Derivative expansions of the exact renormalisation group and SU(N\N) gauge theory

Abstract

We investigate the convergence of the derivative expansion of the exact renormalisation group, by using it to compute the β function of scalar λϕ⁴ theory. We show that the derivative expansion of the Polchinski flow equation converges at one loop for certain fast falling smooth cutoffs. The derivative expansion of the Legendre flow equation trivially converges at one loop, but also at two loops: slowly with sharp cut-off (as a momentum-scale expansion), and rapidly in the case of a smooth exponential cutoff. Finally, we show that the two loop contributions to certain higher derivative operators (not involved in β) have divergent momentum-scale expansions for sharp cutoff, but the smooth exponential cutoff gives convergent derivative expansions for all such operators with any number of derivatives.

In the latter part of the thesis, we address the problems of applying the exact renormalisation group to gauge theories. A regularisation scheme utilising higher covariant derivatives and the spontaneous symmetry breaking of the gauge supergroup SU(N|N) is introduced and it is demonstrated to be finite to all orders of perturbation theory.

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