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Chapter 7: Electron–proton elastic scattering

Chapter 7: Electron–proton elastic scattering

pp. 160-177

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, University of Cambridge
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Summary

In e+e collisions, the initial-state particles are fundamental fermions. Consequently, the cross sections for processes such as e+e annihilation are determined by the QED matrix element and the event kinematics (phase space) alone. Calculations of cross sections for collisions involving protons, for example at an electron–proton collider or a hadron collider, also need to account for the composite nature of the proton. This chapter describes low-energy electron–proton elastic scattering. The main purpose is to provide an introduction to a number of concepts which form the starting point for the description of the high-energy interactions of protons that is the main topic of the following chapter.

Probing the structure of the proton

Electron–proton scattering provides a powerful tool for probing the structure of the proton. At low energies, the dominant process is elastic scattering where the proton remains intact. Elastic scattering is described by the coherent interaction of a virtual photon with the proton as a whole, and thus provides a probe of the global properties of the proton, such as its charge radius. At high energies, the dominant process is deep inelastic scattering, where the proton breaks up. Here the underlying process is the elastic scattering of the electron from one of the quarks within the proton. Consequently, deep inelastic scattering provides a probe of the momentum distribution of the quarks.

The precise nature of the ep → ep scattering process depends on the wavelength of the virtual photon in comparison to the radius of the proton.

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