Hadrons are the physical particles that participate in the strong interaction. The theory of the strong interaction, Quantum ChromoDynamics or QCD, is based upon more fundamental particles called quarks and gluons, of which hadrons are supposed to be composed. In principle, it should be possible to calculate the observed properties of hadrons from those of the quarks and gluons and their interactions. In practice this goal has proved difficult to achieve. Bridging the gap between the underlying field theory and hadronic observables involves understanding how to solve the general problem of a strongly-coupled field theory, which is an issue of great importance across many areas of physics. The only known direct attack on the problem is through numerical solution of the field equations on a discrete space-time lattice, a technique known as Lattice QCD. Important insight can also be obtained using an indirect approach, in which certain simplifying assumptions (i.e. models) to full QCD are used.
The success of the latter approach has been proved many times in other areas such as Atomic Physics and Condensed Matter, where the full theory of Quantum ElectroDynamics (QED) is replaced with an effective theory of nonrelativistic electrons, holes, Cooper pairs, bond potentials, etc. At the scale of nuclear excitations, the strong interaction is described quite well by an effective two-nucleon force. Similarly at the hadron scale, using models that are justified initially by experimental data, one hopes to "crack the code" of QCD and discover the effective degrees of freedom that characterize hadronic excitations.
The majority of these models are decendents of the "quark model" of Gell-Mann and Zweig, whose remarkable success in accounting for the general structure of the light hadron spectrum was an inspiration for the development of QCD. Within the context of QCD one can rationalize the use of a quark model by treating the gluons as a potential field through which the quarks interact, and supposing that relativistic effects can be subsumed into an effective "constituent quark mass." While such models have a range of validity, if QCD is correct then this cannot be the whole story. In particular, states known as exotics with explicit gluonic excitations should appear in addition to those predicted by the primitive quark model. The theoretical and experimental quest for the exotic spectrum and decay properties is the number one motivation for research in hadron spectroscopy today.
The completion of the bridge from QCD to hadrons requires that a successful model be connected to QCD. During the last decade, outstanding progress has been made in Lattice QCD (even for light quarks, which are more difficult to handle than the heavy flavours) owing to the increasing power of affordable computers. The pursuit of these calculations to new levels of precision and better control over systematic errors is a major goal of Nuclear Physics over the coming decade. With this improved calculational apparatus, it will be possible to make an accurate comparison between QCD and experiment for a limited number of hadronic observables. Experiments and phenomenological models will play a pivotal role, both in the selection of appropriate observables and in the determination of their physical values.
| Jetset | (PS202 at CERN) |
| RadPhi | (E94-016 at Jefferson Lab) |
| GlueX | (a new meson photoproduction facility at Jefferson Lab) |
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