Λ and K⁰_S production in Au+Au collisions at 1.23A GeV

In April and May 2012 data on Au+Au collisions at beam energies of E_kin=1.23A GeV were recorded with the High Acceptance Di-Electron Spectrometer, which is located at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. The required energy is not available in binary NN collisions but must be provided by the system e.g. through multi-particle interactions or medium effects like a modified in-medium potential (e.g. KN/ΛN potential). The baryon-dominated systems created in relativistic heavy-ion collisions (HIC) at SIS18 energies reach densities of about 2-3 times ground state density ρ₀ and may be similar to the properties of matter expected in the inner core of neutron stars. It is in particular the behavior of hadrons containing strangeness, i.e. kaons (K^+/-,K⁰_s) and hyperons (Λ,Ξ^-), and their potentials in the dense medium which may have severe implications on astrophysical objects and processes. As ab-initio calculations of quantum chromodynamics (QCD) cannot be performed rigorously on the lattice at finite baryo-chemical potentials due to the fermion sign problem, effective descriptions have to be used in order to model properties of dense systems and the involved particles. The only way to access the in-medium potential of strange hadrons above nuclear ground state density ρ₀ is by comparing data from relativistic HIC to such effective microscopic models. Up to now, not much data on neutral kaons and Λ hyperons are available from heavy collision systems close to their NN production threshold. These two electromagnetically uncharged strange hadrons are in particular well suited to study their potential in a dense nucleon-dominated environment as their kinematic spectra are not affected by Coulomb interactions. After an elaborate improvement of tracking algorithms and a careful event cleaning procedure, in total 2.1 x 10⁹ Au+Au events were analyzed containing the 0-40% most central events. The investigated strange hadrons Λ and K ⁰_s are identified via their weak decays into p-π^- and π⁺-π^- respectively. The relatively large mean decay lengths enable an analysis based on constraints on the decay topology which allows to distinguish between the decay and the primary vertex in order to suppress combinatorial background to the invariant mass spectrum. The data is then analyzed multi-differentially as a function of rapidity y, reduced transverse mass m_t-m₀ and four centrality classes in steps of 10%. For the 0-40% most central collisions multiplicities of Mult_tot = (3.97±0.06^stat±0.06^sys_Cut±0.04^sys_Extrapol) x 10^-2 for Λ hyperons and of Mult_tot = (1.54±0.03^stat±0.05^sys_Cut±0.15^sys_Extrapol) x 10^-2 for K⁰_s mesons were determined. The inverse slope parameters at mid-rapidity T_eff of 93±2±4 for Λ and 97±1±2 for K⁰_s agree within uncertainties. For both particles, a more than linear rise of the mean multiplicity is observed with centrality which is in agreement with measurements by former experiments at higher energies. This indicates that the strength of the rise is only weakly proportional to the excess energy. The experimental data are compared to predictions from three state-of-the-art hadronic transport models: IQMD (c8), HSD (711n) and UrQMD (3.4). All three are semi-classical models simulating HIC on an event-by-event basis. The investigated transport models can be grouped in two relevant fractions: on one side, the HSD and IQMD transport models with a repulsive KN potential of 40 MeV at nuclear ground state density ρ₀ increasing linearly with density as well as an attractive ΛN potential which scales with 2/3 of the strength of the NN potential. On the other side, there is the UrQMD code which employs higher-lying Δ and N* resonances but neither includes a repulsive KN nor an attractive NN/ΛN potential. It is shown, that no model describes all observables at once and that no observable can be described unambiguously by all models. Results from the HSD and IQMD transport models may give the impression that a repulsive KN potential is necessary to describe particle kinematics (p_t spectra), yet, at least partially UrQMD calculations can (over)compensate the effect of the potential via particle production through intermediate resonances but fails to reproduce the scaling of the yields with centrality. On the other hand, p_t spectra of Λ are best described by the UrQMD model. The ambiguities in three out of four observables for each investigated particle hamper conclusions on the KN/ΛN potential and more model-to-data comparison on additional observables are important to rule out further ambiguities. Observing a null result within the attempt to reconstruct the multi-strange Ξ^- hyperon, which would be produced about 840 MeV below its elementary threshold, an upper limit on the production yield can be determined. This limit is derived by using the Feldman-Cousins approach, which is purely based on the statistics of the selected data sample. The confidence interval is chosen to include 99.7% (corresponding to 3σ) of the hypothetical signal providing an upper limit of M_FC<2.32 x 10^-3, being in agreement with former Ξ^- measurements and transport predictions. Considering the upper production limit, a maximum boundary on the ratio of Ξ^- to (Λ+Σ⁰) hyperons is derived to be N_Ξ^-/N_Λ+Σ⁰<5.8 x 10^-2.