{
  "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 $\\sqrt{s_{\\mathrm{NN}}} = 200$~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 $A \\approx 35$ in central collisions from the entropy and participant scaling, indicating the onset of collective behavior. We constructed a kinetic-theory Knudsen-number ($\\mathrm{Kn}$) map to analyze the contributions of the initial-state driver, which grows as $\\mathrm{Kn}^κ$ with $κ\\approx 2$, and a final-state viscous damping of the hydrodynamic response with characteristic scale $\\mathrm{Kn}_0 \\approx 0.27$ for the directed flow slope. This damping scale is found to be about a factor of 2.5 smaller than the $\\mathrm{Kn}_0 \\approx 0.7$ 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.12096",
  "author_total": 2,
  "authors": [
    {
      "name": "Kishora Nayak"
    },
    {
      "name": "Vipul Bairathi"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/7mjmt0s26zhje.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.12096",
  "fulltext_url": null,
  "id": "7mjmt0s26zhje",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.12096",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "hep-ph"
    ],
    [
      "arxiv",
      "nucl-th"
    ],
    [
      "openalex",
      "3106|High-Energy Particle Collisions Research"
    ]
  ],
  "openalex": "W7212684149",
  "pdf_url": "https://arxiv.org/pdf/2609.12096",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-10",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/7mjmt0s26zhje.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:21:07Z",
      "native_id": "2609.12096",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.12096"
    }
  ],
  "subjects": [
    {
      "field": "physics",
      "origin": "arxiv:hep-ph",
      "subfield": "high-energy"
    },
    {
      "field": "physics",
      "origin": "arxiv:nucl-th",
      "subfield": "nuclear"
    }
  ],
  "title": "Driver and damping of the directed-flow response in heavy-ion collisions",
  "title_text": "Driver and damping of the directed-flow response in heavy-ion collisions",
  "type": "preprint",
  "updated_at": "2026-10-30T12:21:07Z",
  "url": "https://research.draconitic.com/paper/7mjmt0s26zhje",
  "venue": "arXiv",
  "versions": []
}