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

Abstract

Recent STAR measurements at fixed-target Beam Energy Scan energies have revealed an unexpectedly large directed flow of $ϕ$ mesons in Au+Au collisions, comparable to that of protons and $Λ$ baryons and much stronger than that of light strange mesons. Since the $ϕ$ 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 $ϕ$ channel. Within the framework of the Parton-Hadron-Quantum-Molecular-Dynamics(PHQMD) model, we demonstrate that in the high baryon density region, $ϕ$ 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 $ϕ$ spectral function, these baryon-driven production channels enhance near-threshold $ϕ$ production and imprint the collective motion of the baryon-rich source on the produced $ϕ$ mesons.