Qualitative exercise on the 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 50 cm in the bubble chamber.

The spectacular knock-on spiralling electron shows that negative particles curve to the right. The collision between the beam and the target proton produces two outgoing charged 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); one goes to the right and the other to the left of the beam. So, to balance momentum, we need one or more neutral particles whose combined momentum is nearly that of the beam.

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