Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / A technique capable of describing in detail the various phenomena arising in the limit of low energy photons because of the polarization of the vacuum is obtained by quantizing the gauge invariant effective interaction Lagrangian. This approach then permits the use of the standard covariant calculational tools of Quantum Electro-dynamics. We first apply this technique to the computation of the differential cross section for low energy photon-photon scattering. The well known Euler cross section is thereby derived in a direct manner. As a second example, the probability of the triple breakup of a free photon because of vacuum polarization is explicitly shown to vanish. This, however, is primarily due to the kinematics of the photon breakup. For a third application we calculate the differential cross section for the scattering of a low energy photon from the Coulomb field of a nucleus, that is, Delbruck scattering. Here the exact low energy differential cross section is obtained. However, though the present technique determines completely the angular dependence, it is necessary to introduce a momentum cutoff in the Coulomb field in order to obtain a finite value for the coefficient in front of the angular dependence. A comparison of the resulting expression for the cross section with that for forward scattering given by Rohrlich and Gluckstern (1952) gives the numerical value of this coefficient. A brief comparison between the angular distribution for Delbruck scattering and that for a combination of an electric and magnetic dipole radiator is presented. / 2031-01-01

We investigate the dynamical generation of fermion mass in quantum electrodynamics (QED). This non-perturbative study is performed using a truncated set of Schwinger-Dyson equations for the fermion and the photon propagator. First, we study dynamical fermion mass generation in quenched QED with the Curtis-Pennington vertex, which satisfies the Ward-Takahashi identity and moreover ensures the multiplicative renormalizability of the fermion propagator. We apply bifurcation analysis to determine the critical point for a general covariant gauge. In the second part of this work we investigate the dynamical generation of fermion mass in full, unquenched QED. We develop a numerical method to solve the system of three coupled non-linear equations for the dynamical fermion mass, the fermion wavefunction renormalization and the photon renormalization function. Much care is taken to ensure the high accuracy of the solutions. Moreover, we discuss in detail the proper numerical cancellation of the quadratic divergence in the vacuum polarization integral and the requirement of using smooth approximations to the solutions. To achieve this, we improve the numerical method by introducing the Chebyshev expansion method. We apply this method to the bare vertex approximation to unquenched QED to determine the critical coupling for a variety of approximations. This culminates in the detailed, highly accurate, solution of the Schwinger-Dyson equations for dynamical fermion mass generation in QED including both, the photon renormalization function and the fermion wavefunction renormalization in a consistent way, in the bare vertex approximation and, for the first time, using improved vertices. We introduce new improvements to the numerical method, to achieve the accuracy necessary to avoid unphysical quadratic divergences in the vacuum polarization with the Ball-Chiu vertex.

Thesis (Ph. D.)--University of Oregon, 2002. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 171-178). Also available for download via the World Wide Web; free to University of Oregon users.

Field output factors (OF) for photon beams from a 6 MV medical accelerator were measured using five different detectors in a scanning water phantom. The measurements were taken for square field sizes of integral widths ranging from 1 cm to 10 cm for two reference source-to-surface distances (SSD) and depths in water. For the diode detectors, square field widths as small as 2.5 mm were also studied. The photon beams were collimated by using either the jaws or the multileaf collimators. Measured OFs are found to depend upon the field size, SSD, depth and also upon the type of beam collimation, size and type of detector used. For field sizes larger than 3 cm x 3 cm, the OF measurements agree to within 1% or less. The largest variation in OF occurs for jawsshaped field of size 1 cm x 1cm, where a difference of more than 18% is observed. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2013.

In 1967 Strocchi established that the quantisation of the electromagnetic field using a vector potential is impossible within the context of conventional field theory. Although this result is frequently referred to its significance is largely misunderstood. The fact that the electromagnetic field cannot be described in conventional field theory reflects more upon conventional field theory than theories of the electromagnetic field. A reappraisal of electromagnetic field theories should therefore be made. It could well be that features of these theories that have been previously regarded as deficiencies are not really deficiencies at all. This thesis is an account of the radiation gauge, Gupta - Bleuler and Fermi methods of quantising the electromagnetic field from that point of view. The radiation gauge and Gupta - Bleuler methods are well established schemes. Our discussion does not yield any results concerning these methods that cannot be found elsewhere. It does, however, serve to place them in a wider context. The Fermi method is little understood and hence most of this work is concerned with it. Even though the various formulations of field theory are by no means equivalent, they all eventually reproduce traditional field theory. Thus if we only require that the theory be rigorously formulated for such examples as the neutral scalar field it does not matter which formulation we choose. The differences are, however, important for applications to the quantisation methods of the electromagnetic field. The formulations have to be modified and the point at which such modifications must be made and their nature depends on both the general formulation and the quantisation method. The formalism that provides the most suitable framework for a rigorous formulation of the Fermi method turns out to be the C * algebra formulation of Segal. Following Segal, the Weyl algebra of the vector potential is constructed. The Fermi method is then related to a certain representation of the algebra. The representation is specified by a generating functional for a state on the algebra. Usually, dynamical and kinematical transformations are represented by unitarily implementable automorphisms of the algebra. We prove that this is not always true in the representation given by the Fermi method. The Weyl algebra of the physical field is then constructed as a factor algebra. Difficulties with both the Fermi and Gupta - Bleuler methods can he attributed to the need to use a factor algebra. The canonical commutation relations [ x [ subscript ] µ , p [ subscript ] ν ] = - i g [ subscript ] µ [ subscript ] ν are formulated as a Weyl algebra. We study the Schrödinger representation of the algebra and find that the Fermi method is just the generalisation of this representation to an infinite number of degrees of freedom. Further analogies are also possible. We can construct factor algebras from the Weyl algebra. The mechanics of such procedures can he studied without the additional complications of an infinite number of degrees of freedom. The Schrödinger representation of the Fermi method is then constructed. We conclude with a discussion of the results that have been obtained and an indication of ways in which the work might be extended. / Thesis (Ph.D.)--Department of Mathematical Physics, 1974.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 49-50). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 49-50). Also available on the Internet.

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