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Pierre Auger Observatory

More muons than predicted by the LHC

Miguel Boyayan Pierre Auger Observatory: up to 60% more muons detected in cosmic-ray showerMiguel Boyayan

Ultrahigh energy cosmic-ray showers apparently create 60% more muons than predicted by models based on data from the Large Hadron Collider (LHC), the main particle accelerator at CERN (European Organization for Nuclear Research). This discrepancy was detected in records obtained by physicists at the Pierre Auger Observatory, in the Argentinean Andes, as part of an international initiative in which Brazil takes part (Physical Review Letters, October 31, 2016). Muons are elementary particles similar to electrons, but their mass is around 200 times greater. When cosmic rays – composed essentially of protons and light nuclei – collide with molecules in the Earth’s atmosphere, a series of interactions generates a cascade of secondary particles. Muons, the vast majority of which reach the ground, are one such type of particle. Using an array of 1,660 water-filled Cherenkov tanks that span 3,000 square kilometers, as well as four sets of telescopes, the Pierre Auger Observatory detects the muons that fall in the Andes. Atmospheric collisions of cosmic rays produce muons with 10 times more energy than those produced at the LHC. These findings suggest that science has only a partial understanding of high-energy elementary particle interactions. This is not the first time that the number of muons measured on the ground fails to match predictions by particle-physics models. In 2000, an experiment with the HiRes-MIA array detected a larger number of muons in cosmic-ray showers than predicted by the theoretical models of that day.