The 2004 Nobel Prize for Physics has been awarded to David Gross, David Politzer and Frank Wilczek for their “discovery of asymptotic freedom in the theory of the strong interaction”. This honour comes a year after they received the High Energy and Particle Physics Prize of the European Physical Society – and just over 30 years since they made the remarkable proposal that the interaction strength between quarks becomes weaker as they come closer together.
Politzer and Wilczek were both still graduate students in June 1973 – Wilczek working with Gross – when their work appeared in two consecutive papers in Physical Review Letters, in fact the last two papers of volume 30. The key factor they discovered was that the beta function, which describes how the coupling constant of an interaction changes with energy, can be negative, contrary to what was generally believed. This means that the interaction strength can decrease with increasing energy, making quarks “asymptotically free” at high energies.
Earlier, in 1970, Kurt Symanzik had shown that only a theory with a negative beta function could lead to the effect of “scaling” derived by James Bjorken (where the probability for the interaction depends on a dimensionless variable), which had been observed in electron-proton interactions at SLAC. However, this was unknown in any other theory. In quantum electrodynamics, the quantum-field theory par excellence, the beta function is positive – charges become free from each other’s grasp as they are separated and the force between them becomes smaller. Could a quantum-field theory for the strong force, with a negative beta function, be found? The question became particularly pressing after Gerard ‘t Hooft’s work in 1971 that overcame problems in the gauge-field theory for the unified electroweak theory.
Indeed, in 1972 in a discussion with Symanzik at a conference in Marseille, ‘t Hooft himself realized what kind of theory could have a negative beta function. Although he did mention it at the conference, he did not follow it up (CERN Courier November 1998 p31). It fell to others to pursue the problem, including Gross’s group at Princeton and Sidney Coleman’s at Harvard, in which Politzer worked. The rest, as they say, is history, as the discovery of Gross, Politzer and Wilczek in a sense not only liberated quarks deep within the proton but also liberated theorists to develop a quantum-field theory of the strong interaction. In particular, it focused attention on the development of quantum chromodynamics (QCD), in which the strong interaction is mediated by massless spin-1 particles, the gluons.
The decrease of the strong coupling constant with energy has since been dramatically confirmed with high precision, most recently at DESY’s electron-proton collider, HERA, and in studies at the mass scale of the Z boson at CERN’s Large Electron Positron (LEP) collider (CERN Courier May 2004 p21). It is fitting that following this clear support for their original ideas, Gross, Politzer and Wilczek have now been rewarded with the Nobel prize.