DAMPE, the Dark Matter Particle Explorer, was launched in December 2015 and has been accumulating high-precision measurements of cosmic ray particle fluxes from orbit. The international mission includes a major contribution from the astrophysics group at the Department of Nuclear and Particle Physics at the University of Geneva (UNIGE), which helped develop one of the instrument's key sub-detectors and led parts of the data analysis.
Cosmic rays are the most energetic particles observed in the universe, reaching energies far beyond anything achievable in ground-based accelerators. Despite more than a century since their discovery, their precise origin remains unresolved. Leading candidates include supernovae, black hole jets, and pulsars. DAMPE was designed in part to probe those sources and to investigate whether dark matter plays any role in their production.
The new results center on what researchers call spectral softening -- a steepening in the rate at which particle counts fall off as energy increases. The team found that this softening occurs at the same magnetic rigidity for all the nuclei examined, at approximately 15 TV (teravolts). Rigidity measures the resistance of a charged particle's path to deflection by a magnetic field and is a fundamental quantity in describing how particles move through the galaxy.
The consistency of the softening threshold across different nuclear species is the critical finding. It strongly supports models in which the acceleration and propagation of cosmic rays are governed by rigidity rather than by energy per nucleon -- the energy divided by the number of protons and neutrons in each nucleus. Alternative models built around energy-per-nucleon as the governing variable are ruled out by the data at a confidence level of 99.999 percent.
"Cosmic rays are primarily composed of protons, but also of helium, carbon, oxygen, and iron nuclei," said Andrii Tykhonov, associate professor at the DPNC and co-author of the study. "These particles are also categorised according to their energy: low, up to a few billion electron-volts; intermediate, from a few billion to several hundred billion electron-volts; and high, from 1,000 billion electron-volts and beyond."
The Geneva team contributed advanced artificial intelligence techniques for reconstructing detected particle events and played a central role in measuring proton and helium fluxes and analysing carbon spectra. The group also led development of DAMPE's Silicon-Tungsten Tracker (STK), which handles precise reconstruction of particle trajectories and charge measurement.
The results place new experimental constraints on acceleration models in astrophysical sources and on how particles propagate through the interstellar medium, offering a sharper framework for modelling the high-energy particle populations that fill the galaxy.
Research Report: Charge-dependent spectral softenings of primary cosmic rays below the knee
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