Search

The separation of contributions from multi- and single-meson production in the missing mass spectrum of the quasi-free pn → pnX reaction constitutes a challenging task when the reaction is studied close to the threshold. This is especially true if the resolution of the mass determination is comparable with the excess energy and if the investigated signal appears close to the kinematical limit. In this paper, we outline a method which permits the extraction of the signal originating from the creation of a single meson without the necessity of conducting model-dependent simulations. For the pd → pnXpspectator reactions, the method allows one to combine events corresponding to multi-meson production at various excess energies with respect to the pn → pn meson process, and hence leads to an increase of the statistics needed for the determination of the shape of the multi-meson background. As an example of the application of the method, we demonstrate that the evaluation of the data from the pd → pnXpsp process according to the described technique enables one to extract a signal of the pn → pnη reaction whose shape is consistent with expectations, supporting the correctness and usefulness of the method introduced.

Utilizing a missing mass technique we investigate the pp → ppX reaction scanning beam energies in the range permitting to create a mass close to that of the f0(980) and a0(980) scalar resonances, but still below the K+K− threshold where they decay dominantly into ππ and πη mesons, respectively. Prior to the data analysis we introduce a notion of the close to threshold total cross-section for broad resonances. We estimated for the overlapping mesons a0 and f0 the total cross-section to be smaller than 430 nb at excess energy of Q &equal; 5 MeV. The experiment has been performed at the Cooler Synchrotron (COSY) using the COSY-11 facility.

Abstract:: The production of K+-mesons in proton-nucleus collisions from 1.0 to 2.5GeV is analyzed with respect to one-step nucleon-nucleon ( NN → NYK +) and two-step Δ-nucleon ( ΔN → K + YN) or pion-nucleon ( πN → K + Y) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final-state interactions. The influence of momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are studied as well as the importance of K+ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental K+ spectra taken at LBL Berkeley, SATURNE, CELSIUS, GSI and COSY-Jülich. It is found that the momentum-dependent baryon potentials affect the excitation function of the K+ cross-section; at low bombarding energies of ∼ 1.0GeV the attractive baryon potentials in the final state lead to a relative enhancement of the kaon yield, whereas the net repulsive potential at bombarding energies ∼ 2GeV causes a decrease of the K+ cross-section. Furthermore, it is pointed out that especially the K+ spectra at low momenta (or kinetic energy T K) allow to determine the in-medium K+ potential almost model independently due to a relative shift of the K+ spectra in kinetic energy that arises from the acceleration of the kaons when propagating out of the nuclear medium to free space, i.e. converting the potential energy to the kinetic energy of the free kaon.

The nonmesonic decay of Λ hyperons has been investigated by the observation of delayed fission of heavy hypernuclei produced in proton–U collisions at T_p &equal; 1.9 GeV. The lifetime of heavy hypernuclei with masses A ≈ 220 of the present work, τ_Λ &equal; (138 ± 6 (stat) ± 17 (syst)) ps, is the most accurate result obtained in proton and antiproton induced collisions on a U target so far. This result together with the values of the lifetime measured previously by the COSY-13 collaboration, for p+Bi (161 ± 7(stat) ± 14(syst)) ps and p+Au (130 ± 13(stat) ± 15(syst)) ps, gives an average value for the lifetime of heavy hypernuclei with masses A > 180: τ_Λ &equal; (145 ± 11) ps. This value was obtained by averaging the above lifetimes with weights taken as reciprocals of the sum of squares of statistical and systematic errors. By comparing the lifetimes of light and heavy hypernuclei we found—on the confidence level of 0.98—a violation of the phenomenological ΔI &equal; 1/2 rule known from the weak mesonic decays of strange particles.

Abstract:: The lifetime of the Λ-hyperon for the nonmesonic decay ΛN → NN has been determined by a measurement at COSY Jülich of the delayed fission of heavy hypernuclei produced in proton-Au collisions at T p = 1.9 GeV. It is found that heavy hypernuclei with mass numbers A≈ (180±5) and atomic numbers Z≈ (74±2) fission with a lifetime τΛ = 130±13 (stat.) ±15 (syst.) ps. This value together with the results obtained for other heavy hypernuclei in previous investigations indicates --on the confidence level of 0.9-- a violation of the phenomenological ΔI = 1/2 rule for the ΛN → NN transitions as known from the weak mesonic decays of kaons and hyperons.

The production ofK+ mesons in proton-nucleus and deuteron-nucleus collisions is analyzed with respect to one-step nucleon-nucleon (NN ? N ?K+),?- nucleon (?N ? N ? K+) and two-step pion-nucleon (pN ? K+?) production channels on the basis of experimental ground state momentum distributions and free on-shell production processes. Whereas forK+ production in proton-nucleus reactions the secondary channelpN clearly dominates at subthreshold energies, meson and nucleon induced channels are of similar magnitude in deuteron-nucleus reactions. Contrary to nucleus-nucleus collisions the? induced reaction channels are found to be of minor importance. The experimental differentiation of the underlying microscopic reaction channels appears possible via differential proton —K+ coincidence measurements as shown in detail by the microscopic simulations including proton rescattering.

Angular distributions of the isospin-forbidden reaction6Li(6Li, d2)10B0+,1 were measured at six beam energies in the range from 3 to 8 MeV. The contribution of two-step transfer mechanism to the reaction under consideration was found to be negligible. Hauser-Feshbach model cross sections, obtained with parameters derived from a previous analysis of isospin-allowed reactions in the6Li +6Li system are consistent with the data, provided a 2% reduction factor reflecting isospin mixing of compound nucleus levels. From this factor an isospin-breaking matrix element of c2>1/2˜13 keV was deduced. The presence of one broad or several resonances with (JP,T)=(1-, 1) or (2-,1) concentrated at an excitation energy of about 30.3 MeV in12C seems to be responsible for the observed shape of angular distributions and their energy dependence.

Elastic and inelastic scattering in the system7Li+7Li was investigated experimentally in the energy range of Elab=8–17 MeV applying the method of kinematic coincidence. Sizeable cross sections were observed in the whole energy range for inelastic excitation of a single7Li nucleus to its first excited state atE*=0.47761 MeV as well as for mutual excitation of both nuclei. Systematic optical model and both one-step and two-step DWBA calculations were performed with the aim of describing in a consistent manner all three channels studied. This was achieved with optical model potentials fitted simultaneously to all elastic scattering data. They were then used to generate form factors for both single and mutual inelastic scattering as well as distorted waves for DWBA description of inelastic scattering considering the same deformation length for all form factors. Potential parameters derived show, despite these constraints, a continuous ambiguity in case of volume absorption while those with surface absorption can be grouped in six distinct families each of them featuring continuous ambiguities. The impact of real and imaginary form factors on single inelastic excitation is reversed for potentials with volume and surface absorption, while mutual excitation is dominated by imaginary form factors.

The differential cross sections for the n+d elastic scattering and for the 2H(n,np)n break-up reaction in the np quasi-free scattering configuration was measured at 67 MeV. The experimental data are compared with results of the calculations based on a rigorous solution of the three-nucleon Faddeev equations using meson-exchange potentials. Good agreement is found between experiment and theory for the n+d elastic-scattering cross section while in the case of np quasi-free scattering the theoretical predictions overestimate the data at neutron angles larger than 40° by about 30%.

We study the production ofΛ hyperons inp + A reactions within the BUU transport approach including elasticΛ N rescattering and extract theΛ sticking probabilities and hypernucleus formation cross sections as a function of bombarding energy and target mass. At 1.5 GeV about 25% of theΛ hyperons are found to be bound in case of heavy nuclei like238U leading to large cross sections forΛ-hypernuclei. The respective hypernuclei show broad distributions in excitation energy, momentum and angular momentum. The decay of theΛ hyperon in case of heavy targets is found to be almost entirely due to nonmesonic decay channels, i.e.ΛN → NN processes.

Angular distributions of the12C(d,7Li)7Be reaction to the ground and to the unresolved first excited states of7Li or7Be nuclei were measured in the entire angular region in the centre of mass system atElab=78.0 MeV. Both the one-step five-nucleon cluster transfer as well as two-step sequential transfers were considered in the theoretical analysis. The former mechanism was found to dominate, but the latter is also not negligible.

Angular distributions of the10B(a,7Li)7Be reaction were measured atElab = 91.8 MeV in the entire center of mass angular region. A DWBA analysis explains the reaction as triton and helion transfer in the forward and backward angular regions, respectively.

We study the production of $\Lambda$ hyperons in $p + A$ reactions on the basis of a BUU transport approach for $^{32}S$, $^{65}Cu$,$^{120}Sn$ , $^{197}Au$, $^{208}Pb$ and $^{238}U$ from 1.1 to 1.7 GeV beam energy and evaluate the properties of the hypernuclei produced with respect to excitation energy, transverse and longitudinal momentum, angular momentum and mass. Furthermore, the decay of the hypernuclei by particle emission in competition with fission channels is followed by a statistical model calculation. We find the system $p + ^{197}Au$ at about 1.5 GeV to be optimally suited for the production of heavy hypernuclei, where the $\Lambda$-decay is expected to be dominated by nonmesonic decay channels .

We study the production ofΛhyperons inp+Areactions on the basis of a BUU transport approach for32S,65Cu,120Sn,197Au,208Pband238Ufrom 1.1 to 1.7 GeV beam energy and evaluate the properties of the hypernuclei produced with respect to excitation energy, transverse and longitudinal momentum, angular momentum and mass. Furthermore, the decay of the hypernuclei by particle emission in competition with fission channels is followed by a statistical model calculation. We find the systemp+197Auat about 1.5 GeV to be optimally suited for the production of heavy hypernuclei, where theΛ-decay is expected to be dominated by nonmesonic decay channels.

We have developed a method to treat multi-nucleon transfer as the transfer of two — possibly different — subclusters, as e.g. with “5Li”=(ap). As a consequence we take into account two reaction mechanisms, the one-step simultaneous and the two-step sequential transfer of the two clusters. We formulate the method of calculation of the simultaneous transfer form factor for two non-identical particles and also of the two-cluster spectroscopic amplitudes from shell model wave functions. We apply the method to the elastic transfer reaction11B(16O,11B)11O together with the single a andp transfer reaction11B(16O,15N)12C forElab between 30 and 60 MeV. We obtain a consistently good description of all the data by reasonable adjustment of the spectroscopic amplitudes. In particular we find that the simultaneous (ap) transfer is considerably more important than the sequential transfer indicating strong five-nucleon correlations in these light nuclei.

Angular distributions of cross sections have been measured for single nucleon transfer reactions induced by 344.5 MeV12C ions on a12C target. DWBA analyses have been performed for transitions to discrete final states. Spectroscopic information has been derived and is compared with results of other studies.

Angular distributions of protons, deuterons, tritons and alpha-particles emitted from the reactions in thed+9Be-system atEd=7 MeV as well as excitation functions at selected angles in the energy rangeEd=6.5–7.5 MeV (LAB) were measured. The potential part of the elastic scattering is described by the phenomenological optical model. The compound nucleus contribution to all exit channels is determined using the Hauser-Feshbach model. The collective excitation of the 2.43 MeV excited state of9Be and transfer processes are analysed within the DWBA formalism. The analyses suggest a significant contribution of five-nucleon transfer to the (d,4He) channel.

Abstract:: It is shown that the mass dependence of the Λ-lifetime in heavy hypernuclei is sensitive to the ratio of neutron-induced to proton-induced non-mesonic decay rates R n/R p. A comparison of the experimental mass dependence of the lifetimes with the calculated ones for different values of R n/R p leads to the conclusion that this ratio is larger than 2 on the confidence level of 0.75. This suggests that the phenomenological ΔI= 1/2 rule might be violated for the nonmesonic decay of the Λ-hyperon.