Two Z particles in the OPAL detector

19 November 1997

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Last week's Picture of the Week showed the decay of a pair of Z particles at CERN, the European Laboratory for Particle Physics. Z particles are carriers of the weak force, one of nature's fundamental forces, and are as heavy as nearly 100 protons. This means that a Z particle has many ways to decay to lighter particles. Here the OPAL detector at CERN has captured a different decay to two Z particles, during the recent high-energy running at CERN's Large Electron Positron Collider (LEP). As in last week's picture, one Z has decayed to an electron and its antiparticle (a positron). They leave tracks in the central part of the detector (at roughly 2 o'clock and 8 o'clock respectively), and then stop in the next layer, where they deposit all their energy (shown as the yellow blocks). The other Z has decayed to a heavier relative of the electron called the tau, together with its antiparticle, the anti-tau. These particles are themselves very short-lived and decay before entering the central tracking chamber. One of them decays to a muon (11 o'clock) and two neutrinos (unseen), the other to particles including a pion which deposits lots of energy shown as the big pink block (5 o'clock).

(The LEP collider and its experiments were shut down to make way for a new machine, the LHC, in November 2000.)

Credit: CERN / OPAL

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