On polarization of strange baryons in reactions p + p↦p + Λ0 + K+ and p + p↦p + Σ0 + K+ near thresholds

Polarization properties of strange baryons produced in pp reactions, p + p↦p + Λ⁰ + K⁺ and p + p↦p + Σ⁰ + K⁺, near thresholds of the final states pΛ⁰K⁺ and pΣ⁰K⁺ are analysed relative to polarizations of colliding protons. The cross-sections for pp reactions are calculated within the effective Lagrangian approach accounting for strong pp rescattering in the initial state of colliding protons with a dominant contribution of the one-pion exchange and strong final-state interaction of daughter hadrons (Eur. Phys. J. A 9, 425 (2000)). Received: 22 October 2001 / Accepted: 14 November 2001     

Chiral dynamics and the reactions pp↦dK+ and pp↦dπ+η

We perform a study of the final-state interactions of the K ⁺ and the d systems in the reactions pp↦dK ⁺ and pp↦dπ⁺η. Since the two-meson system couples strongly to the a ₀(980) resonance, these reactions are expected to be an additional source of information about the controversial scalar sector. We also show that these reactions present peculiar features which can shed additional light on the much debated meson-baryon scalar sector with strangeness -1. We deduce the general structure of the amplitudes close to the dK ⁺ threshold, allowing for primary K ⁺ as well as π⁺η production with the two mesons in relative S- or P-wave. The interactions of the mesons are accounted for by using chiral unitary techniques, which generate dynamically the a ₀(980) resonance, and the d interaction is also taken into account. General formulae are derived that allow to incorporate the final-state interactions in these systems for any model of the production mechanism. We illustrate this approach by considering two specific production mechanisms based on three flavor meson-baryon chiral perturbation theory. It is demonstrated that in this scenario the d interactions are very important and can change the cross-section by as much as one order of magnitude. The amount of π⁺ηversus K ⁺ production is shown to depend critically on the primary mixture of the two mechanisms, with large interference effects due to final-state interactions. These effects are also shown to occur in the event distributions of invariant masses which are drastically modified by the final-state interactions of the two-meson or the d system. Received: 28 September 2001 / Accepted: 26 November 2001     

Production of a0-mesons in the reactions πN↦a0N and pp↦da0 + at GeV energies

We investigate the reactions πN↦a ₀ N and pp↦da ₀ ⁺ near threshold and at medium energies. An effective Lagragian approach and the Regge pole model are applied to analyze different contributions to the cross-section of the reaction πN↦a ₀ N. These results are used to calculate the differential and total cross-sections of the reaction pp↦da ₀ ⁺ within the framework of the two-step model in which two nucleons produce an a₀-meson via π-meson exchange and fuse to a deuteron. The necessity of new measurements on a₀ production and branching fractions (of its decay to the K and πη channels) is emphasized for clarifying the a₀ structure. Detailed predictions for the reaction pp→da ₀ ⁺ are presented for the energy regime of the proton synchrotron COSY-Jülich. Received: 4 August 2000 / Accepted: 17 September 2000     

Measurement of γp→K + Λ and γp→K + Σ 0 at photon energies up to 1.47 GeV

The reactions pK ⁺ and pK ^{+ 0} have been measured with the multiparticle detector system SAPHIR at ELSA in Bonn. Besides the differential cross sections the polarization and, for the first time, the ⁰ polarization have been determined in a photon induced reaction. All data are presented as functions of the photon energy (from threshold up to 1.47 GeV) and of the kaon production angle (0°–180°). The polarization of both and ⁰ is substantial at all energies and varies strongly with the production angle.This work is supported by the Bundesminister für Forschung und Technologie (BMFT), FK 06 BN 621 I     

Study of the reactions γp → K+ Σ± π∓ at photon energies up to 2.6 GeV

The reactions γp → K⁺ Σ^± π^? were studied with the SAPHIR detector using a tagged photon beam at the electron stretcher facility ELSA in Bonn. The decays Σ ^? → nπ ^? and Σ ⁺ → nπ ⁺, pπ ⁰ were fully reconstructed. Reaction cross-sections were measured as a function of the photon energy from threshold up to 2.6 GeV with considerably improved statistics compared to a previous bubble chamber measurement. The cross-sections rise monotonically with increasing photon energy. The two-particle mass distributions of Σ ^± π ^? and K ⁺ π ^? show substantial production of resonant states.     

Production of a0 +-mesons in the reaction pp↦da0 +

We investigate the reaction pp↦da ₀ ⁺ at COSY and SIS energies together with accompanying background reactions and inclusive particle yields. The a ₀ ⁺ is considered as a usual u quark model state with two decay channels a ₀ ⁺↦K ⁺ and a ₀ ⁺↦π⁺η. Calculated cross-sections for a ₀ ⁺ production as well as for the corresponding non-resonant channels pp↦dK ⁺ and pp↦dπ⁺η are compared. Especially, in case of the final channel dπ⁺η, high statistics measurements are necessary to extract the a ₀ ⁺ signal from the high non-resonant background. Received: 20 December 2000 / Accepted: 7 May 2001     

On the production mechanism of Σ-hypernuclear systems in A(K −, π±) reactions

It is shown that the new data on the excitation energy E _exc spectrum of the residual nuclear system in the Σ-hypernuclear region in the reactions (K ^?, π^±) on ⁹Be and in the reaction (K ^?, π⁺) on ⁴He and ¹²C can be described without assuming the existence of excited Σ-hypernuclear states. The basis is formed by a simultaneous consideration of the quasifree Σ production and Σ-nuclear rescattering (elastic and with Σ → Λ conversion) with allowance for the interference of the respective amplitudes. To answer completely the question on the nature of the irregularities in E _exc spectrum, it is proposed to study the picture corresponding to the so-called moving complex singularity of the triangle graph with Σ rescattering: the position and the width of the peak in E _exc distribution should be appreciably changed with momentum transferred from the initial kaon to the final pion.     

\bar K K and\bar p p off-shell annihilations fromK − +p → Λ + pions at 4.2 GeV/c

We have investigated the properties of off-shell \(\bar K\) K and \(\bar p\) p annihilations into pions by using the interactionsK ^? p → Λ + pions at 4.2 GeV/c. The properties considered are multiplicities, hadronic temperatures and angular distributions. These properties appear more akin to those of \(\bar p\) p annihilations in flight than to that of \(\bar p\) p annihilation at rest despite the fact that the annihilation energy is small.     

Study of the reactionsπ − p→K 0 ∑ 0(1385) andπ − p→K + ∑ −(1385) at 3.95 GeV/c

The reactionsπ ^?p→K ⁰ ∑ ⁰(1385) andπ ^? p→K ⁺ ∑ ^?(1385) are studied at an incident momentum of 3.95 GeV/c using data from a high statistics bubble chamber experiment corresponding to approximately 90 events/μb. The total and differential cross sections and the density matrix elements of the Σ(1385) are presented. The results are compared with those obtained for the related processesπ ^pp→K ⁺ ∑ ⁺(1385) and \(K^ - p \to \pi ^ \mp \sum ^ \pm (1385)\) in this energy range. Evidence is presented for the existence of production mechanisms with exotic exchanges in thet channel.     

Asymmetry measurement of the polarized Σ+p elastic scattering and the hyperon-nucleon spin-orbit interaction

In order to study the hyperon-nucleon spin-orbit interaction, asymmetries of the polarized Σ⁺ elastic scattering on protons have been measured. The polarized hyperons were produced through the p(π⁺, K⁺) Σ⁺ reaction in a liquid-scintillator active target, and tracks of Σ⁺p scattering events were recorded with a newly developed track detector named SCITIC (scintillating track image camera). Although analyses of 10 times more data with more sophisticated data-taking systems and more elaborated analyzing tools are under way, data from an early stage of this experiment indicate a large Σ⁺p spin-orbit interaction in contrast to the case of the small ΛN spin-orbit interaction. Received: 27 May 2002 / Accepted: 23 July 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: nakai@post.kek.jp Communicated by Th. Walcher     

On the structure of Σ-hypernuclei

We investigate the-nucleus potential in the framework of a relativistic mean field approximation. As input for the elementaryN interaction we supplement the-repulsion by the exchange of an effective, complex-meson to account for theN N conversion. As the main result we find that the strength of ths absorptive central potential is smaller than the corresponding real part, whereas in the spin-orbit interaction, which is expected to be much weaker than for a nucleon, the imaginary component dominates.This work was partially supported by CNPq, FINEP (Brazil) and by the Bilateral Cooperation W. Germany — Brazil (KFA — CNPq).     

The reaction pp→pΛK + near threshold

We analyze the recent total cross section data for pp→pΛK ⁺ near threshold measured at COSY. Using an effective range approximation for the on-shell pΛ S-wave final state interaction we extract from these data the combination ?= (2|K _s|²+|K _t|²)^−1/2= 0.38 fm⁴ of the singlet (K _s) and triplet (K _t) threshold transition amplitudes. We present an exploratory calculation of various (tree-level) vector and pseudoscalar meson exchange diagrams. Pointlike ω-exchange alone and the combined (ρ⁰,ω,K^*+)-exchange can explain the experimental value of ?. The pseudoscalar meson exchanges based on a SU(3) chiral Lagrangian turn out to be too large. However, when adding π⁰-exchange in combination with the resonant πN→S ₁₁(1650) →KΛ transition and introducing monopole form factors with a cut-off Λ_c= 1.5 GeV one is again able to reproduce the experimental value of ?. More exclusive measurements are necessary to reveal the details of the pp→pΛK ⁺ production mechanism. Received: 28 October 1998 / Revised version: 12 January 1999     

Interference effects inK − p→K − K + Λ at 4.2 GeV/c and the thresholdK\bar K S wave amplitude

We study the interference phenomena between the ? meson and the underlyingK ^? K ⁺ S wave in the reactionK ^? p→K ^? K ⁺ Λ at 4.2GeV/c. We perform an amplitude analysis of the double multipole moments and investigate the \(K\bar K \to K\bar K\) S wave amplitude.     

Mesons of the f0 family in processes ππ↦ππ, K¯K up to 2 GeV and the chiral-symmetry breaking

In a combined analysis of the experimental data on the coupled processes ππ↦ππ, KˉK in the channel with I ^G J ^PC = 0⁺0^{+ +}, the various scenarios of these reactions (with different numbers of resonances) are considered. In a model-independent approach, based only on analyticity and unitarity, a resonance is represented by a pole cluster (poles on the Riemann surface) of the definite type that is defined by the state nature. The best scenario contains the resonances f ₀(665) (with properties of the σ-meson), f ₀(980) (with a dominant sˉs component), f ₀(1500) (with a dominant flavour-singlet, e.g., glueball component) and the f ₀(1710) (with a considerable sˉs component). If the f ₀(1370) exists, it has a dominant sˉs component. The coupling constants of the observed states with the considered channels and the ππ and KˉK scattering lengths are obtained. The conclusion on the linear realization of chiral symmetry is drawn. Received: 25 April 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: surovcev@thsun1.jinr.ru RID="b" ID="b"e-mail: krupa@savba.sk RID="c" ID="c"e-mail: fyzinami@nic.savba.sk Communicated by V.V. Anisovich     

Study of theK + K − π+ π− system centrally produced in the reactions π+ p→π+(K + K − π+ π−)p andpp→p(K + K − π+ π−)p at 85 GeV/c

The reactions π⁺ p→π⁺(K ⁺ K ^? π⁺ π^? p andpp→p(K ⁺ K ^? π⁺ π^?)p where the (K ⁺ K ^? π⁺ π^?) system is centrally produced have been studied at 85 GeV/c. 48% of the final state proceeds through single or double vector meson resonant production i.e.,K ^*0(890) (37%), ? (4%),ρ ⁰ (7%). Evidence is found for associatedK ^*0 (890) \(\bar K\) ^*o (890) production (6.4±1.1%) with a cross section ten times higher than that found for associated ? ? production in the same experiment.     

A measurement of the lifetimes of Ξ0 and Λ hyperons

The mean lifetimes of the Λ and Ξ⁰ hyperons have been measured in a short neutral beam at the CERN Proton Synchrotron. Λ and Ξ⁰ decays have been identified by measuring their decay products in a magnetic spectrometer and in a lead glass hodoscope. The experimental results, based on 53 000 Λ decays and 6300 Ξ⁰ decays are

From the result for τ_Ξ⁰ together with existing data on τ_Ξ^? we obtain a violation of the $\text{ΔI =}\text{1}\text{2}$ rule in non-leptonic Ξ decays.     

The reaction e− + p → Λ + ν and the contributions of the individual form factors to the differential cross section

We calculate the differential cross section for the weak, strangeness changing, electron scattering process, e⁻ + p → Λ + ν, for incoming electron energies of 0.5, 1.0, 2.0, 4.0, and 6.0 GeV. We obtain as well contributions of the individual form factors to the differential cross sections. We find that the differential cross sections peak as the maximal scattering angle for the Λ is approached and that the peak height increases as the electron energy is increased. The behavior of the differential cross section near the maximal angle is discussed as is the possibility of observing this reaction in a facility such as TJNAF.