String-Parton Model Description of Relativistic Heavy-Ion Collisions
Various models have been developed to address the ordinary hadronic physics that occurs in relativistic heavy-ion collisions. These include string-based fragmentation models such as the LUND model1, and its extensions in FRITIOF2, which assume that excited hadrons behave as a chain of color dipoles that move like one-dimensional relativistic strings. Interactions are introduced via multiple small momentum exchanges between the color dipoles of two overlapping strings. Other nondynamical models are the dual-parton model3, in which the strings are formed by soft gluon exchange between the valence partons of the colliding hadrons. The quark-gluon string model4 (QGSM), also based on the dual parton model, has been developed to study soft parton collisions, and includes rescattering. The strings in the above models are in fact one-dimensional constructions in momentum space, and string evolution is carried out in this space. They are sometimes referred to as the longitudinal phase space models. Any coordinate space quantities that these models may study come from transformations from momentum space one-dimensional string coordinates to configuration space. Relativistic quantum molecular dynamics (RQMD) calculations have also been performed to study relativistic collision phenomena5. This approach combines resonance formation and decay of light hadronic states, and one-dimensional string fragmentation (LUND model) for very heavy resonances. RQMD follows the full space-time evolution of the light hadronic states, and uses one-dimensional momentum space evolution for the heavy states via the LUND string description.
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