Dan Hooper

Gamma rays with energies above one hundred trillion electronvolts are like stones thrown with incredible force. For a long time, it was believed that only protons, heavy particles accelerated in cosmic accelerators, could launch such 'stones.' But new work shows that sometimes light electrons are enough.

Buried in Antarctic ice, there's a detector the size of a cubic kilometer. It doesn't catch light but ghostly particles — neutrinos. Yet sometimes, a different beast might flash through: a pair of muons born together and flying apart in a wide fan. No known particle of ordinary matter can paint that pattern.