Long range rapidity correlations and jet production in high energy nuclear collisions

Abstract

The STAR Collaboration at the Relativistic Heavy Ion Collider presents a systematic study of high-transversemomentum charged-di-hadron correlations at small azimuthal pair separation φ in d + Au and central Au + Au collisions at √ s NN = 200 GeV. Significant correlated yield for pairs with large longitudinal separation η is observed in central Au + Au collisions, in contrast to d + Au collisions. The associated yield distribution in η× φ can be decomposed into a narrow jet-like peak at small angular separation which has a similar shape to that found in d + Au collisions, and a component that is narrow in φ and depends only weakly on η, the "ridge." Using two systematically independent determinations of the background normalization and shape, finite ridge yield is found to persist for trigger p t > 6 GeV/c, indicating that it is correlated with jet production. The transverse-momentum spectrum of hadrons comprising the ridge is found to be similar to that of bulk particle production in the measured range (2 < p t < 4 GeV/c).

Azimuthal di-hadron correlations ind+Au and Au + Au collisions atsNN

“…It has also been argued recently that this long-range correlation in η could be caused by long-range structures in the medium, due to density fluctuations in the medium [29] or color flux tubes [30][31][32]. The "ridge"-like correlation structure in η is further explored in other STAR publications [26,[33][34][35].…”

“…A softer fragmentation also implies that a trigger particle of given momentum selects different parton energies in Au + Au collisions than in d + Au collisions, which could explain some of the enhancement of associated yield at lower p assoc T in Au + Au collisions. However, it should be noted that a large part of the increased yield at lower p assoc T is at large η, associated with the ridge effect, and is therefore not necessarily from jet fragments [34]. It is also possible that other sources of particle production, such as parton coalescence and resonance decays, contribute at lower p trig T and may lead to different behavior in d + Au and Au + Au.…”

Schwinger mechanism revisited

“…radial hydrodynamical flow [37,38]). In heavy ion collisions, strong correlations have been observed between pairs of hadrons [39][40][41][42], characterized by a ridge shape, very elongated in the relative rapidity of the two particles, and peaked in their relative azimuthal angle. By causality, the correlations in rapidity have to be created in the very early stages of the collisions [43], and they can be simply understood as a consequence of the near boost invariance of the sources of the incoming nuclei.…”

Two-gluon correlations in heavy–light ion collisions: Energy and geometry dependence, IR divergences, and kT-factorization

“…The discovery of the long-range rapidity correlation known as the 'ridge' in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) [2][3][4][5] spurred, among other things, a flurry of activity [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] aimed at better understanding two-particle correlations in the parton saturation/Color Glass Condensate (CGC) physics framework (see [22][23][24][25][26][27] for reviews of the saturation/CGC field). Apart from quantifying how much of the 'ridge' dynamics, which in the meantime was also observed by experiments at the Large Hadron Collider (LHC) in proton-proton (pp) and protonnucleus (pA) collisions [28][29][30][31], is due to the initial-state saturation effects, the problem of two-gluon production in nucleus-nucleus (AA) collisions is an important theoretical problem in its own right, allowing us to gain a new insight in the nonlinear dynamics of strong gluon fields in the initial stages of heavy ion collisions.…”