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Updated: August 20, 2000
In the early 1970's, evidence that the masses of strongly interacting particles increased without limit as their internal angular momentum increased led the Japanese theorist Yoichiro Nambu to propose that the quarks inside of these particles are "tied" together by strings. Today the string theories which emerged from this idea are being examined as candidates for the ultimate theory of nature, while we know that the strong interactions are instead described by quantum chromodynamics (QCD), the field theory in which quarks interact through a "color" force carried by gluons. Though it is therefore not fundamentally a string theory, numerical simulations of QCD ("lattice QCD") have demonstrated that Nambu's conjecture was essentially correct: in chromodynamics, a string-like chromoelectric flux tube forms between distant static charges, leading to quark confinement and a potential energy between a quark and the other quarks to which it is tied which increases linearly with the distance between them. The phenomenon of confinement is the most novel and spectacular prediction of QCD - unlike anything seen before.
Looking for Glue Degrees of Freedom in Spectroscopy
The ideal experimental test of this new feature of QCD would be to directly study the flux tube of by anchoring a quark and antiquark several fermis apart and examining the flux tube that forms between them. In such ideal circumstances one of the fingerprints of the gluonic flux tube would be the model-independent spectrum shown below. The two degenerate first excited states are the two longest wavelength vibrational modes of this system; is their excitation energy since both the mass and the tension of this "relativistic string" arise from the energy stored in its color force fields. Such a direct examination of the flux tube is of course not possible. In real life we have to be content with systems in which the quarks move. Fortunately, we know both from general principles and from lattice QCD that an approximation to the dynamics of the full system which ignores the impact of these two forms of motion on each other works quite well - at least down to the charm quark mass.
To extend the flux tube picture to yet lighter quarks, models are required, but the most important properties of this system are determined by the model-independent features described above. In particular, in a region around 2 GeV, a new form of hadronic matter must exist in which the gluonic degree of freedom of mesons is excited. The smoking gun characteristic of these new states is that the vibrational quantum numbers of the string, when added to those of the quarks, can produce a total angular momentum J, a total parity (or mirror-inversion symmetry) P, and a total charge conjugation (or quark-antiquark interchange) symmetry C not allowed for ordinary states. These unusual JPC combinations, like 0+ -, 1- +, and 2+ - , are called exotic, and the states are referred to as exotic hybrid mesons.
Not only general considerations and flux tube models, but also first-principles lattice QCD calculations, require that these states are in this mass region, while also demonstrating that the levels and their orderings will provide experimental information on the mechanism which produces the flux tube. Moreover, tantalizing experimental evidence has appeared over the past several years for exotic hybrids as well as for gluonic excitations with no quarks (glueballs). For the last two years a group of 80 physicists from 25 institutions in seven countries has been working on the design of the definitive experiment to map out the spectrum of these new states required by the confinement mechanism of QCD. This experiment is part of the planned 12 GeV Upgrade of the CEBAF complex at Jefferson Lab in Newport News, Virginia.
Our Focus - Light Quark Exotics
Photon beams are expected to be particularly favorable for the production of the exotic hybrids. The reason: the photon sometimes behaves as a virtual vector meson (a qqbar state with the quark spins parallel, adding up to total quark spin S = 1). When the flux tube in this system is excited to the levels shown above, both ordinary and exotic JPC are possible. In contrast, when the spins are antiparallel (S = 0), as in pion or kaon probes, the exotic combinations are not generated. To date, most meson spectroscopy has been done with incident pion, kaon or proton probes. High flux photon beams of sufficient quality and energy have not been available, so there are virtually no data on the photoproduction of mesons below 3 GeV. Thus, experimenters have not been able to search for exotic hybrids precisely where they are expected to be found.
Photon Beam and Virtues of Linear Polarization
Photons will be produced using a "coherent bremsstrahlung" technique by passing a fine electron beam from the CEBAF accelerator though a wafer-thin diamond crystal: At special settings for the orientation of the crystal, the atoms of the crystal can be made to recoil together from the radiating electron leading to an enhanced emission at particular photon energies and yielding linearly polarized photons. With the planned photon fluxes of /sec and the continuous CEBAF beam, the experiment will accumulate statistics during the first year of operation which will exceed extant data with pions by at least an order of magnitude. With this detector, high statistics, and the linear polarization information, it will be possible to map out the full spectrum of these gluonic excitations.
Linear and Circular Polarization
Linear Polarization is Essential
Our detector is optimized for 8-9 GeV photons - this range is the "sweet-spot" balancing the need to go high enough in energy to produce mesons with sufficient yield (need to guarantee that tmin effects are small) and to separate excited baryon from excited mesons with the need to achieve sufficient flux and degree of linear polarization. This photon range also allows an solenoidal geometry (no dipole which would open e+e-pairs) - satisfying the need for linear polarization.
High quality data with sufficient statistics will allow us to
perform a PWA. In the first year of operation we expect to gather an
order of magnitude more statistics than available in extant data from
pion beams in this mass
regime.
Uniqueness of JLab to do this Physics
