Driver and damping of the directed-flow response in heavy-ion collisions
Abstract
The initial-state geometry plays a crucial role in driving directed flow, while the dissipative response of the medium dampens it. Both of these factors influence how the directed-flow slope varies with system size. We developed a method to differentiate between these driving and damping effects on charged-hadron directed flow in O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at √(sNN) = 200~GeV using an improved version of the string-melting AMPT model. We formulated three scaling observables based on the entropy density, the number of participants, and the mass number. A dimensionless ratio was constructed, revealing the threshold mass number A ≈ 35 in central collisions from the entropy and participant scaling, indicating the onset of collective behavior. We constructed a kinetic-theory Knudsen-number (Kn) map to analyze the contributions of the initial-state driver, which grows as Knκ with κ≈ 2, and a final-state viscous damping of the hydrodynamic response with characteristic scale Kn0 ≈ 0.27 for the directed flow slope. This damping scale is found to be about a factor of 2.5 smaller than the Kn0 ≈ 0.7 extracted from the elliptic flow. Furthermore, we determined the ratio of shear viscosity to entropy density, η/s, to be between 0.10 and 0.20, using an alternative method that does not rely on fitting flow harmonics.
Related
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BibTeX
@misc{nayak2026driver,
title = {{Driver and damping of the directed-flow response in heavy-ion collisions}},
author = {Kishora Nayak and Vipul Bairathi},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.12096},
eprint = {2609.12096},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.12096},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Driver and damping of the directed-flow response in heavy-ion collisions
AU - Kishora Nayak
AU - Vipul Bairathi
PY - 2026
DA - 2026/09/10
PB - arXiv
DO - 10.48550/arxiv.2609.12096
UR - https://arxiv.org/abs/2609.12096
AB - The initial-state geometry plays a crucial role in driving directed flow, while the dissipative response of the medium dampens it. Both of these factors influence how the directed-flow slope varies with system size. We developed a method to differentiate between these driving and damping effects on charged-hadron directed flow in O+O, Cu+Cu, Ru+Ru, Au+Au, and U+U collisions at √(s_NN) = 200~GeV using an improved version of the string-melting AMPT model. We formulated three scaling observables based on the entropy density, the number of participants, and the mass number. A dimensionless ratio was constructed, revealing the threshold mass number A ≈ 35 in central collisions from the entropy and participant scaling, indicating the onset of collective behavior. We constructed a kinetic-theory Knudsen-number (Kn) map to analyze the contributions of the initial-state driver, which grows as Kn^κ with κ≈ 2, and a final-state viscous damping of the hydrodynamic response with characteristic scale Kn_0 ≈ 0.27 for the directed flow slope. This damping scale is found to be about a factor of 2.5 smaller than the Kn_0 ≈ 0.7 extracted from the elliptic flow. Furthermore, we determined the ratio of shear viscosity to entropy density, η/s, to be between 0.10 and 0.20, using an alternative method that does not rely on fitting flow harmonics.
ER -