Study of the $Lambda p$ Interaction Close to the $Sigma^+n$ and $Sigma^0p$ Thresholds

The $Lambda p$ interaction close to the $Sigma N$ threshold is considered. Specifically, the pronounced structure seen in production reactions like $K^-d to pi^- Lambda p$ and $ppto K^+ Lambda p$ around the $Sigma N$ threshold is analyzed. Modern interaction models of the coupled $Lambda N - Sigma N$ systems generate such a structure either due to the presence of a (deuteron-like) unstable bound state or of an inelastic virtual state. % A determination of the position of the prominent peak as observed in various experiments for the two aforementioned reactions leads to values that agree quite well with each other. Furthermore, the deduced mean value of $2128.7pm 0.3$ MeV for the peak position coincides practically with the threshold energy of the $Sigma^+ n$ channel. This supports the interpretation of the structure as a genuine cusp, signaling an inelastic virtual state in the $^3S_1-^3D_1$ partial wave of the $Sigma N$ isospin 1/2 channel. % There is also evidence for a second peak (or shoulder) in the data sets considered which appears at roughly 10-15 MeV above the $Sigma N$ threshold. However, its concrete position varies significantly from data set to data set and, thus, a theoretical interpretation is difficult.

Strangeness Production in Hadron Reactions

The paper gives an overview of strangeness-production experiments at the Cooler Synchrotron COSY. Results on kaon-pair and $phi$ meson production in $pp$, $pd$ and $dd$ collisions, hyperon-production experiments and $Lambda p$ final-state interaction studies are presented as well as a search for a strangeness $S=-1$ resonance in the $Lambda p$ system.

Study of the $bm{p+^6}$Li$bm{toeta+^7}$Be reaction 11.3 MeV above threshold

The cross section for the reaction $p+^6text{Li}toeta+^7text{Be}$ was measured at an excess energy of 11.28 MeV above threshold by detecting the recoiling $^7$Be nuclei. A dedicated set of focal plane detectors was built for the magnetic spectrograph Big Karl and was used for identification and four momentum measurement of the $^7$Be. A differential cross section of $frac{dsigma}{dOmega}=(0.69pm{0.20}text{(stat.)}pm 0.20text{(syst.)})text{nb/sr}$ for the ground state plus the 1/2$^-$ was measured. The result is compared to model calculations.

Cross sections of the $ppto K^+Sigma^+n$ reaction close to threshold

The paper is obsolete

Cross Section and Tensor Analysing Power of the $vec{d}dto etaalpha$ Reaction Near Threshold

The angular distributions of the unpolarised differential cross section and tensor analysing power $A_{xx}$ of the $vec{d}dtoalpha eta$ reaction have been measured at an excess energy of 16.6 MeV. The ambiguities in the partial-wave description of these data are made explicit by using the invariant amplitude decomposition. This allows the magnitude of the s-wave amplitude to be extracted and compared with results published at lower energies. In this way, firmer bounds could be obtained on the scattering length of the $eta alpha$ system. The results do not, however, unambiguously prove the existence of a quasi-bound $eta alpha$ state.

Non-equilibrium emission of complex fragments from p+Au collisions at 2.5 GeV proton beam energy

Energy and angular dependence of double differential cross sections d$^2sigma$/d$Omega$dE was measured for reactions induced by 2.5 GeV protons on Au target with isotopic identification of light products (H, He, Li, Be, and B) and with elemental identification of heavier intermediate mass fragments (C, N, O, F, Ne, Na, Mg, and Al). It was found that two different reaction mechanisms give comparable contributions to the cross sections. The intranuclear cascade of nucleon-nucleon collisions followed by evaporation from an equilibrated residuum describes low energy part of the energy distributions whereas another reaction mechanism is responsible for high energy part of the spectra of composite particles. Phenomenological model description of the differential cross sections by isotropic emission from two moving sources led to a very good description of all measured data. Values of the extracted parameters of the emitting sources are compatible with the hypothesis claiming that the high energy particles emerge from pre-equilibrium processes consisting in a breakup of the target into three groups of nucleons; small, fast and hot fireball of $sim$ 8 nucleons, and two larger, excited prefragments, which emits the light charged particles and intermediate mass fragments. The smaller of them contains $sim$ 20 nucleons and moves with velocity larger than the CM velocity of the proton projectile and the target. The heavier prefragment behaves similarly as the heavy residuum of the intranuclear cascade of nucleon-nucleon collisions. %The mass and charge dependence of the total production cross %sections was extracted from the above analysis for all observed %reaction products. This dependence follows the power low behavior %(A$^{-tau}$ or Z$^{-tau}$).

A large acceptance scintillator detector with wavelength shifting fibre read-out for search of eta-nucleus bound states

A large acceptance scintillator detector with wavelength shifting optical fibre readout has been designed and built to detect the decay particles of $eta$-nucleus bound system (the so-called $eta$-mesic nuclei), namely, protons and pions. The detector, named as ENSTAR detector, consists of 122 pieces of plastic scintillator of various shapes and sizes, which are arranged in a cylindrical geometry to provide particle identification, energy loss and coarse position information for these particles. A solid angle coverage of $sim$95% of total 4$pi$ is obtained in the present design of the detector. Monte Carlo phase space calculations performed to simulate the formation and decay of $eta$-mesic nuclei suggest that its decay particles, the protons and pions are emitted with an opening angle of 150$^circ pm 20^circ$, and with energies in the range of 25 to 300 MeV and 225 to 450 MeV respectively. The detailed GEANT simulations show that $sim$ 80 % of the decay particles (protons and pions) can be detected within ENSTAR. Several test measurements using alpha source, cosmic-ray muons etc. have been carried out to study the response of ENSTAR scintillator pieces. The in-beam tests of fully assembled detector with proton beam of momentum 870 MeV/c from the Cooler synchrotron COSY have been performed. The test results show that the scintillator fiber design chosen for the detector has performed satisfactorily well. The present article describes the detector design, simulation studies, construction details and test results.