5th step
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classifying collisions - number of neutral particles from collisions
Neutral particles do NOT leave trails of bubbles (because
bubble formation is initiated by the ionisation of the hydrogen atoms
by the Coulomb interaction). However, an unstable neutral
particle may decay in the bubble chamber into a pair of particles - one
positive and one negative - leaving an easily recognisable letter vee
shape (move cursor 
over the picture).
If the vee tracks happen to cross again, downstream, the line joining the crossing position to the decay position points back to the origin of the neutral particle.
So our neutral decay 'vee' can be seen to be coming from the collision and not from the
kink.