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High-energy neutrinos
Cosmic accelerators make charged pions when their protons hit gas or light; the pions decay to neutrinos, which cross the universe without bending or being absorbed. One in a very large number of them stops in the Antarctic ice: it hits a nucleus, and the charged particles it makes outrun light in the ice and glow (Cherenkov light). A muon draws a long straight track; an electron or the debris of the struck nucleus makes a nearly point-like shower. 5160 light sensors, hung on 86 strings between 1450 and 2450 m deep, watch a cubic kilometre of ice. Up-going neutrinos are the clean ones, because the Earth stops the muons that cosmic rays make in the air; but from about ten TeV the Earth starts to absorb the neutrinos too, first along the diameter, and at 6.3 PeV an antineutrino can make a real W boson on an electron.