Event Display Pictures : Electron-Positron Scattering
This process looks just like a positron bouncing (scattering) off of an electron, or in shorthand:
The initial e+ and e- enter the detector region along the beam pipe, which is perpendicular to the middle of the picture, and they are traveling "into" the plane of the picture so they are not seen. They collide in the middle of the detector region and produce the two final e+ and e-, which are scattered at large angles and can be seen in the detector.
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Event 10128_119_600_z_wab |
Event 34356_3419_600_z |
Two types of process contribute to such events.
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Processes where the initial electron and positron simply deflect one-another because of their opposite electric charges.
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Processes where they annihilate (that is, disappear) so for an immeasurably short instant a Z-particle is present, which then decays producing a new electron and positron which fly out into the detector.
There is no way to separate the contributions of these two possibilities by looking at the events, but the pattern of probability of production at various angles cannot be explained without including both in the calculation.
These charged particles leave single electron-type tracks in the vertex detector and drift chamber. They create an electromagnetic shower in the calorimeter. The direction of the two tracks is opposite. This shows the two particles have opposite charges. The tracks are back-to-back with equal magnitudes. This shows that the two particles have equal magnitude but opposite direction of momentum.
In the pictures above, the electron and positron tracks end in the inner blue section of the calorimeter and the huge amount of energy deposited there is represented pictorially by the long light blue stripes coming from the regions where the electromagnetic showers occurred.
