Introduction

We have discussed charged lepton and (anti)neutrino induced deep inelasticscattering (DIS) off free protons in Chapter 13. The interaction crosssection and the structure functions of nucleons are modified when thescattering takes place from the nucleons bound inside a nucleus. Thereactions are shown in Figure 16.1, where l =e, μ. A is thetarget nucleus and X is the jet of hadrons are the fourmomenta of the initial and the final state particles respectively.Historically, the first observations of modifications of the nuclearstructure

functions were made by the European Muon Collaboration (EMC) at CERN in1981–83. The EMC collaboration studied the ratio of structurefunction F2(x,Q2) per nucleon for iron to deuteriumtargets, that is in the energy region of 120-280 GeV and its deviation fromunity. This effect is known as the EMC effect. Since then, the EMC effecthas been confirmed and studied with improved precision in many DISexperiments using electrons, muons [783, 784, 785, 786, 787, 788],neutrinos, and antineutrinos [789, 790, 791, 792, 793, 794] from differentnuclear targets as well as in the Drell–Yan processes using protonand pions [795, 796, 797]. Some of these results are presented in Figure16.2. From the figure, it may be observed that the ratio is different fromunity in almost the entire region of the Bjorken scaling variable 0< x < 1. From these experiments, somegeneral features of the ratio R(x,Q2) may be inferred:

• • The x dependence ofR(x,Q2) has considerable structure, thatis, it is different in different regions of x.

• • The shape of the effect is almost independent ofA.

• • The functional form ofR(x,Q2) is relatively independent in theregion of high Q2.

Generally, the nuclear medium effects manifested through the ratioR(x, Q2)are broadly divided into four regions of x in which thex dependence is attributed to different physicaleffects.