Klein is an experimental physicist whose research includes the detection of astrophysical neutrinos – ghostly particles that originate outside our solar system – using large detector arrays. He has held several leadership roles in the IceCube Neutrino Observatory experiment, which uses an array of more than 5,400 spherical detector modules embedded in South Pole ice to capture light signals related to neutrino interactions. On IceCube, he leads efforts to use naturally produced, high-energy neutrinos to study the properties of neutrinos’ interactions at energies far beyond those available at human-made accelerators, and he led the effort to make the first measurement of neutrino absorption in the Earth.