[en] It is shown that the off-shell contributions in Compton scattering by pion exist only in the single diagrams. These contributions are canceled completely in the total gauge-invariant amplitude as it confirmed by one-loop calculations. It explains, in particular, some results of the chiral perturbation theory in the order p⁴ but this cancellation has more general reasons, not related with chiral symmetry

[en] Local duality estimates for the ρ-meson leptonic and the spectrum of radial excitations are updated. New perturbative corrections are included in the analysis that gives good agreement with experimental data on low-energy resonances

[en] The GDH experiment has started in May 98 its data taking to measure the helicity dependence of total and partial photon nucleon cross sections. Circularly polarized photons from the tagged photon facility of the Mainz MAMI microtron were used together with longitudinally polarized nucleons in a frozen-spin target. The outgoing particles were detected in the large acceptance detector DAPHNE complemented with forward components to increase its angular acceptance. First preliminary results from the analysis of a small subset of the data taken in the energy range from pion photoproduction up to 800 MeV are presented. (author)

[en] We consider Compton scattering off the nucleon in the framework of fixed-t subtracted dispersion relations. The subtracted dispersion integrals are mainly saturated by πN intermediate states in the s channel, γN → πN → γN, and ππ intermediate states in the t channel, γγ → ππ → NN-bar. In the present formalism the nucleon polarizabilities enter as subtraction constants to be determined by a fit to the Compton data. We show results for Compton scattering discussing in particular the sensitivity on the scalar polarizability difference α - β and the backward spin polarizability γ_π. (author)

[en] The production of Λ and Σ hyperons, as well as η, η', ω, φ mesons has been measured in proton-proton reactions with an incident momentum of 3.67 GeV/c. The normal spin transfer coefficient (D_NN) has been determined for exclusive hyperon production with a polarized beam. A large negative coefficient has been observed, which is qualitatively consistent with expectations for a mechanism dominated by kaon-exchange and rescattering. These measurements also resulted in the first observation of the φ meson near the production threshold and the φ/ω cross section ratio has been compared to predictions based on the OZI rule and on one-boson-exchange models. Results on the total K^- production cross section are also presented. Furthermore, the production of η and η' mesons has been measured, in an effort to gain information on the η'-nucleon coupling. The observed strong forward peaking is consistent with a strong P-wave η'-proton interaction. (author)

[en] We give a general overview of the role and interpretation of fragmentation functions in hard processes. Transverse momentum dependence gives rise to time-reversal odd fragmentation functions contributing at leading order. Final state interactions are necessary to have non-zero time reversal odd functions. We present a model calculation for the special case of interference fragmentation functions in two-hadron inclusive lepton hadron scattering. (author)

[en] Computational particle physics is covering computer intensive calculations occurring in high energy physics (HEP) research. Very detailed simulation programs are needed for the design of HEP experiments as well as for their data analysis. Software packages like GRACE and CompHEP have been developed to automate the lengthy and involved computations appearing at all stages of the evaluation of process observables. Experimental and theoretical meeds may not necessarily be identical, therefore, in this paper, we review the main questions to be addressed concerning the further development of those packages to be used in the framework of the large experimental collaborations. Automatic event generator code generation, computational efficiency, module interface, flexibility and reusability, compatibility issues, version release management and documentation are some of the very important questions facing the implementation of those package in the framework of the HEP experiments at the Large Hadron Collider (LHC) and Linear Colliders (LC). (author)

[en] The development of Extensive Air Showers (EAS) is driven by the hadronic interactions of the primary and secondary particles with the atmospheric nuclei. Hence a careful analysis of the EAS appearance, in particular of the hadronic component, provides valuable information on features of the hadronic interaction. Especially, in ultrahigh energy regions extending the energy limits of man-made accelerators and the experimental knowledge from collider experiments, the hadronic interaction is subject of uncertainties and debates. Since the EAS development is dominantly governed by soft processes, which are not accessible to a perturbative QCD treatment, the interpretation of EAS observations has to rely on QCD inspired phenomenological interaction models, in particular on string models based on the Gribov-Regge theory, like VENUS, QGSJET and SIBYLL. The basic features of such models are sketched from point of view of a practical use as generators for EAS Monte Carlo simulations and analyses of experimental results. Applications are illustrated with various examples of EAS observations, scrutinised as source of information on very-high energy hadronic interaction features. (author)

[en] A new analysis of old data of pp bar annihilation at rest in liquid hydrogen in K_SK_Sπ⁰ final states, gives evidence of a possible narrow resonance decaying into K_SK_S, with a mass of about 1.55 GeV and which was omitted in the original analysis. (authors)

[en] In order to describe physical reality a special (gravity-free) relativity is needed that is founded upon general non-uniform motions as they occur in our environment and hold for the non-inertial reference frame of our laboratory. Such a generalized form of special relativity can be build upon an extension, at relativistic velocities, of Maxwell-Hertz electrodynamics (MHE), which is valid for non-uniform motions, but at small velocities only. The new electromagnetic theory called (in honor to Hertz) Hertz's Relativistic Electrodynamics (HRE), is completely independent and built-up in a completely different way as regards Einstein's Special Relativity (ESR). HRE, a coordinate-free formulation does not need postulates, but confirms the constancy principle of speed of light in vacuum. All experiments of first and second order in v/c are correctly interpreted. To this theory a Hertzian kinematics and dynamics are associated. HRE with its corresponding mechanics forms Hertz's Special Relativity (HSR), as a complementary theory to ESR. According to the principle of complementarity and neglecting the gravitational effects, the Extended Special Relativity (ExSR) is a double faced theory which becomes either ESR, when the motion is inertial or HSR when the motion is non-inertial. The complementarity of both theories assumes that the two descriptions cannot be employed for the same motion, being mutually exclusive. Consequently, to every statement of one of the ExSR corresponds a complementary statement of the other ExSR. The completeness of ESR with HSR ensures an extended view over the relativity in our physical world. (author)