Soft contribution to the hard ridge in relativistic nuclear collisions

Abstract

Nuclear collisions exhibit long-range rapidity correlations not present in proton-proton collisions. Because the correlation structure is wide in relative pseudorapidity and narrow in relative azimuthal angle, it is known as the ridge. Similar ridge structures are observed in correlations of particles associated with a jet trigger (the hard ridge) as well as correlations without a trigger (the soft ridge). Earlier we argued that the soft ridge arises when particles formed in an early Glasma stage later manifest transverse flow. We extend this study to address new soft ridge measurements. We then determine the contribution of flow to the hard ridge.

Long range correlations and hydrodynamic expansion

“…[22]. Measurements at various beam energies, target masses and centralities are in excellent accord with the leading-order dependence [6,7].…”

“…Experimenters measure the quantity ρ sib (φ, η), which represents the distribution of "sibling pairs" from the same event, as a function of relative pseudorapidity and relative azimuthal angle, (6) from Ref. [7]. Dashed lines replace Glasma with independent NN initial conditions.…”

“…We argue that these long range correlations result from Glasma flux tubes that form as the nuclei crash through one another [5][6][7][8]. Color Glass Condensate (CGC) theory predicts an early Glasma stage in the high energy collision in which particles are produced by strong longitudinal color fields.…”

“…[5][6][7][8] we describe both the jet-triggered hard ridge and the untriggered soft ridge as consequences of particle production in an early Glasma stage followed by transverse flow. Our description is part of a broader family of models in which particles are initially correlated at the point of production [9][10][11][12][13][14][15][16].…”

“…Furthermore, fluid cells at a larger radial position have a larger final transverse velocity and the relative angle between parton momentum vectors is narrower. Many of these authors -ourselves included -use blast wave models to characterize the effect of flow [9,10,6,7,[11][12][13].…”