Table of contents

Absolute excitation rate coefficients have been determined experimentally for all excited states up to 1s²5f of the lithium-like ion Si XII using a well diagnosed theta-pinch plasma. A new method has been developed to determine the most crucial quantity of these measurements, the absolute concentration of the silicon ions in the plasma. For this we made use of the fact that sufficiently high lying levels are in partial local thermodynamic equilibrium (PLTE) with the ground state of the next higher ionization stage. A theoretical population model has been developed to study the influence of the different processes that might contribute to the population of the different levels at our plasma parameters. For most levels the theoretical excitation rate coefficients were found to be in fair agreement with the theoretical ones. The error of the experimentally determined excitation rate coefficients is of the order of ±50%.

Based on a two-dimensional description of interacting dipoles and a standard optical effective medium model we estimate structural properties of bacteriochlorophyll c in chlorosome aggregates of Chloroflexus bacteria. The main aim is to see what detailed structural information one can get out of an optical experiment. The measured optical spectrum turns out to be compatible with a distribution of aggregated pigment molecules approximately 9 Å apart from each other. Their interaction is responsible for the red-shift in peak position. A more random distribution of the aggregates in turn gives a broader and slightly asymmetric line shape. These results are in line with available structural determinations using other methods.

A new principle "Cyclotron Maser Cooling" (CMC) of charged particle beams has been tested experimentally. Dramatically rapid cooling of τ_⊥ ≈ 15ns, as predicted, was observed for the transverse energy of electrons. Electrons initially in the range 8-12keV, were all found to accumulate at one discret energy, 23 keV. The agreement between the prediction and the experimental results implies that in principle any kind of charged particle beams with arbitrary energy can be cooled in a single pass through a CMC section.

The principles and history of electron cooling and stochastic cooling are presented, and the characteristics of a number of existing storage rings, for antiproton accumulation and collection, for low energy antiproton physics, and for atomic and nuclear physics are discussed. Finally, three new proposals for the use of phase space cooling at higher energies are mentioned.

COSY-Jülich provides a number of regions, where experiments can be carried out using the internal beam in the storage ring mode. At the internal target positions fiber targets, ribbon targets, gas-jet or cluster targets and polarized atomic beam or gas cell targets can be installed. The size of the effective target spot and the minimum distance between the nozzle and catcher of a gas jet target depend on the beta functions at the target position and on the acceptance of the ring. These quantities will be discussed together with the figure of merit for each target position, including positions where use can be made of a standard COSY dipole as spectrometer. Experiments planned as internal experiments in COSY will be briefly mentioned.

A universal facility is in preparation for the study of particle production in proton-nucleus reactions below the nucleon-nucleon threshold. The device will be located at the internal target position TP2 at COSY-Jülich and will consist of three dipole magnets. This 0° Facility will separate ejectiles from the circulating proton beam and allow momentum and angle analysis both for positively and negatively charged particles. A major goal of the experimental program is the investigation of the K⁺-meson production at projectile energies below the NN threshold. The detector concept for these studies is described and also an overview over further planned experiments is given.

The production of pions in nucleon-nucleon collisions has recently attracted renewed attention because of the advent of highly precise, near-threshold total cross section data in three different channels, and because of the apparent failure of conventional theory to reproduce the magnitude of the s-wave production cross section, leading to the speculation that we might learn something new about the pion field of a nucleon. The onset of higher partial waves is observed in differential cross section and analyzing power measurements. The measurement of spin correlation coefficients close to threshold will become possible in the near future in storage rings with electron cooling and internal targets. Recent data on pion production in the three-nucleon system support the importance of a quasi-free production of pions from nuclei.

A measurement of the total cross section for the reaction pd → pdπ° very close to threshold has been carried out at the IUCF Cooler. The deuteron with its known "nuclear structure" is the simplest system where pion production on nuclei can be investigated and mechanisms other than the quasi-free production may become important. In this contribution the experimental setup and the data analysis are described. Total cross sections for pd → pdπ° in the range 0.1 < η < 1.0 are reported.

The reaction d + d → ⁴He + X at a beam energy of 570 MeV was studied in an exploratory experiment at CELSIUS. With an integrated luminosity of 8.8 × 10³³ cm⁻² 10 events from the reaction were identified.

In ηNN system near η threshold, the S wave resonance (S₁₁) between η and nucleon plays an important role and produces characteristic phenomenon in contrast with πNN system, where the P wave resonance (P₃₃) is dominant between pion and nucleon. Recently, a multi-three-body calculation of the I = 0, J^P = 1⁻ηNN - πNN - NN coupled system has predicted a quasi-bound state of mass 2430 MeV with a narrow width of 20 MeV near η threshold. We review this theoretical approach and show what is the signature of this state in np → np(I = 0), np → ηd, γd → ηd, γd → np(I = 0), np(I = 0) → γd and ηd → ηd reactions.

Inclusive and exclusive (π,η) reactions are studied in a momentum space DWIA model. We assume both reactions are dominated by formation of an N(1535) resonance, taking into account medium corrections to the N* self energy. Cross sections are calculated for (π⁺, η) inclusive reactions on ¹²C and compared with both experiment and previous theoretical calculations. Assuming an N* free width of 150 MeV, we obtain quantitative agreement with experiment, assuming this reaction is dominated by quasi-free single-nucleon knockout. Differential and total cross sections are calculated for the exclusive reaction ¹³C(π⁺, η)¹³N.

The proton-induced η-production is considered. It is shown that the existing experimental data on the η-meson production can be described fairly well in the framework of the simple quasiclassical formula where it is assumed that the η-production is produced in the first inelastic pN-collision. Medium corrections are shown to be important for the reasonable description of the experimental data.

New data on the decay of Δ-hole states in nuclei are presented. The gross features of the decay pattern are reasonably well described in a cascade model with some remarkable exceptions indicating strong medium effects. Both mass and width of the Δ-isobar are modified in the nucleus. The first evidence for the collective pionic mode, the pisobar, is presented.

The pd → npp charge exchange reaction has been studied at 1.15 GeV incident proton energy in a selected kinematical region. The distributions of two-proton invariant mass M_pp, of angle of a single proton and the differential cross section dσ/dt of the (p, n) reaction are presented and compared with model predictions corrected for the acceptance of the experimental apparatus. We see no evidence of narrow (Γ < 30 MeV/c²) peaks in the M_pp interval 2.1-2.3 GeV/c². The production of an intermediate Δ isobar in the primary (p, n) charge exchange and its subsequent absorption in a final state ΔN → pp process together with the background from quasi-free charge exchange scattering reasonably reproduces the general trends of the data.

The EDDA experiment is designed to study p + p excitation functions with high energy resolution and narrow step size in the kinetic energy range from 250 MeV to 2500 MeV at the Cooler Synchrotron COSY. Measurements during the accelertion phase in conjunction with internal targets will allow to achieve a fast and precise energy variation. Prototypes of the detector elements and the fiber target have been extensively tested with proton and electron beams; the detector performance and trigger efficiency have been studied in Monte Carlo simulations. In this contribution, results concerning detector design, prototype studies, Monte Carlo simulations and the anticipated detector resolutions will be reported.

The Δ excitations in nuclei induced by intermediate energy charge exchange reactions are analyzed within the isobar-hole model. It is shown that the isovector spin-longitudinal (S · qT) response function of nuclei in the Δ resonance region is shifted down in energy by ~ 60 MeV in comparison to the spin-transverse (S × qT) response function. This shift is consistent with pion and photon total cross section data. The coherent pion decay of the Δ resonance is studied by means of the ¹²C(p, nπ⁺)¹²C(g.s.) coincidence reaction. The calculations show that the coherent pion production cross section provides a unique signature on the nuclear pionic mode. The coherent pions have a peak energy of E = 250 MeV and a strongly forward peaked angular distribution.

We study the different channels which contribute to the charge exchange reactions of the type (³He, t) (p, n) etc., in the region of excitation of the delta resonance. We show that the shift of the delta peak in nuclei is due to a collaboration of many processes: delta excitation in the projectile, quasi-elastic collisions, virtual pion absorption and coherent pion production. The first three processes are responsible for a considerable shift of strength but do not move the peak position, while the addition of the coherent channel produces finally the shift of the peak.

The problem of narrow resonances in e⁺e⁻ and pp systems is discussed. The theoretical scheme proposed earlier is shown to describe the totality of data without contradictions. The author emphasizes the importance of the problem and calls for accurate experimental studies.

Since the beginning of the 12-GeV proton synchrotron operation, kaon decay studies have been among the central activities at KEK. We discuss here recent experimental results concerning K_μ2 muon polarization measurements and K_L rare decay experiments. We also report on the next program in this field, such as a T-violation search in the K_μ3 decay.

In an experiment at the external COSY beam associated strangeness production will be investigated [1] by measuring total and differential cross sections as well as the polarization of the produced hyperons by means of an almost 4π time-of-flight spectrometer. In a first step we will concentrate on the reaction pp → KΛp. The apparatus will consist of an inner arrangement as start detector and highly granulated scintillator hodoscopes as stop detector. Both parts give information on multiplicity and allow to identify the interesting reaction by means of the delayed charged decay of the Λ. Moreover the tracks of all prompt and delayed reaction products can be reconstructed with high precision. The detector allows the momentum measurement by time-of-flight. To get all this information the inner detector is composed of tiny granulated plastic scintillator detectors, silicon microstrip devices and a scintillating fibre hodoscope. The outer detector barrel with a diameter of about 3m and a length of about 7.5m will consist of three layers of plastic scintillator slabs, two of them bended in helix form. The status of the experiment will be presented. Moreover the results of test measurements for the different detector components will be discussed together with Monte Carlo calculations.

I give a review of several decay modes of light mesons (π, η, η',...) which are rare or forbidden in the standard model of particle physics. In particular, the weak and electromagnetic decays of the π°, π^±, η, and η' mesons are discussed in some detail. Some new results on K decays are also discussed. The sensitivity of these decays to parameters of the standard model as well as to non-standard physics is investigated.

The power value of the A-dependence of the cross-section for 4.5 GeV/nucleon deuteron fragmentation into cumulative pions has been measured on carbon, aluminium, copper and lead nuclei for cumulative numbers within 0.8-1.2. The mean value is equal to 0.27 ± 0.09 in this interval. The target atomic weight dependence significantly differs from the volume type dependence on the atomic weight of fragmenting nucleus.

All presently available data on pionic double charge exchange on nuclei exhibit a resonance-like energy dependence near T_π = 50 MeV, while the angular distributions behave quite regularly. We show that these features find their natural explanation by a narrow resonance in the πNN-subsystem with J^P = 0⁻ and a mass of 2.065 GeV.