K+^+ production in proton-nucleus reactions and the role of momentum-dependent potentials

The production of $K^+$ mesons in proton-nucleus collisions from 1.0 to 2.5 GeV is analyzed with respect to one-step nucleon-nucleon $(NN\to N Y K^+$) and two-step $\Delta$-nucleon $(\Delta N \to K^+ Y N$) or pion-nucleon $(\pi N \to K^+ Y$) 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-J\"ulich. It is found that the momentum-dependent baryon potentials effect the excitation function of the $K^+$ cross section; at low bombarding energies of $\sim$ 1.0 GeV 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 $\sim$ 2 GeV 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 potential energy to kinetic energy of the free kaon.Comment: 11 pages, LaTeX, including 10 postscript figures, submitted to Eur. Phys. J.

Transport analysis of K+ production in proton-nucleus reactions

The production of $K^+$ mesons in proton-nucleus collisions from 1.0 to 2.3 GeV is analyzed with respect to one-step nucleon-nucleon $(NN\to N Y K^+$) and two-step $\Delta$-nucleon $(\Delta N \to K^+ Y N$) or pion-nucleon $(\pi N \to K^+ Y$) production channels on the basis of a coupled-channel transport approach (CBUU) including the kaon final-state-interactions (FSI). Momentum-dependent potentials for the nucleon, hyperon and kaon in the final state are included as well as $K^+$ elastic rescattering in the target nucleus. The transport calculations are compared to the experimental $K^+$ spectra taken at COSY-J\"ulich. Our systematic analysis of $K^+$ spectra from $^{12}C$, $^{63}Cu$, $^{107}Ag$ and $^{197}Au$ targets as well as their momentum differential ratios gives a repulsive $K^+$ potential of $20\pm 5$ MeV at normal nuclear matter density.Comment: 7 pages, 5 figures, submitted to Eur. Phys. J.

On the Delta I = 1/2 rule in the Lambda N ----> N N reaction

It is shown that the mass dependence of the $\Lambda$-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 $\Delta$I=1/2 rule might be violated for the nonmesonic decay of the $\Lambda$-hyperon.Comment: 3 pages, 2 figures, to be published in European Physical Journal

Nonmesonic decay of the Lambda-hyperon in hypernuclei produced by p+Au collisions

The lifetime of the Lambda-hyperon for the nonmesonic decay Lambda N ---> N N has been determined by a measurement at COSY Juelich 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 130ps +- 13ps (stat.) +- 15ps (syst.) . 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 Delta I = 1/2 rule for Lambda N ---> NN transitions as known from the weak mesonic decays of kaons and hyperons. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}Comment: 3 pages, 2 Postscript figures, uses svepj.clo and svjour.cls. submitted to European Physical Journal

The lifetime of the Lambda hyperon bound in hypernuclei produced by p+U collisions

The nonmesonic decay of the Lambda hyperon has been investigated by observation of delayed fission from heavy hypernuclei produced in proton-U collisions at Tp = 1.9 GeV. The lifetime of heavy hypernuclei with masses A approximately 220 obtained in the present work, i.e. tau = 138 +- 6 (stat.) +-m 17 (syst.) ps, is the most accurate result for heavy hypernuclei produced in proton and antiproton induced collisions on a U target so far. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}Comment: 16 pages, 4 Postscript figures, uses file appolb.cls (included), submitted to Acta Physica Polonica B, http://th-www.if.uj.edu.pl/act

Nonmesonic decay of the Lambda hyperon in nuclear matter - implications on the weak Lambda-N interaction

The lifetime of the Lambda hyperon in heavy hypernuclei as measured by the COSY-13 Collaboration in proton - Au, Bi and U collisions has been analysed to yield tau(Lambda) = (145 +- 11) ps. This value for tau(Lambda) is compatible with the lifetime extracted from antiproton annihilation on Bi and U targets, however, much more accurate. We find that the dependence of the lifetime tau(Lambda) on the mass of hypernuclei indicates a violation of the phenomenological Delta I = 1/2 rule known from the weak mesonic decays of strange particles. PACS: {13.30.-a}{Decays of baryons} {13.75.Ev}{Hyperon-nucleon interaction} {21.80}{Hypernuclei} {25.80.Pw}{Hyperon-induced reactions}Comment: 21 pages, 11 PostScript figures, EPJA in prin

Determination of the eta'-proton scattering length in free space

Taking advantage of both the high mass resolution of the COSY-11 detector and the high energy resolution of the low-emittance proton-beam of the Cooler Synchrotron COSY we determine the excitation function for the pp --> pp eta' reaction close-to-threshold. Combining these data with previous results we extract the scattering length for the eta'-proton potential in free space to be Re(a_{p eta'}) = 0+-0.43 fm and Im(a_{p eta'}) = 0.37(+0.40)(-0.16) fm.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev. Let