Light scalar mesons in QCD

TL;DR

This paper reviews the theoretical understanding of light scalar mesons' masses and couplings using QCD sum rules and lattice calculations, discussing their possible compositions and the ongoing puzzles in their internal structure.

Contribution

It provides a comparative analysis of QCD spectral sum rules, low-energy theorems, and lattice results on light scalar mesons, highlighting the potential gluonium nature of the sigma meson.

Findings

The sigma meson may be predominantly a gluonium/glueball.

Light scalar mesons' internal structure remains unresolved.

Progress has been made but further research is needed.

Abstract

I present a mini-review of the masses and couplings of the bare (unmixed) light scalar mesons : $\bar qq, (\overline{qq})(qq), (\bar qq)(\bar qq), gg$ from QCD spectral sum rules (QSSR) and low-energy theorems (LET) which we compare with recent lattice calculations when available. Some unbiased comments on the different scenarios are given. The possiblity for the $\sigma (0.6)$ to be mostly a gluonium/glueball with a huge violation of the OZI rule in its decay is discussed. This review complements and updates the ones presented earlier [1]. Despite some progresses, the internal structure of the light scalar mesons remain puzzling, and some further efforts are required. It will be more fun at LHC if the Higgs of the Standard Model is a $\sigma$-like resonance.