Experimental Status of Exotic Mesons and the GlueX Experiment

Abstract

Abstract. One of the unanswered and most fundamental questions in physics regards the nature of the confinement mechanism of quarks and gluons in QCD. Exotic hybrid mesons manifest gluonic degrees of freedom and their spectroscopy will provide the data necessary to test assumptions in lattice QCD and the specific phenomenology leading to confinement. Within the past two decades a number of experiments have put forth tantalizing evidence for the existence of exotic hybrid mesons in the mass range below 2 GeV. This talk represents an overview of the available data and what has been learned. In looking toward the future, the GlueX experiment at Jefferson Laboratory represents a new initiative that will perform detailed spectroscopy of the light-quark meson spectrum. This experiment and its capabilities will be reviewed. Motivation for the Study of Hybrid MesonsHigh energy experiments have provided clear evidence for significant contributions of gluons to hadronic structure. Evidence for gluons has been found in jet measurements and deep inelastic scattering. However, descriptions of gluonic degrees of freedom in the low energy regime of soft gluons are still unavailable. This description is necessary to better understand the detailed nature of confinement. The nature of this mechanism is one of the great mysteries of modern physics, and in order to shed light on this phenomenon, we must better understand the nature of the gluon and its role in the hadronic spectrum. Confinement within the theory of strongly interacting matter, Quantum Chromodynamics (QCD), arises from the postulate that gluons can interact among themselves and give rise to detectable signatures within the hadronic spectrum. These signatures are expected within hadrons known as hybrids, where the gluonic degree of freedom is excited and can provide for a more detailed understanding of the confinement mechanism in QCD.Gluonic mesons represent a qq system in which the gluonic flux-tube contributes directly to the quantum numbers of the state. In terms of the constituent quark model, the quantum numbers of the meson are determined solely from the quark and antiquark. However, QCD indicates that this simple picture is incomplete. Lattice QCD calculations predict that hybrid states with the flux-tube carrying angular momentum should exist, as well as purely gluonic states (called glueballs). Modern lattice calculations for mesons show that indeed a string-like chromoelectric flux-tube forms between distant static quark charges as shown in Fig. 1a. The non-perturbative nature of the flux-tube leads to the confinement of the quarks and to the wellknown linear inter-quark potential from heavy-quark confinement with dV /dr ∼ 1 GeV/fm (see

Higher hybrid charmonia in an extended potential model

“…Ref. [6] discussed then available experimental evidence for exotic hybrid mesons and described the GlueX experiment in Hall D of Jefferson laboratory as a new initiative for performing detailed spectroscopy of the light-quark meson spectrum. This collaboration aims to investigate the full spectrum of mesonic states upto roughly 3 GeV including hybrid candidates; the photoproduction in it promises to be rich in hybrids, starting with those having exotic quantum numbers.…”

The mass spectra and decay properties of dimesonic states, using the Hellmann potential

“…Recently, in partial wave analysis (PWA) of J/ψ → γηη; BES III collaboration observed most promising candidates for glueballs or multiquark structure below 2.5 GeV [11,12,13]. In π − P → ηπ 0 η channel GAMS group at CERN reported state near 1400 MeV [14,15]. BNLE-852 collaboration reported another state near 1600 MeV in π − p reaction having promising hybrid structure [15,16,17,18].…”

“…In π − P → ηπ 0 η channel GAMS group at CERN reported state near 1400 MeV [14,15]. BNLE-852 collaboration reported another state near 1600 MeV in π − p reaction having promising hybrid structure [15,16,17,18]. Meson spectroscopy, mainly, the light meson spectroscopy gives the information of the nonperturbative regime of QCD.…”