On the glueball spectrum in O(a)-improved lattice QCD

TL;DR

This study computes the glueball mass spectrum in N_f=2 lattice QCD with O(a) improvement, revealing a smaller scalar glueball mass in dynamical quarks compared to quenched QCD and providing evidence for string breaking.

Contribution

First calculation of light glueball spectrum in O(a)-improved N_f=2 lattice QCD at this lattice spacing and quark mass range, including flux tube analysis and string breaking evidence.

Findings

Scalar glueball mass is significantly smaller with dynamical quarks.

Tensor glueball mass remains consistent between dynamical and quenched QCD.

Evidence of string breaking and gauge centre symmetry breaking by sea quarks.

Abstract

We calculate the light `glueball' mass spectrum in N_f=2 lattice QCD using a fermion action that is non-perturbatively O(a) improved. We work at lattice spacings a ~0.1 fm and with quark masses that range down to about half the strange quark mass. We find the statistical errors to be moderate and under control on relatively small ensembles. We compare our mass spectrum to that of quenched QCD at the same value of a. Whilst the tensor mass is the same (within errors), the scalar mass is significantly smaller in the dynamical lattice theory, by a factor of ~(0.84 +/- 0.03). We discuss what the observed m_q dependence of this suppression tells us about the dynamics of glueballs in QCD. We also calculate the masses of flux tubes that wind around the spatial torus, and extract the string tension from these. As we decrease the quark mass we see a small but growing vacuum expectation value for the corresponding flux tube operators. This provides clear evidence for `string breaking' and for the (expected) breaking of the associated gauge centre symmetry by sea quarks.