[Paper Review] Comparison of the space-time extent of the emission source in $d$$+$Au and Au$+$Au collisions at $\sqrt{s_{NN}}=200$ GeV

A. Adare, S. Afanasiev+520 more|arXiv (Cornell University)|Apr 21, 2014

High-Energy Particle Collisions Research4 citations

TL;DR

This study compares the space-time extent of particle emission sources in d+Au and Au+Au collisions at √sNN = 200 GeV using Hanbury Brown-Twiss (HBT) interferometry. It finds that HBT radii scale linearly with the initial geometric size R̄ across d+Au, Au+Au, and Pb+Pb systems, indicating hydrodynamic-like collective expansion driven by final-state rescattering, with similar dynamics in small and large systems.

ABSTRACT

Two-pion interferometry measurements in $d$$+$Au and Au$+$Au collisions at $\sqrt{s_{NN}}=200$ GeV are used to extract and compare the Gaussian source radii R$_{ m out}$, R$_{ m side}$, and R$_{ m long}$, which characterize the space-time extent of the emission sources. The comparisons, which are performed as a function of collision centrality and the mean transverse momentum for pion pairs, indicate strikingly similar patterns for the $d$$+$Au and Au$+$Au systems. They also indicate a linear dependence of R$_{ m side}$ on the initial transverse geometric size $\bar{R}$, as well as a smaller freeze-out size for the $d$$+$Au system. These patterns point to the important role of final-state rescattering effects in the reaction dynamics of $d$$+$Au collisions.

Motivation & Objective

• To investigate whether small systems like d+Au exhibit similar space-time dynamics as large Au+Au collisions.
• To determine if final-state rescattering effects, known in Au+Au, also influence d+Au emission source geometry.
• To test whether HBT radii scale with the initial transverse geometric size R̄ across different collision systems and energies.
• To assess the universality of hydrodynamic-like expansion in high-multiplicity hadronic collisions.
• To provide evidence for collective dynamics in small systems using HBT radii and geometric scaling.

Proposed method

• Measure HBT radii R_side, R_out, and R_inv from two-particle momentum correlations in d+Au and Au+Au collisions at √sNN = 200 GeV.
• Extract emission source sizes using the Gaussian source model and fit to two-particle correlation functions.
• Compare HBT radii as functions of transverse mass mT and average transverse momentum kT across systems.
• Apply geometric scaling by plotting HBT radii versus the initial geometric size R̄, estimated from participant nucleon count N_part.
• Use linear fits to test scaling behavior of R_side and R_inv with R̄ across d+Au, Au+Au, and Pb+Pb systems.
• Compare results with theoretical models predicting τ ∝ R̄ and hydrodynamic scaling patterns.

Experimental results

Research questions

• RQ1 Do d+Au and Au+Au collisions at √sNN = 200 GeV exhibit similar space-time extent in their particle emission sources?
• RQ2 Is the scaling of HBT radii with the initial geometric size R̄ consistent across d+Au, Au+Au, and Pb+Pb systems?
• RQ3 To what extent do final-state rescattering effects shape the emission source geometry in d+Au collisions?
• RQ4 Does the observed R̄ scaling support hydrodynamic-like collective expansion in small systems like d+Au?
• RQ5 How do HBT radii in d+Au compare quantitatively to those in Au+Au at similar N_part and kT?

Key findings

• HBT radii R_side and R_inv scale linearly with the initial geometric size R̄ in both d+Au and Au+Au collisions at √sNN = 200 GeV.
• The slope of the R_side vs. R̄ relation is similar for d+Au and Au+Au systems, indicating comparable expansion dynamics.
• For Pb+Pb at √sNN = 2.76 TeV, the R_side vs. R̄ slope is larger, consistent with faster expansion at higher energy.
• The observed R̄ scaling is consistent with the 1/R̄ scaling of collective anisotropic flow, supporting hydrodynamic behavior.
• The similarity in HBT radius patterns between d+Au and Au+Au suggests that final-state rescattering plays a dominant role in both systems.
• The results imply that hydrodynamic-like collective expansion is not exclusive to large systems and may extend to small systems like d+Au.