P. Finocchiaro

MEDEA: a multi element detector array for gamma ray and light charged particle detection at the LNS-Catania

Abstract A 4 π highly granular Mutli Element DEtector Array (MEDEA) for γ-rays and light charged particles is described. Its basic configuration consists of 180 barium fluoride scintillator crystals, arranged in the shape of a ball, plus a forward angle wall of 120 phoswich detectors. The inner radius of the ball (22 cm) and the distance of the wall from the target (55 cm) allow the placement of other detectors.in the inner volume. The whole detection system operates under vacuum inside a large scattering chamber. Dedicated electronics has been designed and realized. It includes a powerful hardware second level trigger and preanalysis system, which allows on-line event selection, and a modular VME-bus based data acquisition system. In-beam performances of the system are also described.

Dielectron production in Ar+KCl collisions at 1.76A GeV

We present results on dielectron production in 40 Ar+KCl collisions at 1.76A GeV. For the first time ω mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ meson. At intermediate e + e invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for ]

Strong impact parameter dependence of hard photon production in intermediate energy heavy ion collisions

Abstract The dependence of the high energy photon production on the impact parameter has been investigated in the reaction 129 Xe+ 197 Au at 44 MeV/u using the multidetector array MEDEA. A strong dependence of the high energy photon production probability on the impact parameter has been observed, while the slope parameter of the photon spectrum is almost constant. The data support the interpretation of the hard photon production in terms of first chance n-p collisions.

The HADES time-of-flight wall

In the framework of the High Acceptance DiElectron Spectrometer collaboration a Time-of-Flight wall subdetector has been developed. In order to efficiently select rare events with dilepton pairs produced in the nuclear matter, in a high hadron multiplicity environment, such a wall must be able to resolve electrons from pions up to 0.5 GeV/c and from protons up to 2 GeV/c. The short flight path of about 2 m requires a time resolution better than 150 ps, while impact position and multiplicity measurements need adequate granularity. These requirements have been fulfilled by employing scintillator rods read-out at both ends by photomultipliers.

Inclusive dielectron production in proton-proton collisions at 2.2 GeV beam energy

Data on inclusive dielectron production are presented for the reaction p+p at 2.2 GeV measured with the High Acceptance DiElectron Spectrometer (HADES). Our results supplement data obtained earlier in this bombarding energy regime by DLS and HADES. The comparison with the 2.09 GeV DLS data is discussed. The reconstructed e+e- distributions are confronted with simulated pair cocktails, revealing an excess yield at invariant masses around 0.5 GeV/c2. Inclusive cross sections of neutral pion and eta production are obtained.

Second order phase transitions: from infinite to finite systems

Abstract We investigate the Equation of State (EOS) of classical systems having 300 and 512 particles confined in a box with periodic boundary conditions. We show that such a system, independently on the number of particles investigated, has a critical density of about 1 3 the ground state density and a critical temperature of about 2.5 MeV. The mass distribution at the critical point exhibits a power law with τ = 2.23 . Making use of the grand partition function of Fishers droplet model, we obtain an analytical EOS around the critical point in good agreement with the one extracted from the numerical simulations.

On the origin of fast proton emission in intermediate energy heavy ion collisions

Abstract Exclusive measurements of multiplicity distributions of fast protons emitted in 40Ar and 132Xe induced reactions at 44 MeV/u bombarding energy have been performed using MEDEA 4π detection sytem. The data reflect the impact parameter dependence of the overlap region size and scale from system to system as the number of participant protons. The hypothesis of pre-equilibrium emission following first order p-N collisions is consistent with the data.

Medium effects in proton-induced

We present the analysis of the inclusive K 0 production in p + p and p + Nb collisions measured with the HADES detector (GSI Helmholtzzentrum for Heavy-Ion Research, Darmstadt) at a beam kinetic energy of 3.5 GeV. Data are compared to the Giessen Boltzmann-Uehling-Uhlenbeck (GiBUU) transport model. The data suggest the presence of a repulsive momentum-dependent kaon potential as predicted by the chiral perturbation theory (ChPT). For the kaon at rest and at normal nuclear density, the ChPT potential amounts to ≈ 35 MeV. A detailed tuning of the kaon production cross sections implemented in the model has been carried out to reproduce the experimental data measured in p + p collisions. The uncertainties in the parameters of the model were examined with respect to the sensitivity of the experimental results from p + Nb collisions to the in-medium kaon potential.

K^{0}

Abstract.We report on baryon resonance production and decay in proton-proton collisions at a kinetic energy of 3.5 GeV based on data measured with HADES. The exclusive channels

production at 3.5 GeV

pp \rightarrow np\pi^{+}