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Classifying collisions - number of charged tracks
If any beam particle does
NOT continue parallel through the picture, it must have collided with a proton.
(Very occasionally a beam particle
may 'decay' - see glossary.)
The first quantity of interest is the number of charged particles coming
from the collision. This is clearly 4.
ASIDE: Since we are studying the collisions of 
particles with protons, 
, the total charge of the initial state is always zero.
From charge conservation - one of the most important rules of particle
physics – every
collision must produce an equal number of positive and negative particles.
(Every charged particle that leaves
a track in a bubble chamber has a charge of +/- the charge of the electron.)
Let us check charge conservation for our collision. You will see that there
are two positive tracks (curving to the left - active area A),
one negative track (curving to the right, active area B) and one straight
track that kinks suddenly, before travelling far enough to see which way it
was curving (active
area C)
Since the total final charge is zero, this kinking track must be negative.
For an excercise click here