On the phase structure of massless many-flavour QCD with staggered fermions

TL;DR

This paper investigates the phase structure of massless many-flavour QCD using lattice simulations with staggered fermions, aiming to identify the conformal window and understand the nature of the chiral transition.

Contribution

It provides a systematic analysis of the chiral phase boundaries and the interplay between thermal and bulk transitions in lattice QCD with multiple flavors.

Findings

Chiral transition is second order up to the conformal window

Identifies how to distinguish the conformal window from lattice phase transitions

Offers a method to locate the conformal window from lattice data

Abstract

When the number of massless fermions exceeds a critical value $N_f^*$, QCD enters the conformal window and becomes chirally symmetric already in the vacuum. Determining $N_f^*$ from lattice simulations is challenging, since calculations are performed at finite lattice spacing, quark mass, and temporal lattice size, where both a thermal transition and an unphysical bulk transition obscure the conformal behaviour. In this work, we present results on the chiral phase boundaries in the bare lattice parameter space $(N_\tau,\;\beta,\;am,\;N_f)$ of unimproved staggered fermions. Our analysis indicates that the chiral transition in continuum QCD is of second order for all $N_f$ up to the onset of the conformal window. By systematically studying the thermal chiral transition and its interplay with the bulk transition, we obtain a coherent picture of the lattice phase structure and suggest how the onset of the conformal window can be identified from simulations performed away from the chiral and continuum limits.