Bubble chamber picture demonstrating existence of antineutron

The dark lines in this picture are produced by charged particles as they force their way through the bubble chamber liquid. At the arrowed point, an antiproton beam particle that has entered from the top of the picture and annihilated with a proton to produce a neutron and an antineutron .The continues (see dotted line) and itself annihilates with a proton producing a 'star’ of 5 tracks with a total energy consistent with the rest mass of an and a proton being converted into energy.

For a full description of this event, click here.