The “ridge”, an elongated (in rapidity) structure in the two-particle correlations associated with a hard trigger hadron, keeps attracting the attention of theorists. [Stay tuned for a forthcoming review of the experimental status of the “ridge”.] Two recent preprints explore strikingly different explanations. The first,
treats the ridge as an initial-state effect. The authors point out that the enhanced two-particle correlations are a natural consequence of the correlations in the classical color fields responsible for multi-particle production in relativistic heavy-ion collisions. If the fields are denoted by A, then single hadron production is proportional to 〈A4〉 but two-hadron production is proportional to 〈A8〉, always leading to a positive correlation, even for Gaussian intensity fluctuations. Because the fields extend over a wide rapidity range, so do the correlations. The narrow azimuthal extent of the “ridge” is qualitatively, but not quantitatively, explained as a focussing effect caused by the transverse flow of the medium.
The other preprint,
explores a final-state effect as the origin of the ridge. The authors explore the effect of transversely oriented turbulent color fields, generated by instabilities in the preequilibrium phase of the quark-gluon plasma, on the propagation of a jet. They perform the first self-consistent microscopic simulation of this mechanism and show that the jet is preferentially broadened in the longitudinal direction. The authors do not attempt a quantitative comparison with the data.