Introduction

In 1951, Lyman, Hanson, and Scott [541] were the first to observe elasticelectron scattering from different nuclei using a 15.7 MeV beam obtainedwith the help of the betatron accelerator facility at Illinois. Furtherextensive studies using electron beams started with the development of theMark III linear accelerator (LINAC) in 1953 at the High Energy PhysicsLaboratory (HEPL), Stanford. Hofstadter, Fechter, and McIntyre studied theeffect of electron scattering (Ee ~ 125− 150 MeV) on various nuclear targets andconcluded that these nuclei have finite charge distribution. With increasedenergy (550 MeV) available for scattering, the first evidence of elasticscattering from the proton was observed by Chambers and Hofstadter at HEPLin 1956 [542], using a polyethylene target. Assuming that proton had anexponential density distribution, they found the r.m.s. (root mean squared)radius of the proton to be about 0.8 fm. Later, Yearian and Hofstadterperformed electron scattering experiments on deuteron targets and determinedthe magnetic moment of the neutron [543]. At that time, the investigation ofthe structure of the proton and neutron was a major objective of HEPL, whichwas upgraded to achieve electron beams of energies up to 20 GeV; today, HEPLis known as the Stanford Linear Accelerator Center (SLAC) [544]. By the1960s, it became possible to perform both elastic and inelastic scatteringexperiments at high energies and for a wide range of four-momentum transfersquared (Q2). Thus, by the end of the 1960s,nuclear physics entered the ‘deep inelastic scattering’ (DIS)era, when experiments with 20–40 times higher energies were beingperformed at SLAC; it became possible to probe the hadron. DIS is thescattering of charged leptons/neutrinos from hadrons in the kinematic regionof very high Q2 and energy transferν. During the late 1960s, experiments byMIT-SLAC collaboration, led by Taylor, Kendall, and Friedman [544] confirmedthe scaling phenomenon in the deep inelastic region, which was theoreticallypredicted by Bjorken [545, 546]. They received the 1990 Nobel Prize inPhysics for the experiments. These experimental results confirmed that, inthe scaling region, the constituents of protons behave like free particlescalled partons. Charged partons are identified as quarks and neutral partonsare identified as gluons. Later, many experiments using electron and muonbeams were performed at CERN, DESY, Fermilab, etc.