A century after their discovery, cosmic rays—particles of extreme energy originating from the far reaches of the universe—remain a mystery to scientists. The DAMPE (Dark Matter Particle Explorer) space telescope is tackling this phenomenon, particularly investigating the role that dark matter may play in their formation. This international mission, which includes the University of Geneva (UNIGE), has made a major breakthrough by highlighting a universal feature of these particles. The results are published in the journal Nature.
Cosmic rays are the most energetic particles observed in the universe, far surpassing the energies of particles produced by man-made accelerators on Earth. Their exact origin is still under study, and it is believed that they originate from extreme astrophysical phenomena, such as supernovae, black hole jets, or pulsars.
The DAMPE space telescope, launched in December 2015, aims to provide answers regarding the origin and nature of these cosmic rays. This space mission, with the astrophysics group from the Department of Nuclear and Particle Physics (DPNC) at the University of Geneva (UNIGE) being one of its main contributors, has made a crucial breakthrough. Through the analysis of high-precision measurements collected by the telescope, scientists have identified a universal feature in the energy spectra of primary cosmic ray nuclei, ranging from protons to iron.
"Cosmic rays are primarily composed of protons, but also of helium, carbon, oxygen, and iron nuclei,’’ explains Andrii Tykhonov, associate professor at the DPNC in the Faculty of Science at UNIGE, 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."
A New Common Feature
The results show that, for all the nuclei studied, the number of particles decreases more and more rapidly beyond a certain value. This phenomenon is called "spectral softening". Normally, the number of particles already decreases as energy increases, but here, this decrease becomes even more pronounced. This occurs around a rigidity of about 15 TV (teraelectron-volts). The rigidity of a particle measures the resistance of its trajectory to a magnetic field. The observation of a common structure at this rigidity strongly supports models that explain that the acceleration and transport of cosmic rays depend on the particles' rigidity. In contrast, alternative models, which suggest that energy per nucleon (energy divided by the number of nucleons in the particle) is a key factor, are strongly ruled out by these measurements, with a confidence level of 99.999%.
The Geneva team played a central role in this scientific breakthrough. They notably developed advanced artificial intelligence techniques for reconstructing the detected events and contributed to key measurements of proton and helium fluxes, as well as to the analysis of carbon. The group also led the development of one of DAMPE's major sub-detectors, the Silicon-Tungsten Tracker (STK), an essential instrument for the precise reconstruction of particle trajectories and the measurement of their charge.
These results represent a significant step towards a more comprehensive understanding of the origin of cosmic rays and the mechanisms that govern their propagation in the galaxy. They provide new experimental constraints on acceleration models in astrophysical sources and on the transport of particles in the interstellar medium, paving the way for a more accurate description of high-energy particle populations.