The parallel lines are produced by K- particles entering the CERN 2m hydrogen bubble chamber from the right with an energy of 4.2 GeV.
The knock-on electron on the upmost track shows that negative particles turn to the right.
There is an interaction on one of the beam tracks that produces a 4-particle
final state with two positive tracks and two negative tracks.
The dark positive track moving down and to the right is a typical proton.
(For more details on why protons are dark, click
here.)
Close inspection, however, reveals an early kink on this track. This suggests
that we have an example of a sigma+ decaying to p pi0. (Click
here for
more details.) The two photons from the decay of the pi0 have escaped.