Light Quarkonium - Glueball Mixing from a Holographic QCD

TL;DR

This paper uses a holographic QCD model to analyze the mixing of light quarkonium and glueball states, predicting small mixing and identifying the $f_0(1710)$ as mainly a glueball.

Contribution

It introduces a bottom-up holographic model to define pure quarkonium and glueball states and predicts their mixing and masses in realistic QCD conditions.

Findings

Tiny mixing between light quarkonia and glueball at realistic parameters

Predicted scalar masses around 1.25 GeV and 1.77 GeV

Identified $f_0(1710)$ as predominantly glueball

Abstract

We study the mixing structure of isospin-singlet scalars, the light quarkonium $(\bar{q}q)$ and glueball $(gg)$ in two-flavor QCD, based on a holographic model of bottom-up hard-wall type. In the model the pure quarkonium and glueball states are unambiguously defined in terms of the different $U(1)_A$ charges in the restoration limit of the chiral $U(2)_L \times U(2)_R$ symmetry, in which the quarkonium gets massless as the chiral partner of the pion. Hence the $\bar{q}q$-$gg$ mixing arises in the presence of the nonzero chiral condensate or pion decay constant. At the realistic point where the pion decay constant and other hadron masses reach the observed amount, we predict the tiny mixing between the lightest quarkonia and glueball: The smallness of the mixing is understood by the slightly small ratio of the chiral and gluon condensate scales. The low-lying two scalar masses are calculated to be $\simeq 1.25$ GeV and $\simeq 1.77$ GeV, which are compared with masses of $f_0(1370)$ and $f_0(1710)$. Our result implies that $f_0(1710)$ predominantly consists of glueball.