canonical shock acceleration of particles. However, several experiments surprisingly observed changes in the powerlaw spectral indices γ for protons, helium and heavy nuclei. Specifically, the spectra of GCRs become harder by ∆γ ‘ 0.1−0.2 at kinetic energies (or rigidities) of several hundred GeV/n (or GV), and become softer again by ∆γ ‘ −0.3 at energies of 15 − 30 TeV (for protons and possibly helium). The deviations from single power-law of the spectra motivate extensive investigations for deeper understanding of the acceleration and propagation mechanisms or of new possible GCR sources.

Precise measurements of the GCR spectra, particularly for individual species, are mainly from magnetic spectrometers such as the Payload for Antimatter-Matter Exploration and Light-nuclei Astrophysics (PAMELA) and Alpha Magnetic Spectrometer (AMS-02) whose maximum measurable rigidity can reach only few TV. Direct measurements at higher energies were mostly done with balloon-borne calorimeter experiments in the past decades, and the uncertainties (both statistical and systematic) are somewhat large, hindering a good understanding of the spectral features above TeV energies.

The DArk Matter Particle Explorer (DAMPE) is a satellite-borne particle and γ-ray detector launched on December 17, 2015. It consists of a Plastic Scintillator Detector (PSD) for charge measurement, a Silicon Tungsten tracKer-converter (STK) for trajectory measurement, a Bi3Ge4O12 electromagnetic calorimeter (BGO) for energy measurement and electron-hadron discrimination, and a NeUtron Detector (NUD) for additional electron-hadron discrimination. DAMPE is expected to significantly improve the measurement precision of GCR spectra up to 100 TeV energies, due to its large acceptance and a good energy resolution.

The DAMPE data confirm the hardening feature of the helium spectrum reported by previous experiments. The hardening is smooth with a hardening energy of ∼1.3 TeV. The DAMPE data further reveals a softening feature at ∼34 TeV with a high significance of 4.3σ. Combined with the proton spectrum, the softening energy is well consistent with a dependence on particle charge, although a dependence on particle mass can not be ruled out yet. These results will provide important implications in understanding GCR acceleration or propagation processes. Extending the DAMPE measurements to even higher energies is possible with new data and improved analysis performance.

The measurement of the energy spectrum of cosmic ray helium nuclei from 70 GeV to 80 TeV using 4.5 years of data recorded by the DArk Matter Particle Explorer (DAMPE) is reported in this work. A hardening of the spectrum is observed at an energy of about 1.3 TeV, similar to previous observations. In addition, a spectral softening at about 34 TeV is revealed for the first time with large statistics and well controlled systematic uncertainties, with an overall significance of 4.3σ. The DAMPE spectral measurements of both cosmic protons and helium nuclei suggest a particle charge dependent softening energy, although with current uncertainties a dependence on the number of nucleons cannot be ruled out.

Arxiv – Measurement of the cosmic ray helium energy spectrum from 70 GeV to 80 TeV with the DAMPE
space mission