Investigating the transverse-momentum- and pseudorapidity-dependent flow vector decorrelation in p--Pb collisions with a Multi-Phase Transport model

TL;DR

The paper tackles how flow-vector decorrelations in p--Pb collisions depend on transverse momentum and pseudorapidity. It adopts the AMPT framework with varied initial conditions, partonic cross sections, and hadronic interactions to disentangle contributions from early-state fluctuations, parton dynamics, and nonflow. Key findings show that partonic interactions and initial conditions drive the decorrelations, while hadronic rescattering is largely negligible, with long-range jet correlations playing a crucial role in longitudinal decorrelations. These insights help constrain transport properties of the medium in small systems and guide future measurements of decorrelation observables.

Abstract

The event-by-event fluctuations in the initial energy density of the nuclear collisions lead to the decorrelation of second order flow vector, as known as its transverse-momentum ($p_{\mathrm{T}}$) and pseudorapidity ($η$) dependence as observed in high-energy heavy-ion collisions. Existing measurements at the CERN Large Hadron Collider shown that these decorrelations are also observed in small collision systems. In this work, a systematic study of the transverse-momentum- and pseudorapidity-dependent flow vector decorrelation is performed in p--Pb collisions at the 5.02 TeV with A Multi-Phase Transport (AMPT) model using different tunings of the initial conditions, partonic and hadronic interactions. It is found that the string-melting version of the AMPT model provides a reasonable description of the measured flow vector decorrelation as a function of $p_{\mathrm{T}}$ and $η$. We demonstrate that the hadronic scatterings do not have significant impact on decorrelation in p--Pb collisions for different centrality selections, while both initial conditions and partonic interactions influence the magnitude of the decorrelations. In addition, we found that the subtraction of the nonflow, especially the long-range jet correlation, is crucial for the accurate extraction of the flow vector decorrelation in small collision systems. The comparison of data and model presented in this paper provide further insights in understanding the fluctuations of the flow vector with $p_{\mathrm{T}}$ and $η$ in small collision systems and has referential value for future measurements.

Figures (6)

• Figure 1: (Color online) The $v_{2}\{2\}$ and $v_{2}[2]$ as a function of $p_{\mathrm{T}}$ and their ratio obtained from the AMPT model in 0--20%, 20--40% and 40--60% p--Pb collisions at 5.02 TeV, in comparison of the ALICE measurements. The results with the nonflow subtraction obtained from the AMPT model are also shown.
• Figure 2: (Color online) The $p_{\mathrm{T}}$-dependent $v_{2}\{2\}$/$v_{2}[2]$ ratio in 0--20%, 20--40%, 40--60% centrality classes from the AMPT model with different configurations. ALICE data points are shown as black open circles for comparison.
• Figure 3: (Color online) The factorization ratio $r_2$ as a function of $p^{a}_{\mathrm{T}}$, in different centrality and $p^{b}_{\mathrm{T}}$ ranges, obtained from the AMPT model with various configurations. ALICE data points are shown as black open circles for comparison.
• Figure 4: (Color online) The $v_{2}$ as a function of pseudorapidity $\eta$ in in different centrality ranges, obtained from the AMPT model with various configurations. ALICE data points are shown as black open circles for comparison.
• Figure 5: (Color online) The square root of the product of factorization ratios, $\sqrt{r_{2}(\eta^{a},\eta^{b})r_{2}(-\eta^{a},-\eta^{b})}$, as a function of $\eta^{a}$ for 3.0 $< \eta^{b} <$ 4.0 (left) and $4.4 < \eta^{b} < 5.0$ (right), in 0--20% centrality class of p--Pb collisions at 5.02 TeV, obtained from the AMPT model with various tunings. The CMS data points are shown as black open circles for comparison.