This picture was taken in the CERN 2-metre hydrogen bubble chamber expose to a beam of negative kaons with an energy of 4.2 GeV, entering from the bottom These beam particles produce parallel trails of bubbles.
Due to the kaon having interacted with a proton in the hydrogen, the rightmost beam track produces a spray of 4 tracks. The longer highlighted track is clearly dark – it has produced a higher number of bubbles per centimetre than, say, the beam tracks; this tells us that it is moving more slowly. (For details, click here.) Such tracks are a common feature in bubble chamber pictures and usually signify protons.
Notice that there is another very short highlighted proton track on one of the outgoing tracks. This tells us that the particle concerned gave a proton a very gentle impulse as it passed by.
For more details of this picture, click here.
For another example of proton click on the picture below.