Glueballs and k-strings in SU(N) gauge theories : calculations with improved operators

TL;DR

This paper improves the calculation of glueball and k-string properties in SU(N) gauge theories using advanced operators, providing insights into string tensions, quasi-stable strings, and glueball masses across different N values.

Contribution

It introduces improved operators for better overlap with lightest states and offers new results on k-string tensions and glueball masses, with systematic error analysis.

Findings

k-string tensions lie between MQCD and Casimir Scaling

evidence for quasi-stable strings without sources

glueball masses extrapolate consistently to large N

Abstract

We test a variety of blocking and smearing algorithms for constructing glueball and string wave-functionals, and find some with much improved overlaps onto the lightest states. We use these algorithms to obtain improved results on the tensions of k-strings in SU(4), SU(6), and SU(8) gauge theories. We emphasise the major systematic errors that still need to be controlled in calculations of heavier k-strings, and perform calculations in SU(4) on an anisotropic lattice in a bid to minimise one of these. All these results point to the k-string tensions lying part-way between the `MQCD' and `Casimir Scaling' conjectures, with the power in 1/N of the leading correction lying in [1,2]. We also obtain some evidence for the presence of quasi-stable strings in calculations that do not use sources, and observe some near-degeneracies between (excited) strings in different representations. We also calculate the lightest glueball masses for N=2, ...,8, and extrapolate to N=infinity, obtaining results compatible with earlier work. We show that the N=infinity factorisation of the Euclidean correlators that are used in such mass calculations does not make the masses any less calculable at large N.