Rare Events with W and Z Particles

When the LEP experiments had finished their studies of Z particles, the accelerator's energy was raised to produce W particles. In electron-positron collisions, these have to be produced in pairs since they are charged and the law of charge conservation demands that the total charge before the collision is equal to the total charge after. The energy needed to produce a pair of W particles is about 161 GeV.

The event samples below are representative of several years of data taking with the DELPHI detector at LEP at different energies. Have a look at some of them and see if you can see the difference between the higher energy events and those at the Z production threshold of 91 GeV.