A Gauge Invariant Flow Equation

Abstract

Given a Quantum Field Theory, with a particular content of fields and a symmetry associated with them, if one wants to study the evolution of the couplings via a Wilsonian renormalisation group, there is still a freedom on the construction of a flow equation, allowed by scheme independence.

In the present thesis, making use of this choice, we first build up a generalisation of the Polchinski flow equation for the massless scalar field, and, applying it to the calculation of the beta function at one loop for the lf⁴ interaction, we test its universality beyond the already known cutoff independence. Doing so we also develop a method to perform the calculation with this generalised flow equation for more complex cases.

In the second part of the thesis, the method is extended to SU(N) Yang-Mills gauge theory, regulated by incorporating it in a spontaneously broken SU(N/N) supergauge group. Making use of the freedom allowed by scheme independence, we develop a flow equation for a SU(N/N) gauge theory, which preserves the invariance step by step throughout the flow and demonstrate the technique with a compact calculation of the one-loop beta function for the SU(N) Yang-Mills physical sector SU(N/N), achieve a manifestly universal result, and without gauge fixing, for the first time at finite N.

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