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16950 results

Page 267 of 848

  • 18 - The Standard Model and beyond

  • Mark Thomson, University of Cambridge
  • Book:
    Modern Particle Physics
    Published online:
    28 May 2018
    Print publication:
    05 September 2013, pp 499-511

  • Summary

    The success of the Standard Model of particle physics in describing the wide range of precise experimental measurements is a remarkable achievement. However, the Standard Model is just a model and there are many unanswered questions. This short concluding chapter provides a broad overview of the current state of our understanding of particle physics and describes some of the more important open issues.

    The Standard Model

    The ultimate theory of particle physics might consist of a (simple) equation with relatively few free parameters, from which everything else followed. Whilst the Standard Model (SM) is undoubtedly one of the great triumphs of modern physics, it is not this ultimate theory. It is a model constructed from a number of beautiful and profound theoretical ideas put together in a somewhat ad hoc fashion in order to reproduce the experimental data. The essential ingredients of the Standard Model, indicated in Figure 18.1, are: the Dirac equation of relativistic quantum mechanics that describes the dynamics of the fermions; Quantum Field Theory that provides a fundamental description of the particles and their interactions; the local gauge principle that determines the exact nature of these interactions; the Higgs mechanism of electroweak symmetry breaking that generates particle masses; and the wide-reaching body of experimental results that guide the way in which the Standard Model is constructed. The recent precision tests of the Standard Model and the discovery of the Higgs boson have firmly established the validity of the Standard Model at energies up to the electroweak scale. Despite this success, there are many unanswered questions.

  • 7 - Electron–proton elastic scattering

  • Mark Thomson, University of Cambridge
  • Book:
    Modern Particle Physics
    Published online:
    28 May 2018
    Print publication:
    05 September 2013, pp 160-177

  • 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 e−p → e−p scattering process depends on the wavelength of the virtual photon in comparison to the radius of the proton.

Page 267 of 848