New results with colour-sextet quarks

TL;DR

This study investigates the phase transitions of QCD with 2 and 3 flavours of colour-sextet quarks through lattice simulations, aiming to distinguish between walking and conformal behaviors by analyzing confinement and chiral symmetry restoration.

Contribution

It provides new lattice simulation data on phase transitions in colour-sextet quark theories, offering insights into their walking or conformal nature by examining transition behaviors at different temperatures.

Findings

Transitions are far apart for colour-sextet quarks, unlike fundamental quarks.

Beta values increase as lattice spacing decreases, indicating running coupling behavior.

The 3-flavour theory shows strong-coupling behavior consistent with conformality.

Abstract

We study QCD with 2 and 3 flavours of colour-sextet quarks. The 2-flavour theory is a candidate Walking Technicolor theory. Since we are attempting to distinguish whether this theory is walking or conformal, we also study the 3-flavour theory, which is believed to be conformal, for comparison. We simulate lattice QCD with 2 and 3 flavours of colour-sextet staggered quarks at finite temperatures to determine the scales of confinement and chiral-symmetry breaking from the positions of the deconfinement and chiral-symmetry restoration transitions. Unlike the case with fundamental quarks, these transitions are far apart. For 2 flavours the values of beta=6/g^2 for both transitions increase as Ta is decreased from 1/4 to 1/6 to 1/8, as expected for a theory whose coupling runs to smaller values as the lattice spacing is decreased. However, for the chiral transition, the increase in beta between Ta=1/4 and Ta=1/6 is much larger than the increase between Ta=1/6 and Ta=1/8. This suggests that between Ta=1/4 and Ta=1/6 we are at strong coupling where the theory is effectively quenched, while between Ta=1/6 and Ta=1/8 we are emerging into the weak coupling regime. It will require even smaller Ta values to determine whether the running of the chiral-transition coupling is controlled by asymptotic freedom and the theory walks, or if it reaches a non-zero limit when the transition becomes a bulk transition and the theory is conformal. The 3 flavour case at Ta=1/4 and Ta=1/6 behaves similarly to the 2 flavour case. Since this theory is expected to be conformal, the interpretation that we are seeing strong-coupling behaviour, inaccessible from the weak-coupling limit (continuum) is the most likely interpretation.