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Register a new userStrange Mesons as a Probe for Dense Nuclear Matter
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P. Senger
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The production and propagation of kaons and antikaons has been studied in symmetric nucleus-nucleus collisions in the SIS energy range. The ratio of the excitation functions of K^+ production in Au+Au and C+C collisions increases with decreasing beam energy. This effect was predicted for a soft nuclear equation-of-state. In noncentral Au+Au collisions, the K^+ mesons are preferentially emitted perpendicular to the reaction plane. The K^-/K^+ ratio from A+A collisions at beam energies which are equivalent with respect to the threshold is found to be about two orders of magnitude larger than the corresponding ratio from proton-proton collisions. Both effects are considered to be experimental signatures for a modification of kaon properties in the dense nuclear medium.
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We study the properties of $K$ and $bar K$ mesons in nuclear matter at finite temperature from a chiral unitary approach in coupled channels which incorporates the $s$- and p-waves of the kaon-nucleon interaction. The in-medium solution accounts for Pauli blocking effects, mean-field binding on all the baryons involved, and $pi$ and kaon self-energies. We calculate $K$ and $bar K$ (off-shell) spectral functions and single particle properties. The $bar K$ effective mass gets lowered by about -50 MeV in cold nuclear matter at saturation density and by half this reduction at T=100 MeV. The p-wave contribution to the ${bar K}$ optical potential, due to $Lambda$, $Sigma$ and $Sigma^*$ excitations, becomes significant for momenta larger than 200 MeV/c and reduces the attraction felt by the $bar K$ in the nuclear medium.The $bar K$ spectral function spreads over a wide range of energies, reflecting the melting of the $Lambda (1405)$ resonance and the contribution of $YN^{-1}$ components at finite temperature. In the $KN$ sector, we find that the low-density theorem is a good approximation for the $K$ self-energy close to saturation density due to the absence of resonance-hole excitations. The $K$ potential shows a moderate repulsive behavior, whereas the quasi-particle peak is considerably broadened with increasing density and temperature. We discuss the implications for the decay of the $phi$ meson at SIS/GSI energies as well as in the future FAIR/GSI project.
The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, where a proton is knocked-out of the nucleus with high momentum transfer and high missing momentum, show that in 12C the neutron-proton pairs are nearly twenty times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.
Laboratory experiments with high-energetic heavy-ion collisions offer the opportunity to explore fundamental properties of nuclear matter, such as the high-density equation-of-state, which governs the structure and dynamics of cosmic objects and phenomena like neutron stars, supernova explosions, and neutron star mergers. A particular goal and challenge of the experiments is to unravel the microscopic degrees-of-freedom of strongly interaction matter at high density, including the search for phase transitions, which may feature a region of phase coexistence and a critical endpoint. As the theory of strong interaction is not able to make firm predictions for the structure and the properties of matter high baryon chemical potentials, the scientific progress in this field is driven by experimental results. The mission of future experiments at FAIR and NICA, which will complement the running experimental programs at GSI, CERN, and RHIC, is to explore new diagnostic probes, which never have been measured before at collision energies, where the highest net-baryon densities will be created. The most promising observables, which are expected to shed light on the nature of high-density QCD matter, comprise the collective flow of identified particles including multi-strange (anti-) hyperons, fluctuations and correlations, lepton pairs, and charmed particles. In the following, the perspectives for experiments in the NICA energy range will be discussed.
An extended chiral SU(3) model is applied to the description of dense, hot and strange hadronic matter. The degrees of freedom are the baryon octet and decuplet and the spin-0 and spin-1 meson multiplets. The parameters of the model are fitted to the hadron masses in vacumm, infinite nuclear matter properties and soft pion theorems. At high densities the appearance of density isomers cannot be ruled out and extrapolation to finite temperature exhibits a first order phase transition at $T approx 150 MeV$. The predicted dropping baryon masses lead to drastically changed particle ratios compared to ideal gas calculations.
We study the medium modifications of the spectral functions as well as production cross-sections of the strange vector mesons ($phi$, $K^*$ and $bar {K^*}$) in isospin asymmetric strange hadronic matter. These are obtained from the in-medium masses of the open strange mesons and the decay widths $phi rightarrow Kbar K$, $K^* rightarrow Kpi$ and $bar {K^*} rightarrow {bar K}pi$ in the hadronic medium. The decay widths are computed using a field theoretic model of composite hadrons with quark/antiquark constituents, from the matrix element of the light quark-antiquark pair creation term of the free Dirac Hamiltonian between the initial and final states. The matrix element is multiplied with a coupling strength parameter for the light quark-antiquark pair creation, which is fitted to the observed vacuum decay width of the decay process. There are observed to be substantial modifications of the spectral functions as well as production cross-sections of these vector mesons due to isospin asymmetry as well as strangeness of the hadronic medum at high densities. These studies should have observable consequences, e.g. in the yield of the hidden and open strange mesons arising from the isospin asymmetric high energy heavy ion collisions at the Compressed baryonic matter (CBM) experiments at the future facility at GSI.
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