Charm

In August 1964, a paper was published by James Bjorken (1956-) and Sheldon Lee Glashow (1932-), two American theorists working at the University of Copenhagen. It suggested the existence of a wider symmetry named SU(4), of which SU(3) was a sub-group. Despite the fact that it pointed towards a basic unification of weak and electromagnetic interactions, it attracted little attention at the time. This new symmetry implied that there should be a fourth quark. The authors even suggested the name 'charmed' for the new quark, to quote their own words, 'as an expression of pleasure at a delightful idea'. They were, however, careful to point out that 'this model is vulnerable to rapid destruction by experimentalists'.

The rapid destruction of charm did not take place; far from it. Ten years later, there was a dramatic development when, on 11 November 1974, two experimental groups, one under

Samuel Ting (1936-) at Brookhaven and the other led by Burton Richter (1931-) at Stanford, working on completely different experiments, simultaneously announced the observation of a new resonant state, which decayed approximately 10-19 s after creation. It had a mass more than three times the mass of the proton. The discovery of a new resonance in itself was nothing special, but in this case there was something very anomalous in the way it behaved. Resonant states decay in typically 10-22 s, but this particle lived about 1000 times longer than expected. Richter's paper concluded with the statement: 'It is difficult to understand how, without involving new quantum numbers or selection rules, a resonance which decays to hadrons could be so narrow.' In plain English: 'We have discovered some kind of matter which we have never seen before.' In 1976, the Nobel Prize was awarded jointly to Richter and Ting 'for their pioneering work in the discovery of a heavy elementary particle of a new kind'.

Ting called the new particle J, while Richter chose the Greek letter y. The physics community diplomatically adopted the name J/y. More important than the name of the particle was its structure, which seemed most likely to be a combination of a charmed quark and a charmed antiquark.

Just like their charge, the 'charm' of quark and anti-quark 'cancels', so J/y is said to possess hidden charm'.

To make the existence of the charmed quark credible, it was necessary to find particles containing one charmed quark with one or more of the known quarks u, d and s. Throughout 1975 and 1976, experiments at accelerator laboratories on both sides of the Atlantic searched for what was called 'naked' charm. As thousands of bubble chamber pictures and billions of electronic counts were analysed at Stanford, CERN, Brookhaven and Fermilab, more and more evidence of charmed particles began to accumulate. This evidence, although convincing, was based on the characteristics of decay products and was of necessity indirect. The lifetime of charmed particles was expected to be about 10-12 s, which meant that even at the speed of light they would travel no further than a fraction of a millimetre before decay. This distance was far too short to resolve the points of creation and decay using the above techniques.

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