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the quest for understanding of the possible formation and existence of the quark - gluon plasma ( qgp ) , the deconfined phase of quarks and gluons , has been a major area of heavy - ion research during the last couple of decades .
the study of high energy nuclear collisions at the relativistic heavy ion collider ( rhic)[1 ] has opened a new domain in the exploration of strongly interacting matter at very high enegy density .
high temperature and densities may be generated in the most central nuclear collisions at rhic , creating conditions in which a phase of deconfined quarks and gluons may exist [ 2,3 ] .
the understanding of the early phase of the fireball produced in nuclear collisions requires the study of observables like the energy and momentum , produced transversely to the beam direction viz @xmath5 and @xmath6 .
the number of charge particles produced and @xmath5 are closely related to the collision geometry and are of importance in understanding the global properties of the system formed during the collision .
scattering of the partonic constituents of the incoming nuclei in the initial phase added to the rescattering of the produced partons and hadrons results in generation of @xmath5[4,5 ] .
if the fireball of produced quanta breaks apart quickly without significant reinteraction , the observed @xmath7 will be the same as that produced by the initial scattering .
however , with interaction among the produced quanta , the system can achieve equilibrum at a very early state , after which it can expand resulting in a lowering of @xmath7[6,7 ] . to some extent this will be compensated by any transverse hydrodynamic flow[8 ] .
gluon saturation can delay the onset of the above hydrodynamic flow reducing the effective pressure and thereby reducing the difference in the initially produced and the observed @xmath5[9 ] . + here ,
we present preliminary results on @xmath5 , produced in 62.4 gev au+au collisions at rhic .
we present both the hadronic and the electromagnetic components of @xmath5 , which were measured independently by the star detector using the time projection chamber ( tpc ) and the barrel electromagnetic calorimeter ( bemc ) .
a brief description of the star experiment with the detectors used together with the data analysis methods employed is given in section-2 .
in section-3 we present the results ( star preliminary ) with conclusion presented in section-4 .
star[10 ] , is an azimuthally symmetric , large acceptance solenoidal detector comprising of several detector subsystems .
the subsystems relevant for this analysis are a large tpc located inside a 0.5 t solenoidal magnet , the bemc and two zero - degree calorimeters ( zdcs ) for event selection .
the bemc[11 ] is a lead - scintillator sampling electromagnetic calorimeter with equal volumes of lead and scintillator .
the full barrel corresponds to 120 modules .
each module is composed of 40 towers ( 20 towers in @xmath8 by 2 towers in @xmath9 ) , constructed to project to the center of the star detector . for the 2004 run ,
only half of the barrel was instrumented , corresponding to a pseudorapidity coverage of @xmath10 with full azimuthal symmetry .
the transverse dimensions of a tower are approximately @xmath11 , which at the radius of the front face of the detector coresponds to a phase space interval @xmath12 of ( 0.05 , 0.05 ) .
each tower has a depth of 21 radiation lengths(@xmath13 ) corresponding to one interaction length for a hadron .
the bemc with a radius of 2.3 m sits inside the star solenoidal magnet .
the electromagnetic energy resolution of the bemc is @xmath14 .
+ the tpc[12 ] is the primary star detector used for the event reconstruction .
it is a gas chamber , 4.2 m long with inner and outer radii of 50 and 200 cm respectively , placed in an uniform magnetic field of 0.5 t. the particles passing through the active gas medium release secondary electrons that drift to the readout end caps at both ends of the chamber .
the readout system is based on multiwire propotional counters , with readout pads .
there are 45 pad rows between the inner and outer radii of the tpc .
the induced charge from the electrons is shared over several adjacent pads .
the tpc provides up to 45 independent spatial and specific ionization @xmath15 measurements .
the @xmath15 measurements , along with the momentum measurement from the bending of the tracks inside the magnetic field , determine the particle mass within limited kinematic regions .
the tpc covers a pseudorapidity region with @xmath16 with full azimuthal coverage .
+ the event trigger consisted of the coincidence of signals from two zdcs , located at @xmath17 mrad about the beam down stream of the first accelerator dipole magnet and are sensitive to spectactor neutrons .
these calorimeters provide a minimum bias trigger which , after collision vertex reconstruction , coresponds to @xmath18 of the geometric cross section @xmath19 . the events
are analyzed in centrality bins based on the tpc charged particle multiplicity in @xmath20 .
+ the present analysis is based on the minimum bias au+au collisions data at @xmath1 gev , taken by star in the 2004 rhic run . here
the tpc acceptance is limited by the bemc .
the tpc track quality cuts include a ) z - coordinate ( longitudinal axis ) selection of collision vertex within 30 cm of the tpc center and b ) a minimum tpc track space point cut of 10 .      the hadronic part of the transverse energy as measured from the momentum analysed tpc tracks is defined as + @xmath21 where the sum runs for all hadrons produced in the collision , except @xmath22 , @xmath8 and other long - lived neutral hadrons .
@xmath23 is the polar angle with respect to the beam axis and the collision vertex position .
the hadronic energy , @xmath24 , is defined as [ 13,14 ]    @xmath25    with the above definition , @xmath26 where the sum includes all the primary tracks in bemc acceptance . here
, @xmath27 is a correction factor defined as @xmath28 where @xmath29 is the acceptance correction , @xmath30 is the correction for long - lived neutral hadrons not measured by the tpc , @xmath31 corresponding to the tpc low momentum cutoff .
the factor @xmath32 is defined as @xmath33 which includes the uncertainty in the particle i d determination , @xmath34 , the momentum dependent tracking efficiency , eff(pt ) and the momentum dependent background , @xmath35 .
details of the procedure for finding out the correction factors are given elsewhere[13 ] .
@xmath36 is measured from the bemc tower hits corrected for the hadronic contaminations in the calorimeter .
it is defined as @xmath37 where @xmath38 is the electromagnetic energy measure in an bemc tower and @xmath39 is the polar angle of the center of the tower relative to the beam axis and the collision vertex position .
experimentally , @xmath40 is given by @xmath41 where , @xmath29 is the acceptance correction , @xmath42 is the energy measured by an bemc tower and @xmath43 is the total correction for each tower to exclude the hadronic contributions .
@xmath43 is given by @xmath44 where , @xmath45 is the energy deposited by a track projected on a bemc tower as a function of it s momentum @xmath46 , pseudorapidity @xmath8 and distance @xmath47 from the center of the tower to the track hit point .
@xmath48 is the correction to exclude electrons that are misidentified as hadrons .
eff(pt ) is the track reconstruction efficiency and @xmath30 is the contribution to exclude the long - lived neutral hadron contribution .
assuming the correction factors used in this analysis , to be the same as those obtained for 200 gev au+au collisions[13 ] , we have determined the minimum bias distributions of the total tranverse energy , @xmath5 and it s components ( @xmath49 and @xmath40 ) separately .
these are shown in fig.1.(a)-(c )     gev au+au collisions.,title="fig:",width=220 ]   gev au+au collisions.,title="fig:",width=220 ]   gev au+au collisions.,title="fig:",width=220 ]    in fig .
2(a ) , we present @xmath7 per participant pair for the top 5% central collisions , together with results from other experiments from ags to rhic[15 - 18 ] .
the @xmath7 values for this analysis , were calculated from @xmath50 using a factor of 1.18 obtained from hijing simulation to convert @xmath8 to @xmath51 phase space .
we obtained a value of 3.05 @xmath52  0.05  ( stat ) gev , consistent with an overall logarithimic growth of @xmath53 with @xmath54 .
for the same top 5% central events , @xmath55 as obtained from @xmath56 for full azimuthal coverage and one unit @xmath8 interval , is found to be 450 @xmath576  ( stat ) gev .
further , we have also determined the spatial energy density produced in the collision using the bjorken formula[19 ] @xmath58 where , @xmath7 is the primordial rapidity density of transverse energy , r is the transverse system size given by @xmath59 and @xmath60 is the formation time .
assuming @xmath60 = 1 fm / c , we get @xmath61 .
this energy density is significantly higher than the energy density @xmath62 @xmath63 as required for the transition to a deconfined quark gluon plasma , predicted by lattice qcd [ 20 ] .
this estimate is based on the assumption that local equilibrium has been achieved at @xmath64 and then the system expands hydrodynamically .
+     for central events , ( b ) collision energy dependence of electromagnetic fraction of total @xmath5
as given by @xmath65 for a number of systems from sps to rhic for central events , title="fig:",width=336 ]   for central events , ( b ) collision energy dependence of electromagnetic fraction of total @xmath5 as given by @xmath65 for a number of systems from sps to rhic for central events , title="fig:",width=336 ]    in fig .
2(b ) , we have shown the electromagnetic fraction of the total transverse energy for the top 5% central events , as a function of the center of mass energy from sps[21,22 ] to rhic .
this is seen to increase slowly when we go from ags to rhic . at 62.4 gev
this value is @xmath66   ( stat ) . as discussed in ref.[13 ] , the observed electromagnetic fraction of the total transverse energy is strongly dependent on the baryon to meson ratio . at very high energy
it is expected that virtually all the @xmath5 will be carried by mesons , as an almost baryon free region is expected to be created in the central rapidity region , while at lower sps energies , baryon dominance results in a much smaller electromagnetic fraction .
a very high value of the electromagnetic fraction of total transverse energy is expected in case of a long - lived deconfined phase , due to an excess yield of photons[23 ] .
based on the present ratio of @xmath66   ( stat ) it is very difficult to conclude anything regarding the formation of a deconfined qgp phase .
in the present work we have reported priliminary star results on @xmath5 within @xmath10 for au+au collissions at @xmath67 gev . for top 5% central events @xmath68 has been estimated to be 450 @xmath69  ( stat ) gev .
the value of @xmath70 has been found to be @xmath71  ( stat ) gev .
knowing that the observed @xmath5 is lower than the initial values [ 6,9,24 ] , the present value of @xmath71  ( stat ) gev may be considered as a lower bound only . the initial energy density estimated within the framework of boost - invariant hydrodynamics , as given by @xmath72 , has been found to be @xmath73 which is well above that required for the deconfinement phase transition as predicted by lattice qcd[20 ] .
the electromagnetic fraction of the total transverse energy for central events is found to be @xmath74  ( stat ) , consistent with the fact that the final state is dominated by mesons .
finally , this method of independent measurement of @xmath40 and @xmath49 gives an unique opportunity to study event by event fluctuations in these observables and in their ratios .
the correction factors and the systematic errors are yet to be calculated for this analysis .
we thank the rhic operations group and rcf at bnl , and the nersc center at lbnl for their support .
this work was supported in part by the henp divisions of the office of science of the u.s .
doe ; the u.s .
nsf ; the bmbf of germany ; in2p3 , ra , rpl , and emn of france ; epsrc of the united kingdom ; fapesp of brazil ; the russian ministry of science and technology ; the ministry of education and the nnsfc of china ; irp and ga of the czech republic , fom of the netherlands , dae , dst , and csir of the government of india ; swiss nsf ; the polish state committee for scientific research ; staa of slovakia , and the korea sci . & eng . foundation .
the transverse energy distributions ( @xmath0 ) have been measured for au + au collisions at @xmath1 gev by the star experiment at rhic . 
 they have been obtained from two measurements , the hadronic transverse energy ( @xmath2 ) and the electromagnetic transverse energy(@xmath3 ) . 
 @xmath2 has been measured from the tracks obtained by time projection chamber ( tpc ) excluding the electrons and positrons . 
 @xmath3 has been obtained by the star barrel electromagetic calorimeter ( bemc ) which measures the energy of electrons , positrons and photons . 
 the measure of transverse energy gives an estimate of the energy density of the fireball produced in heavy ion collisions . 
 @xmath0 per participant pair gives information about the production mechanism of particles .     
 corresponding author : raghunath sahoo@xmath4 ( email : raghu@iopb.res.in ) ]