Thermalization at RHIC

Ideal hydroynamics provides an excellent description of all aspects of the single-particle spectra of all hadrons with transverse momenta below about 1.5-2 GeV/c at RHIC. This is shown to require rapid local thermalization at a time scale below 1 fm/c and at energy densities which exceed the critical value for color deconfinement by an order of magnitude. The only known thermalized state at such energy densities is the quark-gluon plasma (QGP). The rapid thermalization indicates that the QGP is a strongly interacting liquid rather than the weakly interacting gas of quarks and gluons that was previously expected. Collective flow as an indicator of thermalization Transverse flow in heavy-ion collisions is an unavoidable consequence of thermalization. Thermalization generates thermodynamic pressure in the matter created in the collision, which acts against the surrounding vacuum and causes rapid collective expansion (“flow”) of the reaction zone. Since the quark-gluon plasma (QGP) is a thermalized system of deconfined quarks, antiquarks, and gluons, collective flow is a necessary result of QGP formation in heavy-ion collisions, and its absence could be taken as proof that no such plasma was ever formed. Its presence, however, does not automatically signal