[Paper Review] Azimuthal anisotropy of K^0_S and Lambda production at mid-rapidity from Au+Au collisions at root s_NN = 130 GeV

P. Sørensen|ArXiv.org|Aug 22, 2002

High-Energy Particle Collisions Research3 citations

TL;DR

This paper presents the first measurement of elliptic flow ($v_2$) for identified $K_S^0$ and $ar{ ext{L}}$ particles in Au+Au collisions at $ oot s_{NN} = 130$ GeV, extending to $p_t o 3.0$ GeV/c. It observes a strong $p_t$-dependent $v_2$ up to $p_t o 2.0$ GeV/c, followed by saturation, suggesting incomplete hydrodynamic behavior at high $p_t$ and supporting early thermalization with partonic energy loss effects beyond $p_t \sim 1.5$ GeV/c.

ABSTRACT

We report STAR results on the azimuthal anisotropy parameter v2 for strange particles K0S and Lambda at midrapidity in Au+Au collisions at /s_NN = 130 GeV at RHIC. The value of v2 as a function of transverse momentum pt and collision centrality is presented for both particles and compared to model calculations. A strong pt dependence in v2 is observed up to pt ~2.0 GeV/c where v2 begins to saturate.

Motivation & Objective

• To measure the azimuthal anisotropy ($v_2$) of strange particles $K_S^0$ and $\Lambda$ in Au+Au collisions at $\sqrt{s_{NN}} = 130$ GeV.
• To extend $v_2$ measurements beyond the $p_t$ range where dE/dx particle identification fails, up to $p_t \sim 3.0$ GeV/c.
• To investigate the $p_t$ and centrality dependence of $v_2$ for $K_S^0$ and $\Lambda$ to probe early-stage dynamics and thermalization in heavy-ion collisions.
• To compare results with hydrodynamic model calculations to assess the validity of hydrodynamic descriptions at high $p_t$.
• To determine whether $v_2$ saturation at high $p_t$ signals the breakdown of hydrodynamic behavior or the onset of partonic energy loss mechanisms.

Proposed method

• Reconstructed $K_S^0 \to \pi^+ \pi^-$ and $\Lambda \to p \pi^-$ decay topologies in the STAR Time Projection Chamber (TPC) using invariant mass analysis.
• Applied distance-of-closest-approach (dca) cuts to suppress combinatoric background: dca > 1.0 cm for $K_S^0$ pions, dca > 1.5 cm for $\Lambda$ pions and > 0.8 cm for $\Lambda$ protons.
• Used dE/dx measurements in the TPC to identify particle types and reconstruct $v_2 = \langle \cos(2\phi) \rangle $ in $p_t$ and centrality bins.
• Estimated the event plane using subevent techniques to minimize auto-correlation, with resolution correction factors of 0.681 ± 0.004 for $K_S^0$ and 0.582 ± 0.007 for $\Lambda$.
• Corrected $v_2$ for event plane resolution and applied systematic corrections for particle identification, background subtraction, and non-uniform acceptance.
• Parameterized $v_2(p_t)$ and integrated $v_2$ over $p_t$ using exponential fits to transverse mass spectra, comparing with hydrodynamic model predictions (Huovinen et al.).

Experimental results

Research questions

• RQ1 How does the elliptic flow $v_2$ of $K_S^0$ and $\Lambda$ particles vary with transverse momentum $p_t$ and collision centrality at $\sqrt{s_{NN}} = 130$ GeV?
• RQ2 To what extent do $v_2$ measurements for identified strange particles at $p_t > 0.8$ GeV/c support the hydrodynamic picture of early thermalization?
• RQ3 Does $v_2$ saturation at $p_t \sim 2.0$ GeV/c indicate a breakdown of hydrodynamic behavior or the onset of partonic energy loss effects?
• RQ4 How do the $v_2$ values of $K_S^0$ and $\Lambda$ compare to those of charged particles and hydrodynamic model predictions?
• RQ5 What does the observed mass dependence of integrated $v_2$ imply about the collective velocity and thermalization mechanism in the early QGP?

Key findings

• The $v_2$ of $K_S^0$ and $\Lambda$ increases with $p_t$ up to $p_t \sim 2.0$ GeV/c, indicating strong collective flow in the mid-central collision regime.
• Above $p_t \sim 2.0$ GeV/c, $v_2$ for both particles begins to saturate, suggesting a deviation from pure hydrodynamic behavior at high $p_t$.
• The $v_2$ values for $K_S^0$ and $\Lambda$ are in good agreement with hydrodynamic model predictions (Huovinen et al.) within uncertainties, particularly for integrated $v_2$.
• The integrated $v_2$ for $\Lambda$ is larger than for $K_S^0$, reflecting the higher mean $p_t$ of $\Lambda$ particles and consistent with a common collective velocity across particle masses.
• The $p_t$-dependent $v_2$ for $K_S^0$ and $\Lambda$ shows similar trends to those of $K^\pm$ and negatively charged particles in the overlapping $p_t$ range, validating consistency across particle species.
• Systematic uncertainties in $v_2$ reach up to ±0.035 in the most peripheral collisions (58–85% centrality), with resolution correction being the dominant source of error.