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Richard Williams

Durham University
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SDEs in QED and QCD: Critical couplings; and the qq-bar condensate for massive quarks

Truncation schemes in the SDE approach to non-perturbative studies often break
symmetries of the Lagrangian of the theory, and so one attempts to construct
improved vertex ansatze that better preserve them. Using strongly coupling QED
as a prototype field theory, we show how one may apply multiplicative
renormalisability to further constrain the fermion-vector boson vertex of
unquenched QED, and give explicit numerical calculations.
Following this, we examine the chiral condensates dependence upon the current
quark mass in three models of QCD, via the Schwinger-Dyson equations, with the
aim of determining the ss-bar condensate so necessary an input for QCD sum rules.

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