Photograph of a neutrino interaction event in the 15- foot bubble chamber at the Fermi National Accelerator Laboratory (Fermilab) near Chicago. The neutrino enters the bubble chamber at far right, but does not leave a track itself because it is electrically neutral. It interacts almost immediately with a proton in the liquid filling the bubble chamber, producing a spray of particles that travel towards the left, curving in the chamber's magnetic field. The V- shaped pairs of tracks in the lower half of the picture, which end in characteristic spiral curlicues, are electron- positron pairs produced from gamma-rays.