On the Possibility of Second-Order Phase Transitions in Spontaneously Broken Gauge Theories

In the ``Type-II'' regime, $m_{\rm Higgs}\gap m_{\rm gauge}$, the finite-temperature phase transition in spontaneously-broken gauge theories (including the standard model) must be be studied using a renormalization group treatment. Previous studies within the $(4-\ep)$-expansion suggest a 1st-order transition in this regime. We use analogies with experimentally accessible phase transitions in liquid crystals, and theoretical investigations of superconductor phase transitions to argue that, in this range, the critical behavior of a large class of gauge-Higgs-fermion systems changes from 1st to 2nd-order as a function of Higgs mass. We identify a set of models which, within the $(2+\ep)$-expansion, possess fixed points that can describe this 2nd-order behavior. As usual, a definitive demonstration that the claimed critical behavior occurs (and a reliable estimate of $m_{\rm Higgs}$ at the tricritical point) will probably require numerical simulations. This paper uses the ``phyzzx'' macro package. To obtain figure contact the author.