We have discussed charged lepton and (anti)neutrino induced deep inelastic scattering (DIS) off free protons in Chapter 13. The interaction cross section and the structure functions of nucleons are modified when the scattering takes place from the nucleons bound inside a nucleus. The reactions are shown in Figure 16.1, where l = e, μ. A is the target nucleus and X is the jet of hadrons are the four momenta of the initial and the final state particles respectively. Historically, the first observations of modifications of the nuclear structure
functions were made by the European Muon Collaboration (EMC) at CERN in 1981–83. The EMC collaboration studied the ratio of structure function F2(x, Q2) per nucleon for iron to deuterium targets, that is in the energy region of 120-280 GeV and its deviation from unity. This effect is known as the EMC effect. Since then, the EMC effect has been confirmed and studied with improved precision in many DIS experiments using electrons, muons [783, 784, 785, 786, 787, 788], neutrinos, and antineutrinos [789, 790, 791, 792, 793, 794] from different nuclear targets as well as in the Drell–Yan processes using proton and pions [795, 796, 797]. Some of these results are presented in Figure 16.2. From the figure, it may be observed that the ratio is different from unity in almost the entire region of the Bjorken scaling variable 0 < x < 1. From these experiments, some general features of the ratio R(x, Q2) may be inferred:
• The x dependence of R(x, Q2) has considerable structure, that is, it is different in different regions of x.
• The shape of the effect is almost independent of A.
• The functional form of R(x, Q2) is relatively independent in the region of high Q2.
Generally, the nuclear medium effects manifested through the ratio R(x, Q2) are broadly divided into four regions of x in which the x dependence is attributed to different physical effects.