The "Shoulder" and the "Ridge" in PHENIX

Abstract

The observation of jet quenching in ultra-relativistic heavy ion collisions demonstrates significant energy loss of fast partons when passing through the created medium. Correlations between final-state particles at intermediate transverse momentum (1.0 $\lesssim$ $p_{T}$ $\lesssim$ 4.0 GeV/$c$) allow for study of the medium and its response to deposited energy. Comparison of these measurements in heavy ion collisions with measurements in proton collisions show strong modification of the correlation shape and particle yields. Two new structures are created, both extended in $Δη$, one centered at $Δϕ=0$ (``ridge'') and the other occurring at $Δϕ\approxπ\pm1.1$ (``shoulder''). In these proceedings, we describe the measurements of these structures that show consistency with a scenario of parton-medium interaction and response. We discuss a new analysis which selects on the angle of trigger particles relative to the reaction plane in Run7. New measurements of the centrality and $p_{T}$ dependencies of the structures raise the possibility that the same production mechanism may give rise to both phenomena.