In this unusual picture, the highlighted tracks show a positron e+ colliding head-on with a more-or-less stationary atomic electron and transfering all its momentum to that electron.
This can only happen if the positron has the same mass as the electron. (Imagine snooker balls running head-on onto each other. What happens if one ball is made of polystyrene and the other of lead?
For other momentum conservation examples click on the pictures below.