Shi-Lin Zhu Review Commissioned by International Journal of Modern Physics A
Since LEPS collaboration reported the first evidence of $\Theta^+$ pentaquark in early 2003, eleven other experimental groups have confirmed this exotic state while many other groups didn't see any signal. If this state is further established by future high statistical experiments, its discovery shall be one of the most important events in hadron physics for the past three decades. This exotic baryon with such a low mass and so narrow a width imposes a big challenge to hadron theorists. Up to now, there have appeared more than two hundred theoretical papers trying to interpret this charming state. I will review some important theoretical developments on pentaquarks based on my biased personal views.
E. Klempt PSI Zuoz Summer School on Phenomenology of Gauge Interactions August 13 - 19, 2000
Lattice gauge calculations predict the existence of glueballs. In particular a scalar glueball is firmly expected at a mass of about 1730 MeV. This prediction has led to an intense study of scalar isoscalar interactions and to the discovery of new meson resonances. The number of scalar states observed seems to exceed the number of states which can be accommodated in the quark model even when two states, the a_0(980) and f_0(980), are interpreted as K Kbar bound states and are removed from the list. However, none of these states has a decay pattern which is consistent with that of a pure glueball. A reasonable interpretation of the number of states and of their decay pattern is found only when mixing of scalar q qbar states with the scalar glueball is taken into account.
In this paper we scrutinize the evidence for these states and their production characteristics. The f_0(1370) - a cornerstone of all qbar q-glueball mixing scenarios - is shown to be likely of non-qbar q nature. The remaining scalar states then do fit into a nonet classification. If this interpretation should be correct there would be no room for resonant scalar gluon-gluon interactions, no room for the scalar glueball. We also discuss the status of mesons with exotic quantum numbers, of mesons which cannot possibly have qbar q structure, and argue that these are, most likely, four-quark states and not excitations of the gluon string providing the binding between quark and antiquark.
LANL preprintChris Michael talk presented at Confinement IV, July 2000, Vienna
We review lattice QCD results for glueballs (including a discussion of mixing with scalar mesons), hybrid mesons and exotic mesons (such as $B_s B_s$ molecules). We also discuss string breaking as a mixing between colour flux states and $B \bar{B}$ states.
LANL preprint
T. Barnes Invited contribution to MESON2000 (Cracow, 19-23 May 2000)
In this talk I summarize the status of exotic mesons, including both theoretical expectations and experimental candidates. The current experimental candidates are ``spin-parity exotics''; since these are most often considered possible hybrid mesons, the theoretical discussion will be mainly concerned with hybrids. The exotic meson candidates discussed are the surprisingly light pi_1(1400) and pi_1(1600).
LANL preprint
Curtis Meyer Lectures presented at the 1999 HUGS summer school.
An overview of light quark meson spectroscopy.
postscript
Gunnar Bali
The present knowledge of QCD confining forces between static test charges is summarised, with an emphasis on lattice results. Recent developments in relating QCD potentials to quarkonium properties by use of effective field theory methods are presented. The validity of non-relativistic QCD and the adiabatic approximation with respect to heavy quark bound states is explored. Besides the static potential and relativistic correction terms, the spectra of glueballs and gluinoballs, hybrid excitations of the QCD flux tube between fundamental colour sources, potentials between charges in various representations of the SU(3) gauge group, and multi-particle interactions are discussed. Implications for quarkonia systems and quark-gluon hybrid mesons are drawn.
Philip R. Page Invited talk at XVth Particles and Nuclei International Conference (PANIC 99), Uppsala, Sweden, 10-16 June 1999.
We provide an overview of theoretical developments on hybrid mesons and glueballs in the last year at this turn of the millenium conference. Cracks in potential models of conventional mesons are developing. Hybrid meson adiabatic surfaces have been calculated and interpreted, experimental J^PC exotics have hybrid meson, four-quark state or non-resonant interpretations, and the strong decay mechanism of hybrids has been studied. All theoretical progress on hybrid mesons in the last year is mentioned. Overall features of glueballs are visited: decays and the successes of the large N_c limit. Two promising experimental areas are mentioned: charmonium hybrids at B-factories and s sbar hybrids at Jefferson Lab.
Nora Brambilla and Antonio Vairo Lectures given at the 13th Annual HUGS at CEBAF (HUGS 98), Newport News, VA, 26 May - 2 Jun 1998.
These lectures contain an introduction to the following topics: 1) Phenomenology of the hadron spectrum; 2) The static Wilson loop in perturbative and in lattice QCD. Confinement and the flux tube formation; 3) Non static properties: effective field theories and relativistic corrections to the quarkonium potential; 4) The QCD vacuum: minimal area law, Abelian projection and dual Meissner effect, stochastic vacuum.
J. Kuti Talk given at the XVI International Symposium on Lattice Field Theory, Boulder, CO, USA, July 13-18, 1998
Recent developments in Quantum Chromodynamics (QCD) are reviewed on three major topics where nonperturbative gluon excitations of the QCD vacuum and the physical properties of the confining flux play a central role: (1) New lattice results on the spectrum and wave functions of heavy quark-antiquark molecules, known as heavy hybrids, will be discussed. (2) Recent advances on the glueball spectrum in lattice QCD will be presented with some theoretical observations. (3) Progress in our understanding of the nonperturbative internal structure of the confining flux and its excitation spectrum will be reported.
Michael Teper (Oxford) not submitted for publication Report-no: OUTP-98-88P
We summarise what lattice simulations have to say about the physical properties of continuum SU(N) gauge theories in 3+1 dimensions. The quantities covered are: the glueball mass spectrum, the confining string tension, the temperature at which the theory becomes deconfined, the topological susceptibility, the value of the scale Lambda{MS-bar} that governs the rate at which the coupling runs and the r0 parameter that characterises the static quark potential at intermediate distances.
Stephen Godfrey and Jim Napolitano Submitted to Reviews of Modern Physics, Nov.12, 1998
We survey the current status of light meson spectroscopy. We begin with a general introduction to meson spectroscopy and and its importance in understanding the physical states of Quantum Chromo Dynamics (QCD). Phemenological models of hadron spectroscopy are described with particular emphasis on the constituent quark model and the qualitative features it predicts for the meson spectrum. We next discuss expectations for hadrons lying outside the quark model, such as hadron states with excited gluonic degrees of freedom. These states include so-called hybrids glueballs}, as well as multiquark states. The established meson states are compared to the quark model predictions and we find that most meson states are well described by the quark model. However, a number of states in the light-quark sector do not fit in well, suggesting the existence of hadronic states with additional degrees of freedom. We end with a brief description of future directions in meson spectroscopy.