Dirac Medallists 2004

James D. Bjorken, Stanford Linear Accelerator Center, Menlo Park, USA and
  
Curtis G. Callan, Princeton University, Princeton, USA

Their work was instrumental in the theoretical developments of the late 60's and early 70's that led to the use of deep inelastic scattering for the elucidation of the nature of the strong interactions.

Bjorken was the first to realize the importance of deep inelastic scattering and understood the ("Bjorken") scaling of the cross sections. He made crucial contributions to the phenomenology of these and other experiments.

Callan (along with Symansik, who is unfortunately now deceased) reinvented the perturbative renormalization group (the "Callan-Symansik" equations) and understood them as measures of scale invariance anomalies. He applied these techniques to the analysis of deep inelastic scattering and made substantial contributions to particle physics including, more recently, to string theory.