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STAR STAR PMD Physics Description  CERN.CH
STAR PMD - Physics Description 
  At high temperatures or high baryon number density, QCD describes a world of weakly interacting quarks and gluons very different from the hadronic world in which we live. This raises the possibility of a phase transition as the temperature or density is increased. This subject of phase transition from a state of matter where quarks are confined inside hadrons to one where quarks are free to move around within a large volume - the ''quark-gluon~plasma'' (QGP), is an interesting physics issue. This can be addressed through experimental studies involving relativistic heavy-ion collisions. Lattice gauge theory calculations suggest that critical temperature for such a phase transition is around 150 MeV, corresponding to an energy density of 2-3 GeV/fm3. It has been estimated that the energy density achieved in the central region of nucleus-nucleus collisions could reach as high as 1-10 GeV/fm3, suggesting that such collisions could be used to create matter in the QGP state in the laboratory. This resulted in several generations of experiments at CERN and BNL to search for the formation of QGP at ultra-relativistic energies. The experimental searches were focussed on isolating signatures of two types of phase transitions which might occur in extremely hot and/or dense nuclear matter. One is related to the deconfinement of quarks while the other is related to chiral symmetry restoration. The $''deconfinement''$ phase transition is expected to occur when the hot system of quarks and gluons no longer feel the long range confining force that binds them into hadrons. The other type of phase transition is associated with the restoration of chiral symmetry, corresponding to the melting of ''quark~condensate'' may be found in the ground state of QCD.

The preshower Photon Multiplicity Detector (PMD) allows event-by-event measurement of photon multiplicity and the spatial distribution of photons. 

A thermodynamical picture of strongly interacting matter, fluctuations in various global observables can be related to various thermodynamical properties of matter at freeze out. This in turn can be used to understand thermalisation and critical fluctuations at the QCD phase boundary. We have also seen that fluctuations are sensitive to the nature of phase transition. A detailed study of event-by-event photon multiplicity fluctuations to get some signatures of any possible underlying phase transition is one of the main aims of PMD. 

The PMD allows, by comparing with the charged particle multiplicity measurement, to determine the photon enrichment in an event or event class. This will lead to the understanding of restoration of chiral symmetry will in turn helps in understanding the vacuum structure of strong interaction and the nature of chiral phase transition.
Even if QGP is not formed, it is very important to know the behaviour of matter under high densities and temperatures, which can be reached in heavy ion reactions. The thermodynamical properties of matter in statistical equilibrium is described by equation of state (EOS). The expansion of the system or collective flow is driven by the pressure gradient, depending crucially on the velocity of sound. In a first order phase transition scenario, the reduction of velocity of sound in the transition region is refered to as "softening" of EOS. Centrality and rapidity dependence of flow is sensitive to the equation of state. Photon multiplicity measurements using PMD can be used to study collective flow. 

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Last modified Wednesday 08 November, 2000 by Zubayer Ahammed > []