Strangeness production on the nucleon

Results of a tree-level analysis of p(γ, K)Y and e, e′K)Y observables are presented. It is shown that the background diagrams play a predominant role in the p(γ,K)Y reaction dynamics. It is argued that the electro-production p(e,e′K)Y channel can help in further constraining the parameters required for determining the background strength.     

K0πand K*0Σ+ photoproduction off the proton

The exclusive reactions γp → K ^*0Σ⁺(1189) and γp → K ⁰π⁰Σ⁺(1189) , leading to the p 4π⁰ final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4π geometrical acceptance. Differential and total cross-sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction γp → K ⁰π⁰Σ⁺ suggesting dominant s -channel production. Σ^*(1385) and higher-lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction γp → K ^*0Σ⁺ indicates a t -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.     

Surface and volume effects in the photoabsorption of nuclei

Recent experimental results for meson photoproduction from nuclei obtained with TAPS at MAMI are analyzed in view of the suppression of the second nucleon resonance region in total photoabsorption. The cross-sections can be split into a component from the low-density surface region of nuclei and a component which scales more like the nuclear volume. The energy dependence of the surface component is similar to the deuteron cross-section, it shows a clear signal for the second resonance peak assigned to the excitation of the P₁₁(1440), D₁₃(1520), and S₁₁(1535). The volume component behaves differently, it is lacking the second resonance peak and shows an enhancement at intermediate photon energies.     

Meson photoproduction on the nucleon with polarized photons

Meson photoproduction with polarized photons has proved to be a powerful tool to identify contributions of baryon resonances that are not evident in the differential cross-sections. It provides information that are complementary to those extracted using pion-nucleon scattering data. Extensive results have been produced in the past on beam asymmetries by the Graal collaboration for η and π ⁰ on the proton. New results are now available for the same reactions on the quasi-free neutron and for the K⁺ photoproduction on the proton. Contributions from hitherto undetected baryon resonances may be important to understand the results.     

Isospin symmetry violation, meson production and η-nucleus interaction studies

We have studied isospin symmetry violation in nuclear reactions by measuring simultaneously the cross-section of the following two reactionsp +d →³H π⁺ andp +d →³He π⁰. The experiment was perfomed at the cooler synchrotron accelerator COSY, Jülich at several beam energies close to the correspondingη production threshold. We also have ongoing programmes onη-nucleus final-state interaction studies viap+⁶ Li →⁷ Be +η reactions, high resolution search for dibaryonic resonances and lambda-proton final state interaction studies. The experimental details and results obtained so far are presented here     

On the nature of the and from their behavior in chiral dynamics

We study the behavior with the number of colors (N_c) of the Λ(1405) and Λ(1670) resonances obtained dynamically within the chiral unitary approach. The leading order meson–baryon interaction, used as the kernel of the unitarization procedure, manifests a nontrivial N_c dependence of the flavor SU(3) representation for baryons. As a consequence, the SU(3) singlet (or ) component of the Λ(1405) states remains bound in the large N_c limit, while the other components dissolve into the continuum. Introducing explicit SU(3) breaking, we obtain the N_c dependence of the excitation energy, masses and widths of the physical Λ(1405) and Λ(1670) resonances. The N_c behavior of the decay widths is found to be different from the general counting rule for a qqq state, indicating the dynamical origin of these resonances.     

Exclusive central diffractive production of scalar and pseudoscalar mesons; tensorial vs. vectorial pomeron

We discuss consequences of the models of “tensorial pomeron” and “vectorial pomeron” for exclusive diffractive production of scalar and pseudoscalar mesons in proton–proton collisions. Diffractive production of f₀(980)$f_0\left(980\right)$, f₀(1370)$f_0\left(1370\right)$, f₀(1500)$f_0\left(1500\right)$, η$\eta$, and η^′(958)${\eta }^{\prime }\left(958\right)$ mesons is discussed. Different pomeron–pomeron–meson tensorial coupling structures are possible in general. In most cases two lowest orbital angular momentum–spin couplings are necessary to describe experimental differential distributions. For f₀(980)$f_0\left(980\right)$ and η$\eta$ production reggeon–pomeron, pomeron–reggeon, and reggeon–reggeon exchanges are included in addition, which seems to be necessary at relatively low energies. The theoretical results are compared with the WA102 experimental data. Correlations in azimuthal angle between outgoing protons, distributions in rapidities and transverse momenta of outgoing protons and mesons, in a special “glueball filter variable”, as well as some two-dimensional distributions are presented. We discuss differences between results of the vectorial and tensorial pomeron models. We show that high-energy central production, in particular of pseudoscalar mesons, could provide crucial information on the spin structure of the soft pomeron.     

New physics and technical challenges of ¯ANDA

An antiproton beam of unprecedented intensity and quality will soon be available at the HESR machine foreseen for the new FAIR accelerator complex of Darmstadt. This new facility, together with a properly designed new detector ( ˉANDA), will be the ideal environment to study fundamental questions of hadron and nuclear physics and to carry out precise tests of the strong interaction.     

Strangeness in the nucleon: Newest results from Happex and G0

We present an introduction to accurate measurements of the weak neutral current in the elastic scattering of longitudinally polarized electrons off hadronic targets, and show how they can provide new and stringent limits on the contribution of strange quarks to the nucleon form factors. Such a contribution is of particular interest for the study of nucleon structure, since strange quarks do not contribute to the valence state and give us a direct insight into the dynamics of quark-antiquark fluctuations. We discuss the latest published results of Happex and G0 experiments as well as the perspectives of high precision measurements of parity violating asymmetries.     

Strangeness production in proton—proton and proton—nucleus collisions

We discuss the investigation of the strange meson production in proton-proton (pp) and proton-nucleus (pA) reactions within an effective Lagrangian model. The kaon production proceeds mainly via excitations ofN*(1650),N*(1710), andN* (1720) resonant intermediate nucleonic states, in the collision of two initial state nucleons. Therefore, the strangeness production is expected to provide information about the resonances lying at higher excitation energies. For beam energies very close to the kaon production threshold the hyperon-proton final state interaction effects are quite important. Thus, these studies provide a check on the models of hyperon-nucleon interactions. The inmedium production of kaons shows strong sensitivity to the self-energies of the intermediate mesons     

Bremsstrahlung from colour charges as a source of soft particle production in hadronic collisions

It is proposed that soft particle production in hadronic collisions is dominated by multiple gluon exchanges between partons from the colliding hadrons, followed by radiation of hadronic clusters from the coloured partons distributed uniformly in rapidity. This explains naturally two dominant features of the data: (a) the linear increase of rapidity spectra in the regions of limiting fragmentation and, (b) the proportionality between the increasing width of the limiting fragmentation region and the height of the central plateau.     

Photo- and electro-production of mesons on nucleons and nuclei

In these lectures I will show some results obtained with the chiral unitary approach applied to the photo- and electro-production of mesons. The results for the photo-production of ηπ⁰p and K⁰π⁰Σ⁺, together with related reactions will be shown, having with common denominator the excitation of the Δ(1700) resonance which is one of those dynamically generated in the chiral unitary approach. Then I will show the results obtained for the e⁺e⁻→?f₀(980) reaction which reproduce the bulk of the data except for a pronounced peak, giving support to a new mesonic resonance, X(2175). Results will also be shown for the electromagnetic form factors of the N^∗(1535) resonance, also dynamically generated in this approach. Finally, I will show some results on the photo-production of the ω in the nuclei, showing that present experimental results claiming a shift of the ω mass in the medium are tied to a particular choice of background and are not conclusive. On the other hand, the same experimental results show unambiguously a huge increase of the ω width in the nuclear medium.     

同位旋守恒对奇异性产生的约束

考虑同位旋守恒约束和奇异夸克的产生压低,研究了高能pp碰撞中的K/π比值,给出了K⁺/π⁺和K^－/π^－与末态产生粒子多重数之间的关系,并提出采用一个近似与参数无关的(K⁺/π⁺)/(K^－/π^－)比值来反映这一过程中同位旋守恒的约束.     

The K0 SK0 S final state in two-photon collisions and SU(3) tensor nonets

The K⁰ _SK⁰ _S final state in two-photon collisions is studied with the L3 experiment at LEP, using orbital angular momentum operators. The mass spectrum is dominated by the formation of tensor mesons, their two-photon partial width are determined. A signal at 1700-1800MeV is found to be a new tensor state f ₂(1750) with mass M = (1755±10)MeV and width Γ = (67±12)MeV. All observed tensor resonances obey SU(3) relations. The f ₂(1750) state forms a second tensor nonet together with f ₂(1560) and a ₂(1700). The SU(3) analysis allows us to determine with good accuracy mixing angles between nonstrange and strange components of the isoscalar members of tensor nonets.     

Contributions to the nucleon form factors from bubble and tadpole diagrams

The nonlocal chiral effective theory is applied to investigate the electromagnetic and strange form factors of nucleons. The bubble and tadpole diagrams are included in the calculation. With the contributions from bubble and tadpole diagrams, the obtained electromagnetic form factors are close to the results without these contributions as long as the low energy constants \begin{document}$ c_1 $\end{document} and \begin{document}$ c_2 $\end{document} are properly chosen, while the magnitudes of strange form factors become larger. The electromagnetic form factors are in good agreement with the experimental results, while the magnitudes of strange form factors are larger than the lattice data.     

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.     

Inclusive ω photoproduction from nuclei and ω in the nuclear medium

With the aim of extracting information on the shift of the ω mass in the nuclear medium we analyze data obtained at ELSA from where claims for evidence of a mass shift of the ω have been made. We develop a Monte Carlo simulation code which takes into account the possible reactions in the experimental set-up of (γA → π⁰γX) in the vicinity of the ω production region with subsequent ω → π⁰γ decay. We compare our results with experiment for the distribution of π⁰γ invariant masses and conclude that the distribution is compatible with an enlarged ω width of about 90MeV at nuclear-matter density and no shift in the mass. This change in the width would be compatible with the preliminary results obtained from the transparency ratio in the A-dependence of ω production. The discrepancy of the present conclusions with former claims of an evidence for a shift of the ω mass stems from a different choice of background which is discussed in the paper.