Abstract

We study the SU(3) gauge theory with Nf=12 flavors in the fundamental representation by use of lattice simulations with staggered fermions. With a non-improved action we observe a chiral zero-temperature (bulk) transition separating a region at weak coupling, where chiral symmetry is realized, from a region at strong coupling where chiral symmetry is broken. With improved actions, a more complicated pattern emerges, and in particular two first order transitions in the chiral limit appear. We observe that at sufficiently strong coupling the next-to-nearest neighbor terms of the improved lattice action are no longer irrelevant and can indeed modify the pattern observed without improvement. Baryon number conservation can be realized in an unusual way, allowing for an otherwise prohibited oscillating term in the pseudoscalar channel. We discuss the phenomenon by means of explicit examples borrowed from statistical mechanics. Finally, these observations can also be useful when simulating other strongly coupled systems on the lattice, such as graphene.

Highlights

  • In recent years attention has been drawn to the study of conformal symmetry restoration in non-abelian gauge theories

  • Some of these models live in a quasi-conformal region of the parameter space at the TeV scale, such as walking technicolor or generalizations to composite Higgs models, or conformal symmetry might be thought to play a role at much higher energies

  • In recent years a growing amount of work has been devoted to the analysis of the so-called bulk transition emerging in the lattice phase diagram at strong bare gauge coupling, see Fig. 1

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Summary

Introduction

In recent years attention has been drawn to the study of conformal symmetry restoration in non-abelian gauge theories. Studies based on the strong coupling expansion of QCD predict that chiral symmetry is always broken in the strong coupling limit, regardless of the number of flavors Support for this claim was offered by Damgaard et al [1], who uncovered a bulk transition for sixteen fundamental fermions. A more general line of work involving quantum – or bulk – transitions in a particle physics environment dates back to early studies of QED at strong coupling. The transition in this context has been for a long time investigated in the hope of finding an interacting, non-asymptotically free theory in four dimensions. In this Letter we present our results on the nature of this phase, and we show that it only exists when the fermion sector is improved

The Actions
Action A: the unimproved case
Action D: the improved gauge and fermion action
The spectrum
Discussion
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