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 [1].  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.


[1] J. Adams et al. (STAR Collaboration), Phys. Rev. C 73, 064907 (2006).

[2] J. Adams et al. (STAR Collaboration), J. Phys. G 32, L37 (2006).

[3] T. A. Trainor,  Phys.  Rev. C 80, 044901 (2009).

[4] T. A. Trainor and D. T. Kettler, Phys. Rev. C. 83, 034903 (2011).

[5] T. A. Trainor, J. Phys. G 37, 085004 (2010).