Transverse momentum distributions of hadrons

The study of hadron production in heavy ion collisions is essential to the search for effects beyond independent nucleon-nucleon collisions, for example the predicted phase transition to quark matter. Hadron distributions are known over a large range of transverse momenta for p-p collisions, so a careful study of the differences can be made. The transverse momentum distributions of hadrons may provide global information about p-nucleus and nucleus-nucleus collisions, such as the degree of thermalization achieved, and perhaps provide evidence for collective expansion of the highly excited central region. Comparison of the pt and transverse mass, mt, distributions of different hadronic species are crucial to extract this kind of information. Hadronic pt spectra show effects of the collision dynamics, such as hard scattering processes, and possibly rescattering of partons as well as of the formed hadrons. Such modifications have been observed in p-nucleus collisions, and can be expected to be important in nucleus-nucleus reactions. The spectral shape changes arising in this manner cause a background in efforts to extract global information from hadronic pt spectra. Lastly, there is an excess of pions observed at low pt in p-A and A-A collisions. the origin of these soft pions is not yet well understood. The phenomenon represents a major difference between p-p and nuclear collisions. 31 refs., 8 figs