Recent results from NA44 and a review of HBT

High energy heavy ion collisions provide the opportunity to create hadronic matter at high energy density and study its properties. In order to do this, one must characterize the collisions, ascertain the size and density of the hot system in the central region of the nucleus-nucleus system, and determine the energy density achieved. Furthermore, one needs to determine whether or not the system approaches equilibrium so thermodynamic descriptions may be used. One of the experimental tools available is the study of two-particle correlations to map the space-time extent of the system when the hadrons decouple. Other observables include the flow of energy and charged particles transverse to the beam and the rapidity distribution of protons to indicate the amount of stopping and randomization of the incoming energy. The transverse mass distributions of hadrons reflect the temperature of the system at freezeout and effects of radial expansion. The production ratios of different particles are related to the extent of chemical equilibrium reached in the collision and subsequent evolution of the hadron gas. The NA44 Experiment at CERN can address all of these observables, though here the author focus mainly on correlation measurements. Kaons and pions are emitted rather late in the evolution of a heavy ion collision, at the time of {open_quotes}freezeout{close_quotes} when the hadrons cease to interact. Their correlations reflect the space-time evolution of the later part of the collision. In addition to characterizing the collision, correlations can signal a phase transition as they measure the duration of hadronization and particle emission, which should be long in both a first- or second-order phase transition. Furthermore, correlation measurements offer an important tool to help disentangle effects of expansion from the freezeout temperature reflected in the single particle spectra.