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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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