Light Quark Spectroscopy

The observation, nearly four decades ago, that mesons are grouped in nonets, each characterized by unique values of J^PC – spin (J), parity (P) and charge conjugation (C) quantum numbers – led to the development of the quark model. Within this picture, mesons are bound states of a quark and an antiquark. The three light quark flavors (up, down and strange) suffice to explain the spectroscopy of most – but not all – of the lighter-mass mesons (below 3 GeV/c²) that do not explicitly carry heavy flavors (charm or beauty). Early observa- tions yielded only those J^PC quantum numbers consistent with a fermion-antifermion bound state. The J^PC quantum numbers of a meson system with total quark spin, S, and relative angular momentum, L, are determined as follows: J = L + S, P = (-1)^L+1 and C = (-1)^L+S. Thus J^PC quantum numbers such as 0^--, 0^+-, 1^-+ and 2^+- are not allowed and are called exotic in this context.

Gluonic Excitations: Hybrids and Glueballs

Our understanding of how quarks form mesons has evolved within quantum chromodynamics (QCD) and we now expect a richer spectrum of mesons that takes into account not only the quark degrees of freedom but also the gluonic degrees of freedom. Gluonic mesons with no quarks (glueballs) are expected. These are bound states of pure glue and since the quantum numbers of low-lying glueballs (below 4 GeV/c²) are not exotic, they should manifest themselves as extraneous states that cannot be accommodated within conventional meson nonets. However, their unambiguous identification is complicated by the fact that they can mix with conventional mesons.

Excitations of the gluonic field binding the quarks can also give rise to so-called hybrid mesons that can be viewed as bound states of a quark, anti-quark and valence gluon. An alternative picture of hybrid mesons, one supported by lattice QCD, is one in which a gluonic flux tube forms between the quark and anti-quark and the excitations of this flux tube lead to so-called hybrid mesons. Actually the idea of flux tubes, or strings connecting the quarks, originated in the early 1970’s to explain the observed linear dependence of the mass-squared of hadrons on spin (Regge trajectories). Conventional mesons arise when the flux tube is in its ground state. Hybrid mesons arise when the flux tube is excited and some hybrid mesons can have a unique signature, exotic J^PC. The spectroscopy of these exotic hybrid mesons is simplified because they do not mix with conventional meson states.

There are intriguing experimental results that suggest gluonic excitations do exist thereby validating a fundamental prediction of QCD. These results need confirmation. Furthermore, a comprehensive study of gluonic excitations would provide valuble experimental data which could be use to further our quantitative understanding of QCD.

GlueX

Hybrid mesons, and in particular exotic hybrid mesons, provide the ideal laboratory for testing QCD in the confinement regime since these mesons explicitly manifest the gluonic degrees of freedom. Photoproduction is expected to be particularly effective in producing exotic hybrids but there is little data on the photoproduction of light mesons. GlueX will use the coherent bremsstrahlung technique to produce a linearly polarized photon beam. A solenoid-based hermetic detector will be used to collected data on meson production and decays with statistics after the first year of running that will exceed the current photoproduction data in hand by several orders of magnitude. GlueX will be the defnitive search for hybrid mesons. Some recent articles and talks on the physics of GlueX appear below.

• American Scientist Article
• Science Case for the 12 GeV Upgrade
• CERN Courier Article
• GlueX Physics - DOE Science Review
• GlueX Documents for JLab PAC

CLEO-c

The quanta of the strong force, gluons, can self-interact and form bound states known as glueballs. Current experimental data suggest such just a state with a mass of around 1.5 GeV/c²; however, verification of the observation of a glueball is challenging. As mentioned above, glueballs below 3 GeV/c² have no unique signature, such as exotic quantum numbers. Their existence is inferred by extensively studying the spectrum of light quark mesons in a number of independent production and decay channels. Some production modes are expected to be particularly glue-rich and the flavorless nature of glueballs has implications for their expected decay modes. An overpopulation of quarkonium nonets is also a signature for glueballs.

The CLEO-c detector, collecting data on the ψ′ resonance, will acquire the world's largest sample of χ_c decays, which are are ideal for light quark spectroscopy. Conducting glueball searches in χ_c decay is complementary to the work that has been done by the BES collaboration in J/ψ decay. The χ_c states decay hadronically by annihilation of charm quarks into gluons that subsequently hadronize. Hadronic χ_c decay is therefore a "glue rich" environment. It is becoming increasingly clear that instead of additional statistics, independent experimental checks are needed to clarify the picture with regard to glueballs. A comprehensive study of hadronic χ_c would do just this and could provide evidence necessary to prove that glueballs do in fact exist as expected.