BROOKHAVEN NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY BROOKHAVEN NATIONAL LABORATORY / SCIENCE PHOTO LIBRARY
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Bubble chamber photo of sigma particle decay. A neutrino enters unseen at bottom & collides with a proton in the bubble chamber liquid, producing the sigma & a negative muon (track 4). The sigma lives too briefly to be seen, decaying immediately to 1 negative & 3 positive pions (tracks 1, 2, 3 & 5) & a neutral lambda which leaves no track but betrays itself when it decays to a proton (6) & a negative pion (7). Negative particles curl anticlockwise, positive ones clockwise in the magnetic field. One positive pion (2) is of lower energy & forms the loop; the muon & one of the pions knock out atomic electrons which produce the two spiral tracks.
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