H. J. Stein

ANKE, a new facility for medium energy hadron physics at COSY-Jülich

Abstract ANKE is a new experimental facility for the spectroscopy of products from proton-induced reactions on internal targets. It has recently been implemented in the accelerator ring of the cooler synchrotron COSY of the Forschungszentrum Julich (FZ-Julich), Germany. The device consists of three dipole magnets, various target installations and dedicated detection systems. It will enable a variety of hadron-physics experiments like meson production in elementary proton–nucleon processes and studies of medium modifications in proton–nucleus interactions.

Precision study of the eta He-3 system using the dp ---> He-3 eta reaction

The differential and total cross sections for the d+p->3He+eta reaction have been measured in a high precision high statistics COSY-ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy Q of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of Q to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for Q>4 MeV. The very rapid rise of the total cross section to its maximum value within 0.5 MeV of threshold implies a very large eta-3He scattering length and hence the presence of a quasi-bound state extremely close to threshold.

Measurement of the in-medium phi-meson width in proton-nucleus collisions

The production of φ mesons in the collisions of 2.83 GeV protons with C, Cu, Ag, and Au at forward angles has been measured via the φ → KK decay using the COSY-ANKE magnetic spectrometer. The φ meson production cross section follows a target mass dependence of A in the momentum region of 0.6–1.6 GeV/c. The comparison of the data with model calculations suggests that the in-medium φ width is about an order of magnitude larger than its free value.

a+0 (980)-resonance production in pp-->dK+K-0 reactions close to threshold.

The reaction pp->d K+ Kbar0 has been investigated at an excess energy of Q=46 MeV above the (K+ Kbar0) threshold with ANKE at COSY-Juelich. From the detected coincident dK+ pairs about 1000 events with a missing Kbar0 were identified, corresponding to a total cross section of sigma(pp->d K+ Kbar0)=(38 +/- 2(stat) +/- 14(syst)) nb. Invariant-mass and angular distributions have been jointly analyzed and reveal s-wave dominance between the two kaons, accompanied by a p-wave between the deuteron and the kaon system. This is interpreted in terms of a0+(980)-resonance production.

Evidence of kaon nuclear and Coulomb potential effects on soft K+ production from nuclei

The ratio of forward K+ production on copper, silver and gold targets to that on carbon has been measured at proton beam energies between 1.5 and 2.3 GeV as a function of the kaon momentum p_K using the ANKE spectrometer at COSY-Juelich. The strong suppression in the ratios observed for p_K<200-250 MeV/c can be ascribed to a combination of Coulomb and nuclear repulsion in the K+A system. This opens a new way to investigate the interaction of K+-mesons in the nuclear medium. Our data are consistent with a K+A nuclear potential of V_K~20 MeV at low kaon momenta and normal nuclear density. Given the sensitivity of the data to the kaon potential, the current experimental precision might allow one to determine V_K to better than 3 MeV.

Near-threshold production of phi mesons in pp collisions.

The pp-->ppphi reaction has been studied at the Cooler Synchrotron COSY-Jülich, using the internal beam and ANKE facility. Total cross sections have been determined at three excess energies epsilon near the production threshold. The differential cross section closest to threshold at epsilon=18.5 MeV exhibits a clear S wave dominance as well as a noticeable effect due to the proton-proton final-state interaction. Taken together with data for ppomega production, a significant enhancement of the phi/omega ratio of a factor 8 is found compared to predictions based on the Okubo-Zweig-Iizuka rule.

Polarizing a Stored Proton Beam by Spin-Flip?

We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement of the spin-flip cross section in low-energy electron–proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.

Near threshold η meson production in the dd→4Heη reaction

Abstract.The dd↦4Heη reaction has been investigated near threshold using the ANKE facility at COSY-Jülich. Both total and differential cross-sections have been measured at two excess energies, Q = 2.6 MeV and 7.7 MeV, with a subthreshold measurement being undertaken at Q = - 2.6 MeV to study the physical background. While consistent with isotropy at the lower energy, the angular distribution reveals a pronounced anisotropy at the higher one, indicating the presence of higher partial waves. Options for the decomposition into partial amplitudes and their consequences for the determination of the s-wave ηα scattering length are discussed.

The neutron-proton charge-exchange amplitudes measured in the dp → ppn reaction

Abstract.The unpolarised differential cross section and the two deuteron tensor analysing powers Axx and Ayy of the