S. Merzliakov

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.

A self-triggering silicon tracking telescope for spectator proton detection

With the ANKE spectrometer at the COoler SYnchrotron COSY Juelich the mesonic structure of the nucleon will be studied in polarized proton-proton and proton-deuteron collisions. The identification and tracking of low energy protons permits using deuterium as an effective neutron target. For this purpose, modular self-triggering tracking telescopes built up by double-sided silicon strip detectors inside the accelerator ultra-high vacuum have been developed. Their basic features are /spl Delta/E/E proton identification from 2.5

Near-threshold production of phi mesons in pp collisions.

40 MeV and particle tracking over a wide dynamic range, either 2.5 MeV spectator protons or minimum ionizing particles. By the use of self-triggering read-out chips, the telescopes identify a particle passage within 100 ns and therefore allow the possibility of a fast hit pattern recognition. In combination with a read-out pitch of /spl sim/200 /spl mu/m, they provide a high rate capability. The recent development of very thick (/spl ges/5 mm) double-sided microstructured Si(Li) and very thin (/spl les/65 /spl mu/m) double-sided Si-detectors provides the use of the telescopes over a wide range of particle energies.

Polarizing a Stored Proton Beam by Spin-Flip?

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.

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

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.

The energy dependence of the pp->K+ n Sigma+ reaction close to threshold

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

Production of the S01 diproton in the pp→ppπ0 reaction at 0.8 GeV

\vec dp \to \{ pp\} _s n