Next: Bibliography Up: WA97/NA57 home page
Next: Bibliography
Up: WA97/NA57 home
page
A new state of matter: the Quark-Gluon Plasma (QGP)
The study of relativistic heavy-ion collisions provides a unique opportunity to search for a new predicted state of matter -- the quark-gluon plasma (QGP). A number of experimental signatures of the transition to the QGP phase have been proposed. They are being studied in several experiments at the BNL AGS and CERN SPS.
Strange baryons: a hadronic probe of QGP transition
Strange particles produced in heavy-ion collisions give important information on the collision mechanism. In particular, if a phase transition to a QGP state takes place, one would expect an enhancement in the yields of strange and multi-strange particles in nucleus-nucleus reactions compared with those from proton-nucleus interactions [1,2,3]. In fact, the formation of such a state will lead to equilibration of strange quarks on a time scale of a few fm/c, and to the formation of multi-strange baryons and antibaryons close to thermal and chemical hadronic equilibrium. Their abundances will be frozen at the critical temperature Tc since hadronic reactions are too slow to compete with the rapid collective expansion of the fireball at temperatures below Tc. It is expected that the enhancement should be more pronounced for multi-strange than for singly strange particles [4].
No comparable enhancement is expected in a purely hadronic scenario (i.e. without QGP).
The history and the present of strangeness experiments at the SPS
In the sulphur beam era (1986-1992), two heavy-ion experiments dedicated to the study of strangeness production were performed at the Omega spectrometer: WA85 and WA94. WA85 investigated strangeness production in p-W and S-W collisions at 200 A GeV/c, whereas WA94 studied p-S and S-S interactions at the same beam momentum. Both experiments used multiwire proportional chambers and their acceptance covered transverse momenta above 1 GeV/c at mid-rapidity. The measured particle ratios showed evidence for enhanced production of strange and multi-strange particles in nucleus-nucleus with respect to proton-nucleus collisions [5,6]. Multiwire proportional chambers have been replaced by silicon microstrips and silicon pixels in the lead beam experiments WA97 and its successor, NA57, which aims to study the strangeness production over a larger centrality range than in WA97 and at a lower beam momentum (40 A GeV/c).
Next: Bibliography
Up: WA97/NA57 home
page