Glueballs, Hybrids, Multiquarks. Experimental facts versus QCD inspired concepts

Quantum chromodynamics (QCD), the generally accepted theory for strong interactions, describes the interactions between quarks and gluons. The strongly interacting particles that are seen in nature are hadrons, which are composites of quarks and gluons. Since QCD is a strongly coupled theory at distance scales that are characteristic of observable hadrons, there are no rigorous, first-principle methods to derive the spectrum and properties of the hadrons from the QCD Lagrangian, except for lattice QCD simulations that are not yet able to cope with all aspects of complex and short-lived states. Instead, a variety of “QCD inspired” phenomenological models have been proposed. Common features of these models are predictions for the existence of hadrons with substructures that are more complex than the standard quark-antiquark mesons and the three-quark baryons of the original quark model that provides a concise description of most of the low-mass hadrons. Recently, an assortment of candidates for nonstandard multiquark mesons, meson-gluon hybrids, and pentaquark baryons that contain heavy (charm or bottom) quarks has been discovered. Here the experimental evidence for these states is reviewed and some general comparisons of their measured properties with standard quark model expectations and predictions of various models for nonstandard hadrons are made. The conclusion is that the spectroscopy of all but the simplest hadrons is not yet understood.

https://link.aps.org/accepted/10.1103/RevModPhys.90.015003

Nonstandard heavy mesons and baryons: Experimental evidence

The $\ensuremath{\psi}$ and $\ensuremath{\Upsilon}$ spectroscopies are analyzed in the framework of a recently proposed potential model which incorporates linear confinement and asymptotic freedom. Given the Regge slope ${\ensuremath{\alpha}}^{\ensuremath{'}}$ (${\ensuremath{\alpha}}^{\ensuremath{'}}$ taken to be 1 ${\mathrm{GeV}}^{\ensuremath{-}2}$) and the quantum-chromodynamics (QCD) scale parameter $\ensuremath{\Lambda}$ (${\ensuremath{\Lambda}}_{\stackrel{-}{\mathrm{MS}}}$ taken to be 0.5 GeV, where $\stackrel{-}{\mathrm{MS}}$ refers to the modified minimal-subtraction scheme) the potential is completely determined. Excellent agreement with experiment is found, including in particular leptonic widths and hyperfine splittings. This supports a short-distance behavior of the quark-antiquark potential as predicted by QCD. We also demonstrate in a model-independent way that the $\ensuremath{\Psi}$ and $\ensuremath{\Upsilon}$ spectra provide a lower bound on the QCD scale parameter $\ensuremath{\Lambda}$; we find ${\ensuremath{\Lambda}}_{\stackrel{-}{\mathrm{MS}}}g0.1$ GeV. The properties of ($b\overline{c}$) and possible ($t\overline{t}$), ($t\overline{c}$), and ($t\overline{b}$) spectroscopies are studied, including weak-interaction effects. The implications of the $\ensuremath{\Psi}$, $\ensuremath{\Upsilon}$, and possible heavier quarkonium families for quantitative tests of QCD are discussed. It is shown that a ($t\overline{t}$) system with $m(t\overline{t})\ensuremath{\ge}40$ GeV would provide an accurate determination of ${\ensuremath{\Lambda}}_{\stackrel{-}{\mathrm{MS}}}$.

/pdf/quarkonia-and-quantum-chromodynamics-4f2xlqfnos.pdf

Quarkonia and Quantum Chromodynamics

In a manifestly gauge-independent formalism, all relativistic corrections to the fermion propagation function are determined and the general form of the spin-dependent forces in quantum chromodynamics for heavy-quark-antiquark
				 ( q q ¯ ) systems is derived. For example, the classical spin-orbit and Thomas-precession terms are found to be simple derivatives of the static potential. In addition to expressing the spin-dependent forces in terms of the minimal number of independent potentials, two new applications of this formulation are presented: (1) The effect of pseudoparticle solutions on the spin-dependent forces is analyzed, and (2) an electric-confinement assumption produces a zero-parameter spin-dependent potential. This potential determines the fine structure in heavy
				 q q ¯ systems. Spin splittings in the ϒ system are predicted and the J /ψ system splittings are compared with the experimentally observed values.

Spin-dependent forces in quantum chromodynamics

The new detector for data recording by the CLEO collaboration at the Cornell Electron Storage Ring is described. This detector has been designed to optimize studying e+ e− annihilation into hadronic matter at a total energy of 10 GeV. It consists of high precesion charged particle tracking chambers and an electromagnetic calorimeter together with systems for particle identification. The design of the detector and its performance over the first year and a half of operation are presented.

The CLEO-II detector

Three narrow resonances have been observed in e/sup +/e/sup -/ annihilation into hadrons at total energies between 9.4 and 10.4 GeV. Measurements of mass spacings and ratios of lepton pair widths support the interpretation of these ''UPSILON'' states as the lowest triplet-S levels of the bb-bar quark-antiquark system.

D. Larson

Papers

Observation of three upsilon states