Twenty five years of tau particles

  29 DECEMBER 2000

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On 1 December 1975, a *paper published in Physical Review Letters described the first evidence for a new heavy relation for the electron. Martin Perl and colleagues working at the Stanford Linear Accelerator Center (SLAC) had discovered the tau, a particle that behaves like the electron but which is about 3600 times heavier. The tau, the electron and the muon (which is about 210 times as heavy as the electron) together belong to the family of particles known as leptons. Perl was rewarded for his discovery with the Nobel prize in 1995.

This image shows the decay of a tau particle and a tau antiparticle produced about 20 years later at SLAC, in the SLAC Large Detector (SLD). The tau and antitau - a "tau pair" - result from the annihilation of an electron and positron (antielectron) at the centre of the detector. Both members of the tau pair decay in less than 10-13 seconds. One has produced three charged pions that shoot up towards the top right; it will also have produced a tau neutrino (the tau's uncharged relative) which remains unseen. The other member of the tau pair has produced an electron which travels in the opposite direction to the bottom left, accompanied by two unseen neutrinos (one of the tau variety and one of the electron type).

*(See Physical Review Letters 35 1489 (1975)

Credit: SLAC/SLD