A cosmic ray muon creates an arrow of light in the Super-Kamiokande detector (SK) located 1000 metres under Mt Ikenoyama, Japan. A cylinder of ultra- clean water 41m tall and 39m in diameter, SK is lined with 11200 phototubes to record Cerenkov light produced when charged subatomic particles travel through water faster than light does. Here, coloured dots show tubes that have recorded light. The red end of the spectrum indicates the earliest light to arrive, blue the latest. The muon entered at the bottom of the detector, where the earliest light appears, and exited 120 nanoseconds later through the side wall near centre of the image.
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