: from particles at collider experiments to large scale topological solitons

Student thesis: Master's ThesisMaster of Philosophy


The existence of magnetic poles has been the source of much controversy since the late 19th century. Many theoretical models have been devised to include such objects in nature, most notably in work by Paul Dirac, Julian Schwinger and Gerard ’t Hooft. But no such sources have been observed experimentally. However, the existence of magnetic sources would go a long way to solving one of the most puzzling unknowns in quantum electrodynamics. Indeed, it gives a formal reason for the quantisation of electric charge. This thesis studies the possible existence of such objects.
In this work, an effective theory for spin-0, 1/2 and 1 monopole scattering is presented. The particles are produced at threshold in an effective field theory of dualized quantum electrodynamics with a U(1) gauge symmetry by Drell-Yan and photon fusion processes. Motivated by analyses of classical scattering amplitudes, the effective magnetic coupling g(β) is kept in the perturbative regime due to its dependence on the velocity parameter β. At threshold, the effective coupling is small and perturbative techniques can be used to evaluate kinematic distributions for production processes in collider type experiments. The magnetic moments of spin-1/2, 1 monopoles are left arbitrary by the inclusion of a free phenomenological parameter κ. Kinematic distributions and the unitarity of production processes are discussed. While unitarity is observed only for κ = 0 (spin-1/2) and κ = 1 (spin-1), this is understood as a direct result of the lack of degrees of freedom in the model that will appear after ultraviolet completion. Results from recent searches by the MoEDAL collaboration, which used the model presented in this thesis, placed new bounds on mass limits for monopoles produced at the Large Hadron Collider at CERN.
Monopoles as topological solitons are also discussed in this thesis. These are extended objects which form as a result of the condensation of a scalar field to its vacuum state in a gauge theory with spontaneous symmetry breaking. They are solutions to the classical field equations. The CP1 gauge symmetry of the Standard Model Electroweak action allows topologically stable spherical solutions with magnetic and electric charge to form. These so-called Cho-Maison dyons are energetically unstable due to a singularity in their core. This is an ultraviolet divergence which comes from the hypercharge contribution to the soliton’s mass. In this work, the potential to regularise its divergence using a Born-Infeld extension to the Standard Model is discussed. It is shown that the non-linear Born-Infeld model applied to the SU(2) × UY (1) gauge sectors adds an infinite set of higher order contributions to the kinetic terms of the gauge fields. It appears likely that these terms contribute to the dynamics in such a way as to cancel the UV divergence of the Cho-Maison dyon, rendering it energetically stable.
Date of Award1 Jul 2020
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorNikolaos Mavromatos (Supervisor) & John Ellis (Supervisor)

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