Influence of initial-state momentum anisotropy on the final-state collectivity in small collision systems

Abstract

A multi-phase transport model is used to understand the origin of long-range collective azimuthal correlations in small-system collisions. To disentangle between collectivity associated with initialstate intrinsic momentum anisotropy and the collectivity arising as a final-state response to the collision geometry, we studied the development of collectivity in 5.02 TeV p+Pb collisions with both initial-state and final-state effects included. We find that the initial momentum anisotropy may not be fully isotropized through parton interactions, and the final-state partonic collectivity in general are correlated with both the initial momentum anisotropy and the shape of the collision geometry. The initial momentum anisotropy also influences the event by event fluctuation of collective flow. Therefore the mere evidence of geometry response of the collective flow can not rule out the presence of large contributions from the initial state.PACS numbers: 25.75. Gz, 25.75.Ld, In high-energy hadronic collisions, particle correlations are important tools to study the multi-parton dynamics of QCD in the strongly-coupled non-perturbative regime [1]. Measurements of azimuthal correlations have revealed a strong harmonic modulation of particle densi-where v n and Ψ n represent the magnitude and the phase of the n thorder harmonic, and are often denoted by flow vector V n = v n e inΨn [2]. The azimuthal correlations are found to be collective, involving many particles spread over a wide pseudorapidity range. Such azimuthal anisotropy was first observed in large A+A collision system [2-5], but are then observed and studied in small collision systems such as pp and p+Pb collisions at the LHC [6-13] and p+Au, d+Au and 3 He+Au collisions at RHIC [14][15][16][17][18][19].Although azimuthal anisotropy in A+A collisions is naturally explained as a result of hydrodynamic collective expansion of the hot and dense matter produced in the collision [20], the applicability of hydrodynamic picture for the azimuthal anisotropy in small collision systems such as pp or p+A collisions remains an open question [21][22][23]. It has been argued that the size is too small and life-time is too short for the matter in small system to hydrodynamize and approach local isotropization [24]. Instead, the azimuthal anisotropy may reflect intrinsic long-range momentum correlations of the dense gluon field right after the collision [25][26][27]. The current debate is focused on the timescale for the emergence of * jiangyong.jia@stonybrook.edu † glma@fudan.edu.cn collectivity: Is the collectivity born in the initial state, developed during non-equilibrium transport before the system hydrodynamizes, or arises even later when the system can be described by hydrodynamics? The latter two scenarios lead to a collectivity that correlates with the initial spatial eccentricities, while the first does not.The system produced right after the collision is highly anisotropic in momentum space, and the strong interactions among the constituents of the produce system tend to ...

Effect of non-eikonal corrections on azimuthal asymmetries in the color glass condensate

“…But for small systems such explanation is a matter of active debate, see e.g. the recent works [20,21,22], and initial state dynamics have also been invoked.…”

Testing the collectivity in large and small colliding systems with test particles

“…However, the case 2 corresponds to the case where the test particles have an initial collective flow due to their common initial velocities. Note that the initial collective flow is only introduced to the test particles for case 2, which is different from our previous study where the initial collectivity was introduced to all particles of the system [35]. If we consider that the mass of the test particle is 0.05 GeV (since we always take the test particles as light quarks), with the initial velocity β the transverse momentum p T = β/( 1 − β 2 )m q ≈ 0.1 GeV/c.…”

“…It can be basically classified into two categories depending on whether the origin comes from the initial or final state. The initial state of color glass condensate (CGC) has also been proposed as a possible mechanism, contributing to the experimentally measured 'flow' in small colliding systems [27][28][29][30][31][32][33][34][35]. The final state of hydro- * glma@fudan.edu.cn dynamics also can transform the initial geometric asymmetry into the final momentum anisotropic flow through the pressure gradient of the QGP [36][37][38][39][40][41][42].…”

System dependence of away-side broadening and $α$-clustering light nuclei structure effect in dihadron azimuthal correlations

“…It was found that small collision systems such as p + p and p + A in highmultiplicity present resembling properties with large A + A collisions [39]. Due to minor size and short lifetime, small-size A+A systems may undergo a modified dynamical process and are expected to shed light on initial fluctuation effects on momentum distribution in the final stage [25,26,[40][41][42].…”