Scientific Achievements since the 1989 Long Range Plan

Significant progress has been achieved in two main areas during the last 5 years. First, there have been new measurements and a reanalysis of the nucleon structure functions over a wide range of . New measurements at SLAC at low and by NMC (CERN) at high now connect smoothly and show strikingly precise scaling violations. Reanalysis of older data from both facilities now indicates good agreement among all experiments. The dependence is in excellent agreement with the evolution predicted by the Altarelli-Parisi formalism, and at low is beginning to suggest effects of higher twist. The dependence yields a gluon distribution which is in good agreement with that obtained from deep-inelastic neutrino scattering.

Secondly, a precise determination of the ratio of by NMC has yielded an impressive result for the Gottfried sum:

This sum is related to the asymmetry between the sea of upquarks and the sea of downquarks in the proton:

so the experimental result clearly indicates and asymmetry in the sea distribution. Although there is no fundamental reason to expect many simple models of the nucleon used this assumption which is now seen to be incorrect. More sophisticated models have recently become available which are consistent with the above measurement of , and further experimental work to determine and in more detail is required to provide additional information on this important issue. In fact, a recent determination of at via Drell-Yan production of muon pairs has been reported by CERN experiment NA51.