Scientific Achievements since the 1989 Long Range Plan

In 1989, only limited information was available on the quark distributions in complex nuclei. Deep inelastic scattering results were not in very good agreement and there was an important result from Drell-Yan studies (Fermilab E772) indicating that the sea quark distributions were not enhanced in nuclei as predicted by some models. Since 1989, the experimental situation has been cleared up considerably (there were some problems with the original EMC data at low ), and we now have quite precise information on the modification of in nuclei over a wide range of kinematics. In particular, there has been great progress in measuring the -dependence of deep inelastic structure functions with muon beams at CERN (NMC) and at Fermilab (E665). As shown in Fig. , there is now excellent agreement among the many different experiments and one clearly observes nuclear effects throughout the range.

At high one observes a depletion of the structure function in nuclei. This effect is only partially understood in terms of the kinematic effects of nuclear binding (Fermi motion and off-shell effects), and a quantitative explanation is not available. At low , the important effect of shadowing is observed. Recent data from Fermilab E665 have extended the study of the nuclear dependence of shadowing down to , and the nuclear suppression of the structure function is observed to saturate at the real photon value for . An interesting study of the ratio of dijet events to single jet events by E665 hints at the possibility of gluon shadowing.

In the last few years there has also been a great deal of effort to observe the effects of color transparency. Intriguing results on the reaction from BNL were available in 1989 showing very substantial changes in the transparency of nuclei to protons involved in high energy quasielastic scattering. Important new experimental information on color transparency has recently become available. As shown in Fig. , measurements of the nuclear transparency in the reaction (SLAC NE18) indicate no significant rise in the transparency in the range (GeV/c). This result rules out many models of color transparency that had indicated substantial effects in this range. In addition, a measurement of has recently been reported by Fermilab E665. As shown in Fig. , these data indicate that the nuclear transparency may be starting to rise at large (GeV/c). In this production process, the time dilation factor is much larger than in the SLAC measurement. This could ``freeze'' the small- sized configuration over much greater nuclear distances, thereby enhancing the color transparency effect. As a result of these two experiments theorists have developed more realistic and detailed models of color transparency and more reliable predictions for future experimental studies are now available.