3rd step - classifying collisions - number of charged tracks from collision

If any beam particle does NOT continue parallel throughout the picture, it must have collided with a proton. (Very occasionally a beam particle may "decay" )

The first quantity of interest is the number of charged particles coming from the collision. Since we are studying K- collisions with the protons the total charge before the collision is zero.

From charge conservation - one of the most important rules of particle physics - there must therefore always be an equal number of positive and negative particles after collision. (The possibility of neutral particles being produced will be discussed in the next section.)

Check charge conservation for our collision. You will see that there are two positive tracks (curving to the left), one negative track (curving to the right) and one straight track that kinks suddenly (move cursor above the picture), before travelling far enough to see which way it was curving. Since the total final charge is zero, this kinking track must be positive.