Bubble chamber picture of the decay of a K-

This picture was taken in the CERN 2-metre hydrogen bubble chamber, filled with hydrogen and exposed to a beam with an energy of 4.2 GeV/c

The highlighted tracks show a kink (see glossary) that characterises the decay of charged particles. Since the first track is itself the product of another decay, we know that it must be a or a , depending on whether its parent particle was an or a . Our track kinks quite considerably, telling us that it is a . (The mass of a mu is so close to that of a pi that it could not provide the energy to produce such a sharp kink.)

For full details of this rich event, click here.

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