Effects of compression and collective expansion on particle emission from central heavy-ion reactions.

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Conditions under which compression occurs and collective expansion develops in energetic symmetric reactions of heavy nuclei are analyzed, together with their effects on emitted light baryons and pions. Within transport simulations, it is shown that shock fronts perpendicular to beam axis form in head-on reactions. The fronts separate hot compressed matter from normal matter and propagate into the projectile and target. As the impact parameter increases, the angle of inclination of the fronts relative to beam axis decreases, and in between the fronts a weak tangential discontinuity develops. Hot matter exposed to the vacuum in directions perpendicular to shock motion (and parallel to fronts) starts to expand sideways early within the reactions. Expansion in the direction of shock motion follows after the shocks propagate through nuclei, but due to the delay does not acquire the same strength. Expansion affects angular distributions, mean-energy components, shapes of spectra, and mean energies of different particles emitted into any one direction and further particle yields. Both the anisotropy in the expansion and a collective motion associated with the weak discontinuity affect the magnitude of sideward flow within the reaction plane. Differences in mean particle energy components in and out of the reaction plane in semicentral collisions depend sensitively on the relative magnitude of shock speed in normal matter and speed of sound in hot matter. The missing energy, considered in the past in association with low measured pion multiplicity in central reactions, may be identified with the energy of collective expansion. Relations are established which govern approximately the behavior of density and entropy in the compressed region in reactions with beam energy and impact parameter.