Novel phenomena in particle correlations in relativistic heavy-ion collisions

The novel phenomena observed in particle angular correlations are reviewed. They include the double-peak away-side azimuthal correlations in relativistic heavy-ion collisions and the long-range pseudorapidity near-side (ridge) correlations in heavy-ion as well as in proton-induced collisions. The collision system and energy dependence of these phenomena are examined, wherever possible and most abundantly for the ridge correlations. Their possible theoretical interpretations and what might be learned about the properties of the collision systems from theoretical comparisons are discussed. Prospective future measurements and theoretical undertakings are outlined that might help further the understanding of the physics mechanisms underlying these phenomena.