M. Büscher

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.

Shape of the Lambda(1405) hyperon measured through its Sigma0 pi0 Decay

The pp --> pK(+)Y(0) reaction has been studied for hyperon masses m (Y-0) <= 1540 MeV/c(2) at COSY-Julich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Lambda and Sigma(0) hyperons as well as the Sigma(0)(1385) and Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well separated, though with limited statistics. The shape and position of the Lambda(1405) distribution, reconstructed cleanly in the Sigma(0)pi(0) channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Lambda( 1405) to be a two-state resonance

Lineshape of the Λ(1405) hyperon measured through its Σ0π0 decay

The pp --> pK(+)Y(0) reaction has been studied for hyperon masses m (Y-0) <= 1540 MeV/c(2) at COSY-Julich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Lambda and Sigma(0) hyperons as well as the Sigma(0)(1385) and Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well separated, though with limited statistics. The shape and position of the Lambda(1405) distribution, reconstructed cleanly in the Sigma(0)pi(0) channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Lambda( 1405) to be a two-state resonance

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.

a0(980)–f0(980) mixing and isospin violation in the reactions pN→da0, pd→3He/3Ha0 and dd→4Hea0

Abstract It is demonstrated that f0–a0 mixing can lead to a comparatively large isospin violation in the reactions pN→da0, pd→ 3 He / 3 H a 0 and dd→ 4 He a 0 close to the corresponding production thresholds. The observation of such mixing effects is possible, e.g., by measuring the forward–backward asymmetry in the reaction pn→da 0 0 →dη π 0 .

Is there an ηHe3 quasi-bound state?

Abstract The observed variation of the total cross section for the d p → He 3 η reaction near threshold means that the magnitude of the s-wave amplitude falls very rapidly with the η centre-of-mass momentum. It is shown here that recent measurements of the momentum dependence of the angular distribution imply a strong variation also in the phase of this amplitude. Such a behaviour is that expected from a quasi-bound or virtual η He 3 state. The interpretation can be investigated further through measurements of the deuteron or proton analysing powers and/or spin-correlations.

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.

How to measure the parity of the Θ+ in p→p→ collisions

Abstract Triggered by a recent paper by Thomas, Hicks and Hosaka, we investigate which observables can be used to determine the parity of the Θ + from the reaction p → p → →Σ + Θ + near its production threshold. In particular, we show that the sign of the spin correlation coefficient A xx for small excess energies yields the negative of the parity of the Θ + . The argument relies solely on the Pauli principle and parity conservation and is therefore model-independent.

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.