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"Streamer chamber photo of a pion-muon-electron (pi-mu-e) decay chain resulting from an antiproton annihilation. The antiproton enters from the bottom of the picture & annihilates with a proton in neon gas filling the chamber; their energy materialises as 3 nuclear fragments (thick tracks) & several pions (less dense tracks). One positive pion curls tightly to the right in the chamber's magnetic field before decaying to a muon; the muon forms the spiral at top before decaying to a positron (or anti-electron). At each decay the tracks change direction sharply, indicating emission of an unseen neutrino. Photo taken at Experiment PS 179 at CERN, Geneva." (KEYSTONE/SCIENCE PHOTO LIBRARY/PROF. G. PIRAGINO)