The puzzle of the fall-off of the same-side ridge in relative pseudorapidity, found in unbiased two-particle correlations, is solved. We show that the event-by-event hydrodynamics followed by statistical hadronization with proper charge conservation provides the crucial non-flow component and leads to agreement with the data at soft transverse momenta (p_T < 2 GeV). The fall-off of the same-side ridge follows from the fact that a pair of particles with opposite charges is emitted from the same fluid element, whose collective velocity collimates the momenta of the pair. Basic experimental features of the two-dimensional correlation functions are then represented, including the dependence on the relative charge (like-sign and unlike-sign pairs) and centrality. Related quantities, such as the charge balance functions or the dependence of the harmonic flow coefficients on relative pseudorapidity, are also properly explained in our approach.
Wojciech Broniowski (IFJ PAN & UJK)