The ridge(s) in STAR

The STAR Collaboration has studied the pseudorapidity (η) elongated two-particle correlation structure known as the “ridge” since the first RHIC run. This ridge-like feature is observed in two-particle correlations with and without a selected tag, or trigger particle. Un-triggered correlations reveal the ridge structure in minimum-bias Cu–Cu and Au–Au collisions at and 200 GeV. Accompanying this same-side (relative azimuth <π/2) ridge is an away-side ridge on η. Both the same- and away-side structures have corresponding correlation peaks in two-dimensional transverse momentum (pt) space. The pt and event-plane dependences of the same-side correlation structures have also been studied. A summary of the present experimental information and a discussion of the constraints which this body of data imposes on possible theoretical explanations of the ridge phenomenon are presented.