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Deep Inelastic Scattering of a lepton on a hadron, at leading order in perturbative expansion.

Deep Inelastic Scattering is the name given to a process used to probe the insides of hadrons (particularly the baryons, such as protons and neutrons), using electrons, muons and neutrinos. It provided the first convincing evidence of the reality of quarks, which up until that point had been considered by many to be a purely mathematical phenomenon. It is a relatively new process, first attempted in the 1960s and 1970s. It is conceptually similar to Rutherford Scattering, but with important differences. The reason why this type of scattering is described as "deep" and "inelastic" is discussed at [1].

The Standard Model of physics, particularly given the work of Murray Gell-Mann in the 1960s, had been successful in uniting much of the previously disparate concepts in particle physics into one, relatively straightforward, scheme. In essence, there were three types of particles.

• The Leptons, which were light (as in not particularly massive) particles such as electrons, neutrinos and their antiparticles. They have integer (or no) charge
• The Bosons, which were particles that exchange forces. These ranged from the massless, easy-to-detect photon (the carrier of the electro-magnetic force) to the exotic (though still massless) gluons that carry the strong nuclear force
• The Quarks, which were massive particles that carried fractional charges. They are the "building blocks" of the hadrons. They are also the only particles to be affected by the strong interaction

The leptons had been detected since 1897, when J. J. Thomson had shown that electric current is a flow of electrons. Some bosons were being routinely detected, although the W⁺, W^- and Z⁰ particles of the electroweak force were only categorically seen in the early 1980s, and gluons were only firmly pinned down at DESY in Hamburg at about the same time. Quarks, however, were still elusive.

Drawing on Rutherford's groundbreaking experiments in the early years of the Twentieth Century, ideas for detecting quarks were formulated. Rutherford had proven that atoms had a small, massive, charged nucleus at their centre by firing alpha particles at atoms in gold. Most had gone through with little or no deviation, but a few were deflected through large angles or came right back. This suggested that atoms had internal structure, and a lot of empty space.

In order to enter baryons (where quarks were theoretically to be found), a small, penetrating (ie easily accelerated; in reality this meant charged) and easily produced particle needed to be found. Electrons were considered ideal for the role, and in a series of remarkable technological and engineering leaps, electrons were fired as tiny bullets at protons and neutrons in nuclei. As an added bonus, the electrostatic attraction of the positively charged nucleus and the negatively charged electron increased the speed. Later experiments were conducted with mesons, but the same principles apply.

The collision absorbs some kinetic energy, and as such it is inelastic (this compares to Rutherford Scattering which is elastic, with no loss of kinetic energy, taking into account recoils of the nuclei). The electron emerges from the nucleus, and its trajectory and velocity can be detected.

Analysis of the results led to the following conclusions:

• The hadrons do have internal structure
• In baryons, there are three points of deflection (i.e. baryons consist of three quarks)
• In mesons, there are two points of deflection (i.e. mesons consist of a quark and an anti-quark. The reason they do not consist of two quarks is to do with their colour; see the quark article for more explanation)
• Quarks appear to be point charges, as electrons appear to be, with the fractional charges suggested by the Standard Model

The experiments were important because, not only did they confirm the physical reality of quarks but also proved again that the Standard Model was the correct avenue of research for particle physicists to pursue.