The physics of eight flavours

TL;DR

This paper investigates QCD with eight flavors using lattice simulations, demonstrating chiral symmetry breaking and clarifying the nature of the transition, thus refining the understanding of the conformal window in QCD.

Contribution

It provides the first clear lattice evidence that eight-flavor QCD breaks chiral symmetry and identifies the transition as thermal, constraining the conformal window's lower boundary.

Findings

Eight-flavor QCD breaks chiral symmetry in the continuum.

The transition is thermal in nature.

The conformal window starts after this transition.

Abstract

When the flavour content of QCD is increased sufficiently, the theory develops a non-trivial infra red fixed point. Thus, for a number of flavours above a certain critical value, but not yet so high that asymptotic freedom is lost, QCD becomes a conformal field theory. The location of the lower limit of this conformal window has not yet been unequivocally determined. Using an improved lattice action, and exploiting modern algorithms allowing for larger lattices and lower quark masses, we have shown that the theory of QCD with eight flavours breaks chiral symmetry in the continuum. We present proof that the accompanying transition is thermal in nature and as a consequence, the conformal window of QCD can only start afterwards, corroborating recent analytical studies at the expense of older results.