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Several interesting features of the dependence of particle density in pseudorapidity have been observed in Au+Au collisions from the experiments at the Relativistic Heavy-Ion Collider (RHIC). Particle production is found to follow a unique, collision energy independent, longitudinal scaling in p+p and d+Au, as well as in heavy-ion collisions. Such longitudinal scaling is also found to be independent of collision centrality for photons. The total charged particle multiplicity (integrated over the full pseudorapidity range) per average number of participating nucleon (< Npart >) pair is found to be independent of collision centrality by PHOBOS experiment. However, at mid-rapidity (|eta| <1), the PHENIX experiment showed that charged particle multiplicity per < Npart > is observed to increase from peripheral to central collisions. The charged particle production scales with a combination of < Npart > and average number of binary collisions < Nbin >. These clearly indicates that the mechanism of particle production could be different in different pseudorapidity regions. It is believed that the scaling of particle multiplicity with < Npart > indicates the dominance of soft processes in particle production, whereas scaling with average number of binary collisions (< Nbin >) indicates the onset of hard processes (pQCD jets). In the year 2005, a unique opportunity to investigate the system-size dependence of global observables occurred when Cu+Cu collisions were produced at RHIC. Figure shows the comparison of photon multiplicity per average number of participating nucleon pair vs. < Npart > and the corresponding data for charged particles from the PHOBOS for the range -3.7 < eta < -2.3. Both the photon production, the charged particle multiplicity at forward rapidities are found to scale with < Npart >. For similar < Npart >, the both the photon and charged particle production in the region -3.7 < eta < -2.3 is also found to be similar for Au+Au and Cu+Cu at a given beam energy. This forward rapidity particle production scaling with < Npart > is different from what was observed at midrapidity, where particle production was found to scale with a combination of < Npart > and < Nbin >. This indicates the mechanism of particle production has a rapidity dependence at RHIC energies. Further details can be found in the following STAR paper -
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