Phi meson production in heavy-ion collisions at SIS/GSI energies (∼ 2 GeV/nucleon) is studied in the relativistic transport model. We include contributions from baryon-baryon, pion-baryon, and kaon-antikaon collisions.The cross sections for the first two processes are obtained in an one-bosonexchange model, while that for the last process is taken to be of Breit-Wigner form through the phi meson resonance. The dominant contribution to phi meson production in heavy ion collisions at these energies is found to come from secondary pion-nucleon collisions. Effects due to medium modifications of kaon masses are also studied and are found to reduce the phi meson yield by about a factor of two, mainly because of increased phi decay width as a result of dropping kaon-antikaon masses. In this case, the φ/K − ratio is about 4%, which is a factor of 2-3 below preliminary experimental data from the FOPI collaboration at GSI. Including also the reduction of phi meson mass in medium increases this ratio to about 8%, which is then in reasonable agreement with the data.

Kaon production cross sections in nucleon-nucleon, nucleon-delta and delta-delta interactions are studied in a boson exchange model. For the latter two interactions, the exchanged pion can be on-mass shell, only contributions due to a virtual pion are included via the Peierls method by taking into account the finite delta width. With these cross sections and also those for pion-baryon interactions, subthreshold kaon production from heavy ion collisions is studied in the relativistic transport model.Typeset using REVT E X 1

Phi meson production in pion-baryon and baryon-baryon interactions near the production threshold is studied in a boson-exchange model. Parameters in this model are either taken from the Bonn model for the nucleon-nucleon potential or determined from the measured phi meson decay width and the experimental data for the reactions π − p→φn and pp→ppφ. The isospinaveraged cross sections for the reactions πN →φN , π∆→φN , N N →N N φ, which are needed in transport models to study phi meson production in heavyion collisions at subthreshold energies, have been evaluated. For the reactions N ∆→N N φ and ∆∆→N N φ, which become singular when the exchanged pion is on-shell, the Peierls method is used to calculate their cross sections. We find that the cross section for the reaction πN →φN is much larger than the other cross sections.

Dilepton spectra from the decay of phi mesons produced in heavy-ion collisions at SIS/GSI energies (∼ 2 GeV/nucleon) are studied in the relativistic transport model. We include phi mesons produced from baryon-baryon, pion-baryon, and kaon-antikaon collisions. The cross sections for the first two processes are obtained from an one-boson-exchange model, while that for the last process is taken to be the Breit-Wigner form through the phi meson resonance. For dileptons with invariant mass near the phi meson peak, we also include contributions from neutron-proton bremsstrahlung, pion-pion annihilation, and the decay of rho and omega mesons produced in baryon-baryon and meson-baryon collisions. Effects due to medium modifications of the kaon and vector (rho, omega and phi) meson properties are investigated. We find that the kaon medium effects lead to a broadening of the dilepton spectrum as a result of the increase of phi meson decay width. Furthermore, the dropping of phi meson mass in nuclear medium leads to a shoulder structure in the dilepton spectrum besides the main peak at the bare phi meson mass. The experimental measurement of the dilepton spectra from heavy-ion collisions is expected to provide useful information about the phi meson properties in dense matter.