A novel view of the flavor-singlet spectrum from multi-flavor QCD on the lattice

TL;DR

This study uses lattice simulations to analyze the flavor-singlet spectrum in multi-flavor QCD, revealing a novel hierarchy with light scalars and insights into the dynamics of strongly interacting sectors relevant for beyond Standard Model physics.

Contribution

The paper provides a comprehensive lattice analysis of flavor-singlet scalar and pseudoscalar channels across different flavor numbers, highlighting new spectral hierarchies and their implications for new physics models.

Findings

Identification of a new hierarchy with light scalar states

Influence of many light flavors on pseudoscalar dynamics

Comparison of spectra across Nf=4, 8, 12 theories

Abstract

SU(3) gauge theories with increasing number of light fermions are the templates of strongly interacting sectors and studying their low-energy dynamics and spectrum is important, both for understanding the strong dynamics of QCD itself, but also for discovering viable UV completions of beyond the Standard Model physics. In order to contrast many-flavors strongly interacting theories with QCD on a quantitative footing, we use Lattice Field Theory simulations. We focus on the study of the flavor-singlet spectrum in the scalar and pseudoscalar channels: this is an interesting probe of the dynamics of the strongly interacting sector, as reminded by the QCD case with the $f_0(500)$ ($\sigma$) and $\eta^\prime$ mesons. The hierarchy of the spectrum of a strongly coupled new gauge sector of the Standard Model defines the potential reach of future colliders for new physics discoveries. In addition to a novel hierarchy with light scalars, introducing many light flavors at fixed number of colors can influence the dynamics of the lightest flavor-singlet pseudoscalar. We present a complete lattice study of both these flavor-singlet channels on high-statistics gauge ensembles generated by the LatKMI collaboration with 4, 8, and 12 copies of light mass-degenerate fermions. We also present other hadron masses on the lightest ensemble for $N_f=8$ generated by the LatKMI collaboration and discuss the chiral extrapolation of the spectrum in this particular theory. We contrast the results to $N_f=4$ simulations and previous results of $N_f=12$ simulations.