voices: strong interaction

More than thirty years after publishing two scientific papers on the theory of the strong interaction, David Gross, David Politzer, and Frank Wilczek received the 2004 Nobel Prize in Physics. symmetry asked two physicists—a particle physics theorist and a nuclear physics experimentalist—to explain how the papers paved the way for one of the most successful theories in physics and how new experiments are testing its predictions.

logarithms are confused with belittles the achievement and other preasymptotic effects; makes it seem like a banality. at high energy, a large lever It is often said that the force arm in energy is required to between quarks is weak at observe these logarithmic short distances and strong effects. So it took some time at long distances; and on until these logarithmic effects Nobel morning, more than one were confirmed. commentator described the The work of many people analogy of a stretched rubber was required to turn the band whose restoring force Photo: Fermilab fledging theory into a reliable diminishes as the two ends calculable structure. David are brought closer together. The Advent of QCD Politzer was particularly active But in fact, asymptotic freedom I was delighted to wake up on in this stage, teaching us only provides a logarithmic the morning of October 5 and that perturbation theory con- variation on top of the QCD find that David Gross, David tained both long distance equivalent of the Coulomb Politzer and Frank Wilczek had parts and short distance parts, force. The force between been awarded the 2004 Nobel which need to be separated. quarks does not become weak Prize for the discovery of Since, at useful energy scales, at short distances; it is only asymptotic freedom in the the- the coupling was about 10 slightly weaker than it might ory of the strong interaction. times larger than the QED have been with a fixed cou- Their discovery in 1973 resur- coupling, it was quickly appre- pling. So it is more correct to rected the role of field theory ciated that radiative corrections say that the coupling gets in strong interactions and would be of great importance smaller at high energy. It is initiated the development of in the verification of this theory. the smallness of this coupling Quantum Chromodynamics In this regard, I would like to which allows us to predict (QCD). In one fell swoop, all acknowledge the contribution phenomena at the Tevatron the methods of Quantum of William Caswell, who showed and the Large Hadron Collider. Electrodynamics (QED) could that the behavior found by These predictions are the be applied in the new theory, Gross, Wilczek and Politzer true legacy of the discovery by albeit with a number of subtle persists when the calculation Gross, Politzer and Wilczek. differences. is improved by including radia- R. Keith Ellis is a theoretical physicist at However, the discovery of tive effects. Further details Fermilab. He is the co-author, with Bryan asymptotic freedom did not of the joy and frustrations of Webber of Cavendish Laboratory, mean that Quantum Chromo- QCD radiative corrections University of Cambridge, and James Stirling, University of Durham, of QCD dynamics was immediately can be found in the poignant and Collider Physics. accepted as the theory of the obituary of Caswell, written strong interaction. Richard by Frank Wilczek and Curt Physics

Feynman, in particular, was Callan, and published in symmetry | volume 01 issue 02 dec 04/jan 05 unconvinced until about 1980, Today in December 2001. believing that a correct theory One of the problems of should quickly explain and asymptotic freedom is the dif- predict many phenomena, as ficulty of providing a cogent it had with QED. The effects and simple explanation of predicted by QCD vary only the phenomenon. To say that as the logarithm of the energy it is all to do with a minus scale. At low energy the sign, which is true, somehow

4 Super Proton Synchrotron conditions are well within the during the 1980’s. predicted range for producing In the wake of these prom- the quark-gluon plasma. ising first experiments, the US Detailed studies of these phe- Department of Energy con- nomena include the observa- structed the Relativistic Heavy tion of high-energy “jets” of Ion Collider (RHIC) at particles emerging from deep Brookhaven. RHIC was within the hot dense matter designed specifically to exploit produced in the collisions.

Photo: BNL this new window into the These jets are the manifesta- strong interaction by colliding tion of very energetic head-on two beams of heavy nuclei collisions between pairs of Testing QCD with (gold ions) at energies previ- quarks, or quarks and gluons. Nuclear Collisions ously accessible only for the Their properties can be accu- The discovery that earned David highest energy elementary rately calculated using QCD Gross, David Politzer, and Frank particle collisions. At these theory, and the alterations they Wilczek the 2004 Nobel Prize high energies, the predictions experience while traversing in Physics—that the strong, of QCD come directly into play. the dense plasma provide or “color,” force binding quarks These extraordinary colli- a sensitive probe of this new together grows weaker as two sions yield a new testing state of matter. quarks become closer, and is ground for QCD, providing an Such measurements, along stronger as quarks are pulled environment in which quarks with improved understanding apart—spawned a new physics and gluons might be freed of the theory as more precise theory with a profound impact from their normal confinement data become available, are on experimental nuclear and inside protons, neutrons or providing the keys to identify- particle physics. Many aspects other strongly interacting parti- ing and understanding the of the new theory have been cles. Scientists expect the new form of matter unveiled confirmed, and new insights collisions to create a state of by the theory of QCD. They revealed, in high-energy colli- matter consisting of a simple, reveal the fundamental proper- sions of elementary particles large system of quarks and ties of the strong interaction. (electrons, protons, and anti- gluons—a “quark-gluon Thomas Ludlam is a nuclear physicist at protons) carried out using plasma.” The existence of the Brookhaven National Laboratory, working large accelerators at labora- quark-gluon plasma is a key on the STAR experiment. From 1999 tories around the world. prediction of the QCD theory. to 2001 he served as deputy associate director for High Energy and Nuclear Motivated by this new The production of this state Physics at BNL. theory, known as Quantum under controlled conditions Chromodynamics (QCD), a could open a powerful new completely new class of high- avenue for research into the energy collision experiments basic structure of the strongly has become a major part of interacting particles that com- the international effort, with prise atomic nuclei, accounting beams of heavy nuclei pro- for essentially all of the observ- duced at the highest energies able mass in our universe. available at today’s accelera- After three years of opera- tors. The collisions create new tion, the early RHIC results forms of matter at tempera- have begun to verify the QCD tures and densities character- prediction of this new state istic of the very early universe, of matter. The data show that a few millionths of a second the concentration of energy,

after the big bang. Over the or “energy density,” produced symmetry | volume 01 issue 02 dec 04/jan 05 past two decades physicists in the collisions is up to 50 have adapted existing major times greater than the density accelerators to handle beams of a normal nucleus. The tem- of large nuclei. High-energy perature at the center of these nuclear beams became avail- collisions is about one trillion able at Brookhaven Lab’s degrees Celsius, ten thousand Alternating Gradient times hotter than the center Synchrotron and CERN’s of the sun. These extreme

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