Abstract: Viscous Hydrodynamic Description of Untriggered Two Particle Correlations in Nuclear Collisions
At RHIC, measurements of particle production associated with a high pt particle reveal a ridge like
broadening in pseudo-rapidity on the near-side in azimuth . Although this 'e2'80'9chard ridge'e2'80uc0u157 structure has been
associated with the wake of a jet though the medium, two particle correlation measurements, without jet
triggers, show a similar ridge like feature . This 'e2'80'9csoft ridge'e2'80u157 broadens in pseudo-rapidity and narrows
in azimuthal angle with increasing centrality.
In this talk we will show that hydrodynamic flow can explain the soft ridge and extend
this work to address the full behavior of two particle correlations in rapidity and azimuthal angle
. We solve viscous hydrodynamic equations that account for radial and elliptic flow. Our results are
consistent with the centrality dependence found in average transverse momentum, and elliptic flow, v2, measurements.
This model allows us to build a picture of the soft ridge that is consistent with
known distributions. We argue that correlation measurements can greatly reduce the uncertainty in the measured shear
viscosity by explaining the rapidity broadening of the soft ridge with viscous diffusion and we extract
a value of the shear viscosity to entropy ratio. Finally, for comparison, we discuss the relation
of the soft ridge to the jet-associated ridge.
 J. Putschke, et al. (STAR Collaboration), Proceedings of QM06; arXiv:nucl-ex/0701074.
 J. Adams et al. [STAR Collaboration], J. Phys. G32, L37, (2006); arXiv:nucl-ex/0509030.
 G. Moschelli and S. Gavin, in progress; S. Gavin and M. Abdel-Aziz, Phys. Rev. Lett.
97 (2006) 162302.