Z particles, California style

13 May 1998

Fifteen years after the observation of the first Z particle at CERN, near Geneva, physicists at the Stanford Linear Accelerator Center (SLAC) in California are breaking records with their studies of Zs (neutral carriers of the weak force). The world's first linear electron-positron collider, the SLAC Linear Collider (SLC), has been operating there for nearly 10 years, and since 1991 the SLAC Large Detector (SLD) experiment has been recording the decays of the Z particles produced in the electron-positron annihilations. Recently, the number of Z particles collected since the current "run" began in July 1997, rose beyond the 250,000 mark, which means that SLD had recorded more Z particles in 10 months than in the previous 5 data-taking years combined. This week's image shows the decay of a Z particle in SLD. Here the Z has produced three jets of particles, flying off left, down and towards the top right. Two of the jets are from a quark and an antiquark, into which the Z has decayed. The third jet comes from a gluon, the carrier of the strong force that acts between quarks, which has been radiated by either the quarks or the antiquark. The SLC provides a unique highly-polarised electron beam, which together with other features allows SLD to make tests of the Standard Model of particle interactions that are complementary to those made in unpolarised electron-positron collisions at the LEP collider at CERN.

Credit: SLAC/SLD

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