Fluctuation probes of early-time correlations in nuclear collisions

Abstract

Correlation measurements imply that anisotropic flow in nuclear collisions includes a novel triangular component along with the more familiar elliptic-flow contribution. Triangular flow has been attributed to event-wise fluctuations in the initial shape of the collision volume. We ask two questions: 1) How do these shape fluctuations impact other event-by-event observables? 2) Can we disentangle fundamental information on the early time fluctuations from the complex flow that results? We study correlation and fluctuation observables in a framework in which flux tubes in an early Glasma stage later produce hydrodynamic flow. The calculated collision-energy dependence of multiplicity and transverse momentum fluctuations is in good accord with data from 62.4 GeV Au+Au up to 2.76 TeV Pb+Pb.

Event-by-event mean $$\varvec{p}_{\mathbf {T}}$$ p T fluctuations in pp and Pb–Pb collisions at the LHC

“…In a recent approach [24], initial spatial fluctuations of glasma flux tubes have been related to mean transverse momentum fluctuations of final state hadrons via their coupling to a collective flow field. A comparison of these calculations to data from ALICE and STAR is shown in [24].…”

“…A number of possible mechanisms have been proposed to explain this behavior, such as string percolation [21] or the onset of thermalization and collectivity [22,23], but no strong connection to critical behavior could be made. It was recently suggested [24,25] that initial state density fluctuations [26] could affect the final state transverse momentum correlations and their centrality dependence.…”

Onset of the ridge structure inAA,pA, andppcollisions

“…The origin of long range rapidity correlations is similar in heavy ion and proton-proton collisions and it is explained, in the glasma picture of the Color Glass Condensate (CGC), due to the saturation of color flux tubes correlated in the transverse space with a length 1/Q s determined by the saturation momentum Q s [18][19][20][21][22][23][24]. On the contrary, not much attention has been paid to the onset of the ridge structure.…”

“…More recently, a sizable ridge has been seen in p-Pb collisions at √ s = 5.02 TeV [9][10][11][12][13]. Much attention has been paid to understand whether these structures are due to initial state or to final state effects that are amenable to a hydrodynamic description [14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”

Initial state fluctuations and final state correlations in relativistic heavy-ion collisions

“…In [148], the final transverse momentum correlations are related to the density correlation function in the initial profile. The transverse momentum fluctuations have also been studied in [149] in an event-by-event viscous hydrodynamic approach and the relation to structures in the initial state profile has been confirmed.…”