ultra-high-energy cosmic ray (UHECR)
partowhâ-ye keyhâni-ye ultar-meh-kâruž
Fr.: rayons cosmiques de très haute énergie
A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 1020 → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.