Bjorken and Callan are being honoured for their theoretical investigations in the 1960s and 1970s, which led to the use of deep inelastic scattering for shedding light on the nature of strong interactions.
Bjorken was the first to realize the importance of deep inelastic scattering and the first to understand the scaling of cross sections, an insight that ultimately bore his name – the Bjorken scaling of cross sections.
Callan, together with Kurt Symanzik (now deceased), reinvented the perturbative renormalization group (in a form that now bears the name Callan-Symanzik equations) and recognized these groups as measures of scale invariance anomalies. Callan has applied these techniques to analyses of deep inelastic scattering and has made substantial contributions to particle physics and, more recently, string theory.
The annual award, sponsored by the Abdus Salam International Centre for Theoretical Physics (ICTP) in Trieste, Italy, is given to scientists who have made significant contributions to theoretical physics and mathematics. Each winner receives a cash award of US$5000 and will present a Dirac Medal lecture at ICTP at a later date. Recipients of the Nobel Prize, Fields Medal or Wolf Foundation Prize are not eligible.
The announcement of the Dirac Medal is made 8 August, the birth date of the great 20th century physicist P.A.M. Dirac, who won the Nobel Prize in 1933. Dirac was a close associate and friend of the ICTP from the Centre's launch in the early 1960s until his death in 1984.
For additional information about the Dirac Medal and list of previous winners, see www.ictp.trieste.it/~sci_info/awards/Dirac/DiracMedal.html.