Resolving the $\phi$-meson directed-flow puzzle by multi-step meson--baryon dynamics

Yingjie Zhou; Taesoo Song; Susanne Glässel; Jiaxing Zhao; Christoph Blume; Iouri Vassiliev; Vadim Voronyuk; Yaping Wang; Nu Xu; Jörg Aichelin; Elena Bratkovskaya

arXiv · 2608.24573

Resolving the $\phi$-meson directed-flow puzzle by multi-step meson--baryon dynamics

Yingjie Zhou·Taesoo Song·Susanne Glässel·Jiaxing Zhao·Christoph Blume·Iouri Vassiliev·Vadim Voronyuk·Yaping Wang·Nu Xu·Jörg Aichelin·Elena Bratkovskaya

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Abstract

Recent STAR measurements at fixed-target Beam Energy Scan energies have revealed an unexpectedly large directed flow of $\phi$ mesons in Au+Au collisions, comparable to that of protons and $\Lambda$ baryons and much stronger than that of light strange mesons. Since the $\phi$ is a hidden-strangeness meson with relatively weak interactions with non-strange hadrons, this observation has been interpreted as a possible signal of unconventional baryonic dynamics or exotic baryonic resonances coupled to the $\phi$ channel. Within the framework of the Parton-Hadron-Quantum-Molecular-Dynamics(PHQMD) model, we demonstrate that in the high baryon density region, $\phi$ mesons are produced predominantly through multi-step meson--baryon and meson--hyperon reactions, whose transition amplitudes are constrained by a coupled-channel $T$-matrix calculation based on an extended SU(6) chiral effective Lagrangian. Together with the in-medium broadening of the $\phi$ spectral function, these baryon-driven production channels enhance near-threshold $\phi$ production and imprint the collective motion of the baryon-rich source on the produced $\phi$ mesons.

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Yingjie Zhou, Taesoo Song, Susanne Glässel, Jiaxing Zhao, Christoph Blume, Iouri Vassiliev, Vadim Voronyuk, Yaping Wang, Nu Xu, Jörg Aichelin, Elena Bratkovskaya. 2026-08-25. Resolving the $\phi$-meson directed-flow puzzle by multi-step meson--baryon dynamics. https://arxiv.org/abs/2608.24573

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