First observation of phi-meson directed flow v1 comparable in magnitude to protons and Lambdas but much larger than K0S, with baryon-like energy dependence at high baryon density.
Mean-field update in the JAM microscopic model: Mean-field effects on collective flow in high-energy heavy-ion collisions at $\sqrt{s_{NN}}=2-20$ GeV energies
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abstract
The beam energy dependence of the directed flow is a sensitive probe for the properties of strongly interacting matter. We consider different implementations of momentum-dependent hadronic mean fields in the relativistic quantum molecular dynamics (RQMD) framework. First, Lorentz scalar implementation of a Skyrme type potential is examined. Then, full implementation of the Skyrme type potential as a Lorentz vector in the RQMD approach is proposed. We find that scalar implementation of the Skyrme force is too weak to generate repulsion explaining observed data of sideward flows at $\sqrt{s_{NN}}<10$ GeV, while vector implementation gives collective flows compatible with the data for a wide range of beam energies $2.7 <\sqrt{s_{NN}}<20$ GeV. We show that our approach reproduces the negative proton directed flow at $\sqrt{s_{NN}}>10$ GeV discovered by experiments. We discuss the dynamical generation mechanisms of the directed flow within a conventional hadronic mean field. A positive slope of proton directed flow is generated predominantly during compression stages of heavy-ion collisions by the strong repulsive interaction due to high baryon densities. In contrast, in the expansion stages of the collision, the negative directed flow is generated more strongly than the positive one by the tilted expansion and shadowing by the spectator matter. At lower collision energies $\sqrt{s_{NN}}<10$ GeV, the positive flow wins against the negative flow because of a long compression time. On the other hand, at higher energies $\sqrt{s_{NN}}>10$ GeV, negative flow wins because of shorter compression time and longer expansion time. A transition beam energy from positive to negative flow is highly sensitive to the strength of the interaction.
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2026 2representative citing papers
A LightGBM model trained on pion observables from one transport model predicts impact parameters in Au+Au collisions at 4 and 11 GeV with 0.2-0.4 fm error, generalizing to data from other models where polynomial fits fail.
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Observation of Strong $\phi$-meson Directed Flow at High Baryon Density
First observation of phi-meson directed flow v1 comparable in magnitude to protons and Lambdas but much larger than K0S, with baryon-like energy dependence at high baryon density.
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Machine learning the impact parameter in heavy-ion collisions at $\sqrt{s_{\rm NN}}$ = 4 and 11 GeV: a cross-check study with UrQMD, AMPT, and JAM
A LightGBM model trained on pion observables from one transport model predicts impact parameters in Au+Au collisions at 4 and 11 GeV with 0.2-0.4 fm error, generalizing to data from other models where polynomial fits fail.