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star focus: Long range rapidity correlations
Highlights from the STAR papers: Long range rapidity correlations and jet production in high energy nuclear collisions and Growth of Long Range Forward-Backward Multiplicity Correlations with Centrality in Au+Au Collisions at sqrt(sNN) = 200 GeV . Submitted for publication to Physical Review C and Physical Review Letters respectively.

Posted: Sep 5, 2009

The STAR experiment has now reported two interesting results on long range correlations in rapidity. One of the experimental observation is from a correlation study in azimthal angle and pseudorapidity for produced charged hadrons with respect to a particle with larger transverse momentum. Such studies revealed a jet-like correlation at small pair phase space separation (in azimuth and pseudorapidity - near side) which seems to be unmodified in central Au+Au collisions relative to d+Au and a significant correlated yield in central Au+Au collisions at large pair separation in pseudorapidity (the RIDGE). The ridge is observed in Au+Au collisions and not observed in d+Au collisions (See figures).


Several models have been proposed to explain the observed broadening of the near-side distributions and the occurrence of the ridge since we first reported this at RHIC. Models based on radiative partonic energy loss suggest that the ridge arises from the coupling of induced gluon radiation to the longitudinal flow of bulk matter, or from the coupling of radiation to transverse chromo-magnetic fields. Other models attribute the ridge to the effect of elastic scattering of the jet in the flowing medium, to medium heating by a jet,to radial flow of bulk matter in coincidence with a jet trigger bias due to energy loss,or to long-range rapidity correlations arising from a Color Glass Condensate initial state. For knowing further details of the properties of the ridge please read the STAR paper [1].


We also obtain the long range correlations strength (b) by measuring the magnitude of the forward-backward multiplicity correlation over a long range in pseudorapidity. The correlation strength is found (see figure) to be almost constant as a function of gap in pseudorapidity. A decrease in such correlations as a function of gap in pseudorapidity would have indicated presence of strong short range correlations. The magnitude of the correlations in central Au+Au is much larger compared to p+p collisions. Models like the Dual Parton Model and Color Glass Condensate argue that the such long range correlations are produced by multiple parton- parton interactions. For further details of the results take a look at STAR paper [2].

Further details can be found in the following STAR papers -
Long range rapidity correlations and jet production in high energy nuclear collisions - arXiv:0909.0191 and Growth of Long Range Forward-Backward Multiplicity Correlations with Centrality in Au+Au Collisions at sqrt(sNN) = 200 GeV - arXiv:0905.0237.


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