The dark lines in this picture are produced by charged particles as they
force their way through the bubble chamber liquid. The highlighted line shows
of a set of more or less parallel antiproton beam particles that have entered
from above. At the arrowed point it meets a proton and
annihilates into two neutral particles – a neutron and
an antineutron .
The antineutron travel forward a bit before annihilating and producing a characteristic 'star’.
For a full description of this event, click here.