The first appearances of charm

10 November 1999

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The particle that writes its own name! On 10 November 1974, physicists working on the SPEAR machine at the Stanford Linear Accelerator Center (SLAC) in Caliifornia were in a state of euphoria. They realised that they had discovered a remarkable new particle, which they named after the Greek letter "psi". The following morning they discovered that a different experiment, at the Brookhaven National Laboratory in New York had also discovered the same particle, which the team at Brookhaven called J. The J/psi, as it became known, turned out to be the first example of a particle containing the charm quark - in fact, a charm quark bound with its antiquark. Before this, only three quarks were known (up, down and strange). In the image shown here, from later studies*, the Mark I detector at SPEAR reveals the decay of a heavier relation, the psi', into a J/psi plus two charged pions. The J/psi itself decays into an electron (e-) and a positron (e+), and the four charged particles together write out the sign of the Greek letter psi in the detector!

(*see W.Braunschweig et al, Physics Letters 57B 407 (1975) )

Credit: SLAC