Particle interactions in matter at the terascale: The cosmic-ray experience

Cosmic-rays with energies up to 3×1020 eV have been observed, as have astrophysical neutrinos with energies above 1 PeV. In this talk, I will discuss some of the unique phenomena that occur when particles with TeV energies and above interact with matter. The emphasis will be on lepton interactions. The cross-sections for electron bremsstrahlung and photon pair conversion are suppressed at high energies, by the Landau-Pomeranchuk-Migdal (LPM) effect, lengthening electromagnetic showers. At still higher energies (above 1020 eV), photonuclear and electronuclear interactions dominate, and showers become predominantly hadronic. Muons interact much less strongly, so can travel long distances through solids before losing energy. Tau leptons behave similarly, although their short lifetime limits how far they can travel. The hadronic interaction cross-section is believed to continue to increase slowly with rising energy; measurements of cosmic-ray air showers seem to confirm this prediction.