CP3 Lunch - The Physics of Glueballs - Vincent Mathieu

by Dr Vincent Mathieu

Wednesday 21 Jan 2009, 13:00 → 13:50 Europe/Brussels

Cycl.06

Quantum Chromodynamics (QCD) is the modern theory of the strong interaction. QCD allows the coupling of gluons, the gauge particles of the interaction, which can form a observable state. Glueballs, bound states of gluons, are then a beautiful consequence of the QCD. I review the recent developments on glueballs spectroscopy with a special emphasis on constituent models. The problematic on the mixing with nearby mesons lead to difficulties in the identification of physical states. I then restrict my models to pure gauge QCD. The pure gauge spectrum of quarkless QCD was investigated by Morningstar and Peardon on a lattice. They identified 15 glueballs below 4 GeV both in $C=+$ and $C=-$. In a constituent picture, the low-lying states are bound states of two gluons with $C=+$. A negative charge conjugation requires at least three gluons. In the usual picture, the gluon is a heavy spin-1 particle since the gauge boson gains a dynamical mass induced by non-perturbative effects. This consideration leads to gluonium pictures for two- and three-gluon glueballs which are extrapolation of meson and baryon systems. However I show that the usual $LS$-coupling scheme cannot reproduce the lattice spectrum. This formalism is indeed irrelevant to handle transverse particles such as gluons. The implementation of the helicity formalism for two-gluons glueballs leads to a spectrum in perfect agreement with the lattice results even without the inclusion of short-range spin-splitting potentials.