Sigma+ produced in a high energy numu interaction

This is an exceptional event, produced by a muon neutrino with an energy of about 50 GeV. The very straight outgoing track was identified as a because it escaped from the bubble chamber and was picked up in an electronic detector called a multi-wire-proportional chamber. Any charged particle other than a muon would have been absorbed by the material in the way.

The remarkable feature of this picture is that a visual analysis enables a very large number of particles to be identified - so this is for connoisseurs! Here we draw attention to a part of the whole event – the production and decay of a , which is followed by a sequence of further decays and interactions. Photos of the event, which was taken in the Big European Bubble Chamber (BEBC) at CERN filled with a mixture of neon and hydrogen, were taken on four views. The decay story has been highlighted on each view.

Click here for another view.

1st view

2nd view

3rd view

The event looks quite different on different views because of the optics - the cameras are more separated than they are in the CERN 2m chamber and they have fish-eye lenses.

On view one we have highlighted a 'Compton' electron which shows that negative particles turn to the right.

We see:

• the track kinks to the right; the outgoing charged track is a , and it is accompanied by a neutron which does not leave a track;
• the travels a short distance, stops, and decays to a short track;
• this moves a very short distance to the right before stopping and decaying into a spiralling positron .

While this sequence of decays was taking place, the neutron from the decay of the was carrying most of the sigma's momentum forward – the neutron mass is about 85% that of the sigma mass.. This neutron then collided with a proton, giving it enough momentum to leave a short dark stopping track. This collision also yielded a which decayed into two photons , both of which produced pairs. The full reaction, with details of the decay sequence is

For details of the click here.

For a full discussion of this event, including a discussion of the other unseen neutral particles, measurements, and calculation of masses, click here.