You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
inclusive charged particle multiplicity measurements at rhic have so far revealed a lot of information on the nature and dynamics of particle production in heavy ion collisions  @xcite . at midrapidity
a significant increase in charged particle production normalized to the number of participating nucleons ( @xmath3 ) has been observed for central au+au collisions compared to peripheral au+au and p+p collisions  @xcite .
this has been attributed to the onset of hard scattering processes , which scale with the number of binary collisions .
alternatively , in the color glass condensate  @xcite picture of particle production at midrapidity , the centrality dependence could reflect increasing gluon density due to the decrease in the effective strong coupling constant . limiting fragmentation ( lf )  @xcite behaviour of inclusive charged particles
have been studied at rhic .
it has been observed that inclusive charged particles follow a energy independent and centrality dependent limiting fragmentation scenario  @xcite .
it will be interesting to study the above physics aspects with inclusive photon multiplicity measurements  @xcite at rhic . in this paper
we present the first photon multiplicity measurements in the forward rapidity at rhic for the au + au collisions at @xmath2 = 62.4 gev in the star experiment .
the photon multiplicity measurements were done in the star experiment  @xcite at rhic by a highly granular gas based photon multiplicity detector ( pmd ) in pseudorapidity region 2.3 @xmath0 @xmath1 @xmath0 3.7 .
the details of the construction and design of pmd can be found in the following ref .
@xcite . the data presented here corresponds to 0 to 80% of au+au hadron cross section .
the minimum bias trigger was obtained from the information of the following trigger detectors .
an array of scintillator slats arranged in a barrel surrounding the time projection chamber ( tpc ) , called central trigger barrel ( ctb ) , which measures charged particles and two zero degree hadronic calorimeters ( zdcs ) at @xmath4 18 m from the detector center  @xcite.the charged particles from tpc within @xmath5 @xmath6 0.5 was used for the centrality selection for data and simulation in the present paper .
@xmath7 for various event centrality classes compared to hijing and ampt model calculations . ]    @xmath7 for various event centrality classes compared to hijing and ampt model calculations . ]    in the present analysis , only the data from the preshower plane of the pmd has been used .
the data analysis proceeded through the following steps : ( a ) calibration of gain of all cells of pmd , ( b ) clustering of hits on pmd and ( c ) photon - hadron discrimination .
details of each of the above steps of analysis of can be found in ref .
fig .  [ fig1 ] shows the minimum bias distribution of @xmath8 along with results from hijing  @xcite and ampt  @xcite models .
we observe that hijing underpredicts the measured photon multiplicity .
ampt slightly overpredicts the total measured photon multiplicity for central collisions .
however , within the systematic errors  @xcite it is difficult to make a firm conclusion . the top @xmath9 central multiplicity distribution ( open circles )
is fitted to a gaussian with a mean of 252 .
variation of @xmath7 and @xmath10 normalized to @xmath11 with @xmath1 - y@xmath12 for central collisions at various collision energies .
the error bars shown are systematic errors . ]
variation of @xmath7 and @xmath10 normalized to @xmath11 with @xmath1 - y@xmath12 for central collisions at various collision energies .
the error bars shown are systematic errors . ]
[ fig2 ] shows the pseudorapidity distribution of photons for various event centrality classes .
the errors shown are a quadratic sum of the systematic and statistical errors .
the results from hijing are systematically lower compared to data for mid - central and peripheral events .
the results from ampt compare well to the data .
[ fig3 ] shows the variation of total number of photons per participant pair in the pmd coverage as a function of the number of participants .
@xmath11 is obtained from glauber calculations  @xcite .
we observe that the total number of photons per participant pair is approximately constant with centrality .
the values from hijing are lower compared to the data .
the values from ampt agree fairly well with those obtained from the data .
[ fig4 ] compares the photon spectra in au + au collisions at @xmath2 = 62.4 gev , with the top sps energy photon data for pb + pb collisions  @xcite and charged particle data at @xmath2 = 130 and 200 gev  @xcite for au+au as a function of @xmath1 - y@xmath12 for central collisions .
the sps and rhic photon results are consistent with each other , suggesting that photon production follows the lf behavior .
however , the photon multiplicity values are lower compared to charged particles .
variation of @xmath7 scaled down to reflect @xmath13 contribution and @xmath14 normalized to @xmath11 for central collisions at various collision energies with y - y@xmath12 .
the error bars include the systematic errors . ]
variation of @xmath7 scaled down to reflect @xmath13 contribution and @xmath14 normalized to @xmath11 for central collisions at various collision energies with y - y@xmath12 .
the error bars include the systematic errors . ]    in fig .
[ fig5 ] we study the centrality dependence of the lf behavior for inclusive photons and compare the photon spectra with those for charged particles . in the forward @xmath1 region ,
the photon production cross section as a function of @xmath1 - y@xmath12 is independent of centrality .
the dependence of lf spectra on the collision system is established by the comparison of charged particles spectra from @xmath15 and @xmath16 collisions at @xmath2 = 53 and 200 gev , respectively , and @xmath17 at 546 gev  @xcite .
we observe that the photon results in the forward rapidity region from @xmath16 collisions at @xmath2 = 546 gev are in close agreement with the measured photon yield in au+au collisions at @xmath2 = 62.4 gev within the lf scenario .
[ fig6 ] shows the total charged pion rapidity density from sps  @xcite and scaled photon rapidity density at @xmath2 = 62.4 gev as a function of y - y@xmath12 .
hijing calculations indicate that about 93 - 96% of the photons are from @xmath13 decays .
the photon results in fig .
[ fig6 ] have been scaled down accordingly to reflect approximately twice the @xmath18 spectrum .
the results show that pion production in heavy ion collisions in the fragmentation region agrees with the lf picture .
in summary , we have presented the first results of photon multiplicity measurements at rhic in the pseudorapidity region 2.3 @xmath0 @xmath1 @xmath0 3.7 .
the pseudorapidity distributions of photons have been obtained for various centrality classes .
photon production per participant pair is found to be approximately independent of centrality in this pseudorapidity region .
comparison with photon and charged pion data at rhic and sps energies shows , for the first time in heavy ion collisions , that photons and pions follow an energy independent limiting fragmentation behavior , as has been previously observed for inclusive charged particles .
however photons , unlike charged particles , follow a centrality independent limiting fragmentation scenario .      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 ; sfom of the czech republic , fom and uu of the netherlands , dae , dst , and csir of the government of india ; the swiss nsf .
we acknowledge the help of cern for use of gassiplex chips in the pmd readout .
9    i.g .
bearden et al . , ( brahms collaboration ) , 2001 _ phys . lett .
_ b * 523 * 227 ; 2002 _ phys .
lett . _ * 88 * 202301 ; i. arsene et al .
, nucl - ex/0410020 .
back et al . , ( phobos collaboration ) , 2001 _ phys .
lett . _ * 87 * 102303 ; 2003 _ phys .
lett . _ * 91 * 052303 ; nucl - ex/0410022 .
we present the first measurement of multiplicity and pseudorapidity distributions of photons in the pseudorapidity region 2.3 @xmath0 @xmath1 @xmath0 3.7 for different centralities in au + au collisions at @xmath2 = 62.4 gev . 
 the pseudorapidity distribution of photons , dominated by neutral pion decays , has been compared to those of identified charged pions , photons , and inclusive charged particles from heavy ion and nucleon - nucleon collisions at various energies . 
 scaling of photon yield with number of participating nucleons and limiting fragmentation scenario for inclusive photon production has been studied .