V. Koptev

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

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.

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.

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

Production of the SIGMA{sup +} hyperon through the pp->K{sup +}nSIGMA{sup +} reaction has been investigated at four energies close to threshold, 1.826,1.920,1.958, and 2.020 GeV. At low energies, correlated K{sup +}pi{sup +} pairs can only originate from SIGMA{sup +} production so their measurement allows the total cross section for the reaction to be determined. The results obtained are completely consistent with the values extracted from the study of the K{sup +}-proton correlation spectra obtained in the same experiment. These spectra, as well as the inclusive K{sup +} momentum distributions, also provide conservative upper limits on SIGMA{sup +} production rates. The measurements show a SIGMA{sup +} production cross section that varies roughly like phase space, and, in particular, none of the three experimental approaches used supports the anomalously high, near-threshold pp->K{sup +}nSIGMA{sup +} total cross section previously reported [T. Rozek et al., Phys. Lett. B 643, 251 (2006)].

Study of ω-meson production in pp collisions at ANKE

Abstract.The production of ω-mesons in the pp → ppω reaction has been investigated with the COSY-ANKE spectrometer for excess energies of 60 and 92MeV by detecting the two final protons and reconstructing their missing mass. The large physical background was subtracted using an event-by-event transformation of the proton momenta between the two energies. Differential distributions and total cross-sections were obtained after careful studies of possible systematic uncertainties in the overall ANKE acceptance. The results are compared with the predictions of theoretical models. Combined with data on the φ-meson, a more refined estimate is made of the Okubo-Zweig-Iizuka rule violation in the φ/ω production ratio.

Coupled-channel effects in the pp→ppK+K− reaction

The cross sections for the pp→ppK+K−pp→ppK+K− reaction were measured at three beam energies 2.65, 2.70, and 2.83 GeV at the COSY-ANKE facility. The shape of the K+K−K+K− spectrum at low invariant masses largely reflects the importance of KK¯ final state interactions. It is shown that these data can be understood in terms of an elastic K+K−K+K− rescattering plus a contribution coming from the production of a K0K¯0 pair followed by a charge-exchange rescattering. Though the data are not yet sufficient to establish the size of the cusp at the K0K¯0 threshold, the low mass behaviour suggests that isospin-zero production is dominant.

The pp → K+nΣ+ reaction near threshold

Inclusive K+ production in proton-proton collisions has been measured at a beam energy of 2.16 GeV using the COSY-ANKE magnetic spectrometer. The resulting spectrum, as well as those corresponding to K+ p and K+ pi(+) correlated pairs, can all be well described using consistent values of the total cross sections for the pp -> K+ p Lambda, pp -> K+ p Sigma(0), and pp -> K+ n Sigma(+) reactions. While the resulting values for Lambda and Sigma(0) production are in good agreement with world data, our value for the total Sigma(+) production cross section, sigma (pp -> K+ n Sigma(+)) = (2.5 +/- 0.6(stat) +/- 0.4(syst)) mu b at an excess energy of epsilon = 129 MeV, could only be reconciled with other recently published data if there were a highly unusual near threshold behaviour. (c) 2007 Elsevier B.V. All rights reserved.

Precision Measurement of the Quasifreepn→dϕReaction Close to Threshold

The quasifree pn-->dvarphi reaction has been studied at the Cooler Synchrotron COSY-Jülich, using the internal proton beam incident on a deuterium cluster-jet target and detecting a fast deuteron in coincidence with the K+K- decay of the varphi meson. The energy dependence of the total and differential cross sections are extracted for excess energies up to 80 MeV by determining the Fermi momentum of the target neutron on an event-by-event basis. Though these cross sections are consistent with s-wave production, the kaon angular distributions show the presence of p waves at quite a low energy. Production on the neutron is found to be stronger than on the proton but not by as much as for the eta meson.

Phenomenological analysis of K + -meson production in proton-nucleus collisions

= 0.8 and 2.9 GeV, coveringthe transition across the free nucleon-nucleon threshold at 1.58 GeV. No clear evidence for theexpected change of the dominant reaction mechanism from two-step to direct kaon productionis found. It is suggested that further data, in particular at forward angles, are taken in order toclarify the situation. It also is shown that, independent of the beam energy and emission angle,the invariant K