Walking signals in Nf=8 QCD on the lattice

TL;DR

This study examines the chiral and conformal properties of Nf=8 lattice QCD, finding evidence of spontaneous chiral symmetry breaking and signs of near-conformal behavior with a large anomalous dimension, relevant for walking technicolor models.

Contribution

First detailed lattice investigation of Nf=8 QCD revealing both chiral symmetry breaking and near-conformal features with a large anomalous dimension.

Findings

F_pi non-zero in chiral limit, consistent with chiral symmetry breaking.

rho meson mass remains finite in chiral limit.

Evidence of large anomalous dimension gamma_m ~ 1 near the chiral limit.

Abstract

We investigate chiral and conformal properties of the lattice QCD with eight flavors (Nf=8) through meson spectrum using the Highly Improved Staggered Quark (HISQ) action. We also compare our results with those of Nf=12 and Nf=4 which we study on the same systematics. We find that the decay constant F_pi of the pseudoscalar meson "pion" is non-zero, with its mass M_pi consistent with zero, both in the chiral limit extrapolation of the chiral perturbation theory (ChPT). We also measure other quantities which we find are in accord with the pi data results: The rho meson mass is consistent with non-zero in the chiral limit, and so is the chiral condensate, with its value neatly coinciding with that from the Gell-Mann-Oakes-Renner relation in the chiral limit. Thus our data for the Nf=8 QCD are consistent with the spontaneously broken chiral symmetry. Remarkably enough, while the Nf=8 data near the chiral limit are well described by the ChPT, those for the relatively large fermion bare mass m_f away from the chiral limit actually exhibit a finite-size hyperscaling relation, suggesting a large anomalous dimension gamma_m ~ 1. This implies that there exists a remnant of the infrared conformality, and suggests that a typical technicolor ("one-family model") as modeled by the Nf=8 QCD can be a walking technicolor theory having an approximate scale invariance with large anomalous dimension gamma_m ~ 1.