Qualitative exercise on momentum conservation

Picture from CERN 2-metre hydrogen bubble chamber exposed to a beam of negative kaons () with energy 4.2 GeV.This piece corresponds to about 80cm in the bubble chamber.

Knock-on spiralling electrons show that negative particles curve to the right. One beam particle interacts producing two outgoing tracks, one positive and one negative.

Educationally, this event is of interest because it provides a way of discussing momentum conservation in a qualitative way. Both outgoing tracks have low momentum compared with that of the beam (they are much more curved), also they both go to the right of the beam. So, to balance momentum, we need one or more neutral particles that together have

• some momentum to the left of the beam to balance the two low momentum tracks to the right
• considerable momentum in the beam direction to conserve the “forward” momentum.

From a particle physics point of view we cannot say very much. The outgoing tracks are not particularly dark and are probably pions. So, a tentative interpretation would be

+ neutrals

Possible questions:

1. How many beam tracks enter the bubble chamber?
2. How many tracks that are not original beam tracks enter the bubble chamber?
3. Try to identify some electrons. What is the direction of the magnetic field?
4. How many beam particles interact with protons?
5. Look at the collision at the centre of the picture.
How many charged particles are there in the final state of the collision?
6. What are their charges?
7. Look at the momenta of these two particles. What conclusions can you draw?
8. What can you tell about neutral particles here? Can you estimate the direction in which the neutral particle is travelling (assume there is only one neutral particle)?

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