The international collaboration of the DArk Matter Particle Explorer (DAMPE) satellite, for the first time, directly observed charge-dependent spectral softenings of five primary cosmic-ray nuclei, i.e. protons, helium, carbon, oxygen, and iron. This observation confirms the charge-dependent acceleration model first proposed in the 1960s, offering a key clue to solve the longstanding mystery of Galactic cosmic-ray origin. The study has been published in Nature on April 29.

Cosmic rays (CRs) are energetic particles traveling through space at speeds close to that of light. They consist of atomic nuclei, electrons and positrons, high-energy gamma rays, and neutrinos, and are thought to originate from extreme astrophysical objects such as supernova remnants, rapidly rotating neutron stars, and accreting black holes. Although CRs has been studied for more than a century, fundamental questions—how and where they are produced, how they are accelerated, and how they propagate and interact in interstellar space—remain unresolved.

The energy spectrum of CRs, which reflects how particle numbers vary with energy, encodes rich physical information. Precise measurements of the spectra of individual CR components are essential for understanding those fundamental questions of cosmic ray physics.

DAMPE, also known as "Wukong," is designed to study high-energy CRs and indirectly probe dark matter. Since launched in late 2015, it has been operated flawlessly for over a decade, and has recorded about 18.5 billion high-energy particle events. DAMPE has an excellent energy resolution, a good particle identification capability, and a reasonably large acceptance, which makes it suitable for the study of spectral structures of CRs, particularly in the tera- to peta-electronvolt range.

Based on DAMPE's nine years of on-orbit data, the DAMPE collaboration precisely measured the spectra of the five most abundant cosmic-ray nuclei, and for the first time, directly detected distinct spectral softenings in the spectra of carbon, oxygen, and iron nuclei, by extending the measurements to peta-electronvolt energy range. Combined with the updated proton and helium spectra, the collaboration found that the spectral softening appears universally at a rigidity of about 15 teravolts and a nuclei-mass-dependent softening is rejected at a confidence level of >99.999%.

These findings combined with large-scale anisotropy measurements indicated that there exists a nearby cosmic-ray accelerator, with the universal spectral softening marking its charge-dependent energy limit. The DAMPE observation provides the first experimental verification for a 1961 hypothesis "Peters cycle," which posited that particle acceleration in magnetic field should obey a charge-dependent limit. DAMPE is expected to shed new light on the fundamental questions about cosmic ray physics.