The "soft ridge" -- is it initial-state geometry or modified jets?, T. Trainor, University of Washington, USA − A 2D peak at the angular origin is a prominent feature of all angular correlations on pseudorapidity eta and azimuth phi from nuclear collisions. In p-p collisions and more-peripheral A-A collisions the 2D peak properties are consistent with minimum-bias jets. In more-central Au-Au collisions the peak becomes strongly elongated in the eta direction but remains quite narrow in the phi direction . The detailed shape on eta depends strongly on the presence or absence of applied (trigger-associated) pt cuts. The elongated 2D peak without pt cuts has been referred to as the "soft ridge." It has been conjectured that the ridge formation mechanism may be some aspect of initial-state collision geometry coupled with radial flow. Possible initiating structures include Glasma flux tubes and initial-state overlap-geometry fluctuations. However, some systematic properties of the 2D peak remain consistent with jet formation, albeit with substantial modification of the fragmentation process [2,3,4]. And some measured peak properties seem to contradict hydro-based interpretations [5,6]. In this talk I will review arguments for several interpretations in comparison to available spectrum and correlation data.
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