Forward dispersion relations and Roy equations in <Emphasis Type="Italic">ππ</Emphasis> scattering

We first review the results of an analysis of ππ interactions in S, P and D waves for the two-pion effective mass from threshold to about 1.4GeV. In particular, we show a recent improvement of this analysis above the Kˉ threshold using more data for phase shifts and including the S0-wave inelasticity from ππ→Kˉ. In addition, we have improved the fit to the f ₂(1270)-resonance and used a more flexible P-wave parametrization above the Kˉ threshold and included an estimation of the D2-wave inelasticity. The better accuracy thus achieved also required a refinement of the Regge analysis above 1.42GeV. Finally, in this work we check that the ππ scattering amplitudes obtained in this approach satisfy remarkably well forward dispersion relations and Roy's equations.     

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     

The <Emphasis Type="Italic">Φ</Emphasis>→K<Superscript>0</Superscript>¯<Emphasis Type="Italic">γ</Emphasis> decay

The scalar contributions to the radiative decay φ→K ⁰ˉ⁰ γ are studied within the framework of the Linear Sigma Model (LσM). Theoretical predictions for the associated subprocesses φ→f ₀ γ and φ→a ₀ γ as well as the ratio φ→f ₀ γ/a ₀ γ are also given.     

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     

The K * Kπ and ρππ couplings in QCD

The light cone QCD sum rules are derived for the K ^* Kπ coupling g _K ^* Kπ and the ρππ coupling g _ρππ. The contribution from the excited states and the continuum is subtracted cleanly through the double Borel transform with respect to the two external momenta, p ₁ ², p ₂ ²= (p−q)². Our result g _K ^* Kπ= (8.7 ± 0.5) and g _ρππ= (11.5 ± 0.8) is in good agreement with the experimental value. Received: 31 July 1998 / Revised version: 20 November 1998     

Comparative Regge analysis of Λ,ΣΛ(1520) and Θ<Superscript>+</Superscript> production in γp,πp and pp reactions

Using the Quark-Gluon Strings Model --combined with Regge phenomenology-- we perform a comparative analysis of Λ, Σ⁰, Λ(1520) and Θ⁺ production in binary reactions induced by photon, pion and proton beams on the nucleon. We find that the existing experimental data on the γp↦K⁺Λ differential and total cross-sections can be described very well by the model for photon energies 1-16 GeV and - t < 2 GeV² assuming a dominant contribution of the K^* Regge trajectory. Moreover, using the same parameters we also reproduce the total γp↦K⁺Σ⁰ and γp↦K⁺Λ(1520) cross-sections suggesting a “universality” of the Regge model. In order to check the consistency of the approach we evaluate the differential and total cross-sections for the reaction π^-p↦K⁰Λ which is also found to be dominated by the K^* Regge trajectory. Using the apparent “universality” of the Regge model we extend our scheme to the analysis of the binary reactions γp↦¯⁰Θ⁺, π^-p↦K^-Θ⁺ and pp↦Σ⁺Θ⁺ as well as the exclusive and inclusive Θ⁺ production in the reactions pp↦p¯⁰Θ⁺ and pp↦Θ⁺X. Our detailed studies demonstrate that Θ⁺ production does not follow the “universality” principle, thus suggesting an essentially different internal structure of the exotic baryon relative to conventional hyperons or hyperon resonances.     

K-matrix analysis of the (IJ<Superscript>PC</Superscript> = 00<Superscript>+ +</Superscript>)-wave in the mass region below 1900 MeV

We present the results of the current analysis of the partial wave IJ ^PC = 00^{+ +} based on the available data for meson spectra ( ππ, KˉK,ηη,η,ππππ). In the framework of the K-matrix approach, the analytical amplitude has been reconstructed in the mass region 280 MeV < < 1900 MeV. The following scalar-isoscalar states are seen: comparatively narrow resonances f ₀(980), f ₀(1300), f ₀(1500), f ₀(1750) and the broad state f₀(1200-1600). The positions of the amplitude poles (masses and total widths of the resonances) are determined as well as pole residues (partial widths to meson channels ππ, KˉK,ηη,η,ππππ). The fitted amplitude gives us the positions of the K-matrix poles (bare states) and the values of bare-state couplings to meson channels thus allowing the quark-antiquark nonet classification of bare states. On the basis of the obtained partial widths to the channels ππ, KˉK,ηη,η, we estimate the quark/gluonium content of f ₀(980), f ₀(1300), f ₀(1500), f ₀(1750), f₀(1200-1600). For f ₀(980), f ₀(1300), f ₀(1500) and f ₀(1750), their partial widths testify the qˉq origin of these mesons though being unable to provide precise evaluation of the possible admixture of the gluonium component in these resonances. The ratios of the decay coupling constants for the f₀(1200-1600) support the idea about the gluonium nature of this broad state. Received: 14 May 2002 / Accepted: 20 August 2002 / Published online: 11 February 2003 RID="a" ID="a"e-mail: anisovic@thd.pnpi.spb.ru Communicated by A. Sch?fer     

Study of the proton weak decay of 12ΛCg.s. with FINUDA

The FINUDA experiment studies the Λ -hypernuclei formation and decay using the reaction ( K ^- _stop,π^- induced by the low-momentum K^- from the deacy of the φ -meson produced at the (e ⁺, e ^- collider DAφ NE at the Laboratory Nazionali di Frascati of INFN. In this paper we present new data on the proton spectra following the weak decay of ¹² _ΛC .     

KNΛ and KNΣ coupling constants from the Chiral Bag Model

The KNΛ and KNΣ coupling constants have been calculated in the framework of the Chiral Bag Model(CBM). We find −3.88 ≤g _KNΛ≤−3.67 and 1.15 ≤g _KNΣ≤ 1.24 by taking into account pseudoscalar mesons (π, K) and vector mesons (ρ, ω, K ^*) field effects. Particularly, it is shown that vector mesons make significant contributions to the coupling constants g _KNΛ and g _KNΣ. Our values are existing within the experimental limits compared to the phenomenological values extracted from the kaon photoproduction and kaon-nucleon scattering experiments. Also, form factors are suggested for the πNN, πNΔ, KNΛ and KNΣ couplings. Received: 17 August 1998 / Revised version: 22 December 1998     

The pp→K<Superscript>+</Superscript><Emphasis Type="Italic">Σ</Emphasis><Superscript>+</Superscript>n cross-section from missing-mass spectra

We utilize existing inclusive data on K⁺-meson momentum spectra of the reaction pp→K ⁺ X at T _p = 2.3-2.85GeV to deduce total cross-sections for pp→K ⁺ Σ ⁺ n. The method used to extract those cross-sections is explained and discussed in detail. Our result for T _p = 2.85GeV is consistent with the data point from a direct measurement at the same beam energy. The cross-section obtained for T _p = 2.3GeV is with 13.7±2.3μb considerably smaller than the value found in a recent experiment by the COSY-11 Collaboration at a somewhat lower beam energy, indicating that the pp→K ⁺ Σ ⁺ n reaction cross-section could exhibit a rather unusual energy dependence.     

Meson exchange hyperon-nucleon interaction based on correlated ππ/K¯ exchange

We present a new model for the hyperon-nucleon (ΛN , ΣN interaction, derived within the meson exchange framework. The model incorporates the standard one-boson exchanges of the lowest pseudoscalar and vector meson multiplets with coupling constants fixed by SU(6) flavor symmetry relations. As a new feature, the contribution in the scalar-isoscalar (σ sector is derived from a microscopic model of correlated ππ and Kˉ exchange. The same model is also used to constrain the interaction resulting from the vector-isovector (ρ exchange channel. Additional short-ranged ingredients of the model in the scalar-isovector (a₀) and scalar-isospin-1/2 (κ channels are likewise viewed as arising from meson-meson correlations but are treated phenomenologically. With this model a satisfactory reproduction of the available hyperon-nucleon data is achieved.     

<Emphasis Type="Italic">Λ</Emphasis>N scattering length from the reaction <Emphasis Type="Italic">γ</Emphasis>d→K<Superscript>+</Superscript><Emphasis Type="Italic">Λ</Emphasis>n

The prospects of utilizing the strangeness production reaction γd→K ⁺ Λn for the determination of the Λn low-energy scattering parameters are investigated. The spin observables that need to be measured in order to isolate the Λn singlet (¹ S ₀) and triplet (³ S ₁) states are identified. Possible kinematical regions where the extraction of the Λn scattering lengths might be feasible are discussed.     

Forward analysis of πN scattering with an expansion method

The π N forward-scattering data are analyzed using an expansion method, where the invariant amplitudes are represented by expansions satisfying the forward dispersion relations. The experimental errors of the data are taken into account through the covariance matrix of the coefficients of the expansions in a careful error analysis. From the results, some coefficients, c _n0 ^± , of the subthreshold expansions have been calculated with proper error bars.     

Search for d' in pion inclusive spectra from pp-interactions

We report on our measurement of the inclusive spectrum of positive pions from the reaction pp→π⁺+X on a CH ₂ internal target of the ITEP proton synchrotron at 1.5 GeV/c with a missing mass resolution of about 1 MeV/c². The goal of our experiment was the search for the hypothetical narrow πNN resonance (0.5\ MeV/c² width and a mass of 2.06\ GeV/c²) with I(J ^P P)=even(0⁻), called d′. We do not see any statistically significant d′-signal. The upper limit for d′ production cross-section in pp-interactions is near 2μb at 84%C.L. Received: 5 November 1997     

Measurement of pionium lifetime with the Dirac spectrometer

Pionium ( π ⁺ π ^- bound state) lifetime is measured with improved precision with respect to earlier work, and the ππ s-wave scattering length difference between I = 0 and I = 2 amplitudes | a ₀ - a ₂| is determined to 5% precision.     

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 kaonic hydrogen. Phenomenological quantum field theoretic model revisited

We argue that due to isospin and U-spin invariance of strong low-energy interactions the S-wave scattering lengths a ⁰ ₀ and a ¹ ₀ of ˉN scattering with isospin I = 0 and I = 1 satisfy the low-energy theorem a ⁰ ₀ +3a ¹ ₀ = 0 valid to leading order in chiral expansion. In the model of strong low-energy ˉN interactions at threshold (Eur. Phys. J. A 21, 11 (2004)) we revisit the contribution of the Σ(1750) resonance, which does not saturate the low-energy theorem a ⁰ ₀ +3a ¹ ₀ = 0, and replace it by the baryon background with properties of an SU(3) octet. We calculate the S-wave scattering amplitudes of K^-N and K^-d scattering at threshold. We calculate the energy level displacements of the ground states of kaonic hydrogen and deuterium. The result obtained for kaonic hydrogen agrees well with recent experimental data by the DEAR Collaboration. We analyse the cross-sections for elastic and inelastic K^-p scattering for laboratory momenta 70MeV/c < p _K < 150MeV/c of the incident K^--meson. The theoretical results agree with the available experimental data within two standard deviations.     

Chiral symmetry amplitudes in the S-wave isoscalar and isovector channels and the σ, f0(980), a0(980) scalar mesons

We use a non-perturbative approach which combines coupled channel Lippmann-Schwinger equations with meson-meson potentials provided by the lowest order chiral Lagrangian. By means of one parameter, a cut off in the momentum of the loop integrals, which results of the order of 1 GeV, we obtain singularities in the S-wave amplitudes corresponding to the σ, f₀ and a₀ resonances. The ππ → ππ, phase shifts and inelasticities in the T = 0 scalar channel are well reproduced as well as the π⁰η and mass distributions in the T = 1 channel. Furthermore, the total and partial strong decay widths of the f₀ and a₀ resonances are properly reproduced. The results seem to indicate that chiral symmetry constraints at low energy and unitarity in coupled channels is the basic information contained in the meson-meson interaction below GeV.     

ππ scattering and the meson resonance spectrum

A ππ, ˉKK, and ρρ(ωω) fully coupled channel model is used to predict the lowest isospin S, P, D, F-wave phase shifts and inelasticities for elastic ππ scattering from threshold to 2.0 GeV. As input the S-matrix is required to exhibit poles corresponding to the meson resonance table of the Particle Data Group. As expected, the ππ inelasticity is very strongly related to the opening of the ˉK channel near 1 GeV, and the opening of ρρ(4π) and ωω(6π) channels in the 1.5 GeV region. The predictions of this model are compared to the various elastic ππ→ππ amplitudes, that were obtained from analyses of π⁻ p →π⁻π⁺n data. The role of the various resonances, in particular the glueball candidate f ₀(1500) and the f _J(1710) is investigated. Received: 19 November 1997     

IR-renormalon contributions to the structure functions g 3 and g 5

We calculate the leading 1/N _f perturbative contributions to the polarized nonsinglet structure functions g ₃ and g ₅ to all orders in α_s. The contributions from the first renormalon pole are determined. It is a measure for the ambiguity of the perturbative calculation and is hypothetically assumed to dominate the power corrections. The corrections Δg ₃ and Δg ₅ are given as functions of the Bjorken variable x and turn out to be negligible. The anomalous dimensions of the leading twist operators are obtained in the next-to-leading order. Received: 10 June 1998 / Revised version: 25 September 1998