The quenching of nucleon yields in the nonmesonic weak decay of Λ -hypernuclei and the three-body weak decay process

Recent exclusive coincidence measurements of non-mesonic weak decays (NMWD) reported for the ratio of the partial decay width of neutron-induced-to-proton-induced NMWD, Γ_n/Γ_p , values of 0.45±0.11±0.03 and 0.51±0.13±0.04 for 5lam and ¹² _Λ C , respectively. These observations agree well with the improved theoretical Γ_n/Γ_p ratios which are in the range of 0.3-0.7. It appears that the long-standing discrepancy between the experimental and theoretical values of Γ_n/Γ_p has finally been solved. However, when compared to the results of intra-nuclear cascade (INC) calculations, the observed numbers of both single nucleons and coincident nucleon pairs are strongly quenched. The quenching of the proton yield observed previously has been interpreted as an increase of the Γ_n/Γ_p ratio. On the other hand, significant contributions from the two-nucleon-induced three-body process ΛNN → nNN are predicted. Indeed, the angular correlation of the emitted nucleon pairs in the NMWD of ¹² _Λ C showed not only decay events in back-to-back kinematics, but also events with non-back-to-back kinematics. In this paper we show that the difficulties to extract the correct Γ_n/Γ_p ratio from the proton spectra is related to the three-body weak-interaction process which strongly quenches the nucleon yields.     

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.     

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     

Phenomenological πN→ππN amplitude and the modelling of the Olsson–Turner and Chew–Low approaches

The phenomenological amplitude for the reaction πN→ππN fixed by fittings to the experimental data in the energy region 0.300 ≤P _Lab≤ 500 MeV/c is used for modelling the Chew–Low extrapolation and Olsson–Turner threshold approach. It is shown that the uncritical application of the former results in enermous theoretical errors, the extracted values being in fact random numbers. The results of the Olsson–Turner method are characterized by significant systematic errors coming from unknown details of the isobar physics. Received: 10 December 1997     

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     

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.     

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.     

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 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.     

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.     

On the possibility of f 0 observation in low energy pp collisions

Within the meson-exchange model we calculate f ₀-meson production cross section in πN and NN reactions and investigate the possibility for f ₀ observation via the KˉK decay mode in pp collisions. Our studies indicate that an extraction of the f ₀ signal is unlikely due to the large background from other reaction channels. Received: 14 October 1998 / Revised version: 17 November 1998     

Deformed C λ-Extended Heisenberg Algebra in Noncommutative Phase-Space

We construct a deformed C _λ-extended Heisenberg algebra in two-dimensional space using noncommuting coordinates which close an algebra depends on statistical parameter characterizing exotic particles. The obtained symmetry is nothing but an exotic particles algebra interpolating between bosonic and deformed fermionic algebras. PACS numbers: 03.65.Fd, 02.40.Gh, 05.30.Pr     

Neutron-neutron scattering length from the reaction γd → πnn employing chiral perturbation theory

We discuss the possibility to extract the neutron-neutron scattering length a_nn from experimental spectra on the reaction γd → π⁺ nn . The transition operator is calculated to high accuracy from chiral perturbation theory. We argue that for properly chosen kinematics, the theoretical uncertainty of the method can be as low as 0.1 fm.     

<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.     

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 .     

Bound-state versus collective-coordinate approaches in chiral soliton models and the width of the Θpentaquark

We thoroughly compare the bound-state and rigid-rotator approaches to three-flavored chiral solitons. We establish that these two approaches yield identical results for the baryon spectrum and kaon-nucleon S-matrix in the limit that the number of colors (N_C) tends to infinity. After proper subtraction of the background phase shift the bound-state approach indeed exhibits a clear resonance behavior in the strangeness S = + 1 channel. We present a first dynamical calculation of the widths of the Θ⁺ and Θ^* pentaquarks for finite N_C in a chiral soliton model.     

Decay asymmetry in non-mesonic weak decay of light Λ -hypernuclei

The proton decay asymmetry, α^NM _p , of the polarized Λ -hypernuclei, ⁵ _ΛHe , ¹² _ΛC and ¹¹ _ΛB , has been investigated to understand the reaction mechanism of the non-mesonic weak-decay process. These Λ -hypernuclei were produced in the highest statistics ever via the (π⁺, K ⁺) reaction at 1.05GeV/c by using the SKS spectrometer at KEK 12GeV PS. The results show that the α^NM _p are very small for these s -shell and p -shell hypernuclei.     

Scalar isoscalar part of the hyperon-nucleon interaction

We study the central part of the ΛN potential by considering the correlated and uncorrelated two-meson exchange besides the ω exchange contribution. The correlated two-meson is evaluated in a chiral unitary approach. We find that a short-range repulsion is generated by the correlated two-meson potential which also produces an attraction in the intermediate distance region. The uncorrelated two-meson exchange produces a sizeable attraction in all cases which is counterbalanced by ω exchange contribution.