Open and hidden strangeness production in heavy-ion collisions

Strangeness production in heavy-ion collisions reveals the modification of the properties of strange hadrons in hot and dense nuclear matter. Adopting in-medium properties of antikaons ($\bar{K}=K^-,\bar{K}^0$) described by the self-consistent coupled-channel unitarized scheme based on a SU(3) chiral effective Lagrangian (G-matrix), we study strangeness production in heavy-ion collisions within the off-shell Parton-Hadron-String Dynamics (PHSD) transport approach. The in-medium modification of kaons ($K=K^+,K^0$) are accounted for via the kaon-nuclear potential, which is proportional to the local baryon density. Our results are found consistent with the experimental data on (anti)kaon production from the KaoS, FOPI and HADES Collaborations. Moreover, we demonstrate the sensitivity of kaon observables to the equation-of-state of nuclear matter. We also study hidden strangeness production with in-medium effects realized by a collisional broadening of spectral function, which reflects the partial chiral symmetry restoration. Implementing novel meson-baryon and meson-hyperon production channels for $\phi$ mesons, calculated within a T-matrix coupled-channel approach based on the extended SU(6) chiral effective Lagrangian model, along with the collisional broadening of the $\phi$ meson in-medium spectral function, we find a substantial enhancement of $\phi$ meson production in heavy-ion collisions, especially at sub- and near-thresholds, as shown by the experimental data at the HADES collaboration. This allows to describe the experimentally observed strong enhancement of the $\phi/K^-$ ratio at low energies without including hypothetical decays of heavy baryonic resonances to $\phi$ as in alternative approaches. Our results support that the modifications of open and hidden strange hadrons in the nuclear medium are necessary to understand various experimental data.