A quarter of a century ago, several theorists published an unexpected result which opened the door to quantum chromodynamics (QCD), the unusual field theory that describes quarks and gluons. This year’s international QCD conference was able to look back as well as forward.
Like members of a close-knit family, quarks and gluons behave like free particles when they are very close together, but feel much stronger forces if they are separated. This result, paradoxical at first sight and embodied in the term “asymptotic freedom”, is basically why quarks and gluons cannot be isolated as free particles. They can only be studied in their native habitat, using high-energy particle beams to spy on them deep inside protons and neutrons and other strongly interacting particles.
The idea was pointed out in two landmark papers by David Gross and Frank Wilczek at Princeton and by David Politzer at Harvard, published in the June 1973 issue of Physical Review Letters. In his presentation at the 1992 history seminar at Stanford, now published in The Rise of the Standard Model (edited by Lillian Hoddeson, Laurie Brown, Michael Riordan and Max Dresden, Cambridge University Press, 1997), David Gross described this breakthrough: “For me, the discovery of asymptotic freedom was totally unexpected. Like an atheist who has just received a message from a burning bush, I became an immediate true believer.”
But the idea had been noticed elsewhere. In his book In Search of the Ultimate Building Blocks (Cambridge University Press, 1997), Gerard ‘t Hooft relates: “In 1972 a small conference took place in Marseille. On arriving at Marseille airport, I discovered that (prominent field theorist) Kurt Symanzik and I had shared the same plane… He had been trying to understand Bjorken scaling [the behaviour seen in high-energy scattering when the incoming projectile particle transfers a lot of momentum to the target Ed.] in a quantum field theory, but had limited himself to what he considered to be the prototype of all field theories, a simple spin zero model. Unfortunately it had the wrong scaling behaviour.
” ‘If only I could turn this scaling behaviour round, ‘ Symanzik said,’then you would get a theory where particles at close distance behave almost as free particles, but when they separate to larger distances they would feel much stronger forces.’
” ‘Well, ‘ I [‘t Hooft] cried,’that is exactly what you get in a Yang-Mills (spin one) gauge theory!’
“Symanzik replied: ‘You should publish this quickly, because this would be very important.’ ”
“Much to my later regret, I did not follow this sensible advice, ” says ‘t Hooft, whose 1971 work on the renormalizability of Yang-Mills theories had underlined the importance of gauge fields for understanding particle behaviour. However,’t Hooft did air his idea for spin one fields following Symanzik’s talk at the Marseille meeting.
This idea was the key to quantum chromodynamics (QCD), the field theory of quarks and gluons. A host of theoreticians contributed to the subsequent development of the theory, many of whom, including Gross (now at Santa Barbara) and ‘t Hooft (Utrecht), attended the 6th international QCD conference (QCD 98) held in Montpellier from 28 July. The QCD series, now an established event in the particle physics calendar, is run by Stephan Narison of Montpellier.