The ridges and the cone as “QGP corona" effects

Abstract

Two experimentally observed structures, called a “cone” and a “hard ridge”, have been discovered in dihadron correlation function with large-pt trigger, while “soft ridge” is a similar structure seen without hard trigger. All three can be viewed as traces left by a moving charge in matter, on top of overall expansion. A puzzle is why those perturbations are apparently rather well preserved till the time of the fireball freezeout, instead of spreading out with the speed of sound. The proposed answer is related to new “magnetic picture of the near-Tc matter, also known as M-phase. Because of small density of free quarks the electric screening mass is also small, thus we argue the (gluo)electric field remains in it for the duration of the process. The consequence is existence of a “QGP corona, not dissimilar to that of the Sun, in which electric fields influence propagation of perturbations and even form metastable flux tubes. The natural tool for its description is (dual) magnetohydrodynamics, which predicts that instead of sound there are two modes, one with larger and one with smaller velocity. The latter can be zero, hinting for formation of pressure-stabilized flux tubes. We argue that those flux tubes are related with cone and ridges. We also find that, contrary to naive expectations, near Tc these tubes (or color strings) should have tighter tension and longer life time than in vacuum: this agrees with PHOBOS clustering data. At the end we discussed expected dependence of ridges and cone on the collision energy, in connection to RHIC energy scan run.