International Journal of Modern Physics A

Predictions of the existence of well-defined deeply bound pionic atom states for heavy nuclei and the eventual observation of such states by the (d,³He) reaction have revived interest in the pion-nucleus interaction at threshold and in its relation to the corresponding pion-nucleon interaction. Explanation of the 'anomalous' s-wave repulsion in terms of partial restoration of chiral symmetry and/or in terms of energy-dependence effects have been tested in global fits to pionic atom data and in a recent dedicated elastic scattering experiment. The role of neutron density distributions in this context is discussed in detail for the first time.

The research activity in the field of hadronic atoms performed at Laboratori Nazionali di Frascati by an International Collaboration is reviewed. The results already obtained for Kaonic Hydrogen are presented and the future programs are discussed.

Multiple-scattering approximations to Faddeev calculations of the K^-d scattering length are reviewed and compared with published fully reactive Faddeev calculations. A new multiple-scattering approximation which goes beyond the 'fixed-center' assumption for the nucleons is proposed, aiming at accuracies of 5–10%. A precise value of the K^-d scattering length from the measurement of the K^-d 1s atomic level shift and width, planned by the DEAR/SIDDHARTA collaboration, plus a precise value for the K^-p scattering length by improving the K^-p atom measurements, are essential for extracting the K^-n scattering length, for resolving persistent puzzles in low-energy phenomenology and for extrapolating into -nuclear systems.

Physics results achieved with the use of the FINUDA detector at DAΦNE are presented and discussed. They span from Hypernuclear Spectroscopy to the search for neutron rich Hypernuclei and to the solution of the puzzle about the existence of narrow Deeply Bound States.

The Pauli Exclusion Principle is a basic principle of Quantum Mechanics, and its validity has never been seriously challenged. However, given its fundamental standing, it is very important to check it as thoroughly as possible. Here we describe the VIP (VIolation of the Pauli exclusion principle) experiment, an improved version of the Ramberg and Snow experiment (E. Ramberg and G. Snow, Phys. Lett. B238, 438 (1990)); VIP has just completed the installation at the Gran Sasso underground laboratory, and aims to test the Pauli Exclusion Principle for electrons with unprecedented accuracy, down to β²/2 ≈ 10^-30 - 10^-31. We report preliminary experimental results and briefly discuss some of the implications of a possible violation.

The HERMES experiment at DESY has been designed to study the spin structure of the nucleon. Impressive results have been obtained not only in inclusive processes but also for semi-inclusive and exclusive processes in deep-inelastic lepton scattering, helping to clarify the spin puzzle of the nucleon.

I report on recent work done in collaboration with Irinel Caprini and Gilberto Colangelo¹. We observe that the Roy equations lead to a representation of the ππ scattering amplitude that exclusively involves observable quantities, but is valid for complex values of s. At low energies, this representation is dominated by the contributions from the two subtraction constants, which are known to remarkable precision from the low energy theorems of chiral perturbation theory. Evaluating the remaining contributions on the basis of the available data, we demonstrate that the lowest resonance carries the quantum numbers of the vacuum and occurs in the vicinity of the threshold. Although the uncertainties in the data are substantial, the pole position can be calculated quite accurately, because it occurs in the region where the amplitude is dominated by the subtractions. The calculation neatly illustrates the fact that the dynamics of the Goldstone bosons is governed by the symmetries of QCD.

We present recent results on CP violation in the B meson system from the BABAR experiment at the PEP II asymmetric e⁺e^- collider. We discuss the study of CP violation in B-mixing and present measurements of unitarity-triangle angles α, β, and constraints on γ.

The Belle experiment has started in 1999 and has collected more than 600 fb^-1 data at the ϒ(4S) resonance with the KEK-B asymmetric energy e⁺e^- collider. In this contribution, the results of CP-violation measurement in B meson decays are presented. The large CP asymmetry arises thorough the interference between mixing and decay. The results yield constraints on the angles of the unitarity triangle, which is related to the Kobayashi-Maskawa quark mixing matrix. We discuss the status of the measurements on ϕ₁, ϕ₂ and ϕ₃ with various methods.

We investigate by global and local fits whether existing data on pp → π⁺d and np → π⁰d have indications on charge dependent forces other than the Coulomb force and the effect of differing thresholds. We find evidence in the energy (or pion momentum) dependence of the total cross sections close to threshold.

Pionium (π⁺π^- bound state) lifetime is measured with improved precision with respect to earlier work, and ππ scattering length differences |a₀ - a₂| are determined to 5% precision.

The isobar models η-MAID and η′-MAID have been used to analyze new data on quasi-free η photoproduction on the deuteron from Bonn and recent η′ data on the proton from Jlab. In η photoproduction on the neutron a bump around W = 1700 MeV was observed which could possibly arise from a narrow P₁₁ state that is discussed as a non-strange member of the Θ⁺ antidecuplett. In η′ photoproduction on the proton resonance contributions are found that can be attributed to missing resonances in the energy region around W = 1900 MeV.

The low emittance and small momentum spread of the proton and deuteron beams of the Cooler Synchrotron COSY combined with the high mass resolution of the COSY-11 detection system permit to study the creation of mesons in the nucleon-nucleon interaction down to the fraction of MeV with respect to the kinematical threshold. At such small excess energies, the ejectiles possess low relative momenta and are predominantly produced with the relative angular momentum equal to zero. Taking advantage of these conditions we have performed investigations aiming to determine the mechanism of the production of η and η′ mesons in the collision of hadrons as well as the hadronic interaction of these mesons with nucleons and nuclei. In this proceedings we address the ongoing studies of the spin and isospin dependence for the production of the η and η′ mesons in free and quasi-free nucleon-nucleon collisions.

New results on the spin observables for the reaction, combined with the previously determined total cross section isospin dependence, reveal a statistically significant indication that the excitation of the nucleon to the S₁₁(1535) resonance, the process which intermediates the production of the η meson in the nucleon-nuleon interactions, is predominantly due to the exchange of the π meson between the colliding nucleons.

The pp → ppϕ and quasi–free pn → dϕ reactions have been studied at the Cooler Synchrotron COSY–Jülich, using the internal beam and the ANKE facility. Total cross sections in the pp entrance channel have been determined at three excess energies ∊ in the range of 18.5–75.9 MeV. In case of the pn entrance channel the energy dependence of the cross section up to 80 MeV has been extracted by exploiting the intrinsic momentum of the neutron inside a deuterium target. Taken together with data for ω–production, a significant enhancement of the ϕ/ω ratio for both entrance channels of a factor 8 is found compared to predictions based on the Okubo–Zweig–Iizuka rule.

Some experimental studies of η–production and η–interactions performed or presently under way by the GEM collaboration at COSY Jülich are reviewed. The Germanium Wall, a stack of annular detectors made of high purity Germanium, was used to measure a series of differential and total cross sections for η–production with proton and deuteron beams on light nuclei. The unique combination of a high resolution magnetic spectrograph (BIG KARL) and 7an electron cooled beam delivered by COSY is ideally suited to perform high precision experiments like the η–mass determination. In a self calibrating experiment three particles from two reactions were detected simultaneously. Kinematical coincidence of ³H and π⁺ from p + d → ³H + π⁺ reaction allowed to calibrate the spectrograph and determine the beam momentum with great accuracy. The η–meson was seen clearly as a sharp missing-mass peak on a slowly varying background in the p + d → ³He + X reaction. A new value for the η-mass has been derived with extremely small error bars.

We suggest that the γp → ϕηp reaction cross section, in the kinematics where the ηp invariant mass in the final state lies between the threshold value (m_p+m_η) and the N*(1535) resonance mass, is largely determined by the ηN scattering amplitude close to threshold. The initial photon energy is chosen in the range , in order to reach low (absolute) values of the squared 4-momentum transfer from the initial photon to the final ϕ-meson. In these conditions, we expect the t-channel π⁰- and η-meson exchanges to drive the dynamics underlying the γp → ϕηp process. We show that the η-exchange is the dominating contribution to the cross section while the π⁰-exchange is negligible. The η-π⁰ interference is of the order of 20 – 30%. The sign of this term is not known and alters significantly our results. Data on the γp → ϕηp process would be therefore very useful to help unravelling the behavior of the ηp scattering amplitude close to threshold and assessing the possibility of producing η-nucleus bound states.

Using a Deuterium target, the GRAAL experiment has collected meson photoproduction data. The data have been analysed in the quasi free regime. The results of the quasi free process on the proton reproduce quite well the results previously obtained with a Hydrogen target. The preliminary results on the quasi free neutron for π⁰ and η photoproduction are presented and discussed.

The Crystal Ball spectrometer, with its nearly complete angular coverage, is an efficient detector of photon and neutron final states. While installed in the C6 beamline of the Alternating Gradient Synchrotron (AGS) of Brookhaven National Laboratory (BNL), this feature was used in a series of precise measurements of reactions with all-neutral final states. Here we concentrate on the analysis of data from the pion-induced reactions: π^-p → γn, π⁰n, ηn, and π⁰π⁰n.

The KLOE experiment has finished its data taking at the e⁺e^- collider DAΦNE in Frascati. KLOE has collected 2.5 fb^-1 at the peak of the ϕ resonance, and 250 pb^-1 at . The latest experimental results based on a sample of 450 pb^-1 are presented in this paper: they mainly concern the neutral and charged kaon decays, and the radiative ϕ decays into scalar and pseudoscalar mesons.

Recent experimental studies of the synthesis and properties of deeply bound antikaon mediated nuclear systems are reviewed. Following a brief introduction in the basic properties of the antikaon–nucleon interaction which may lead to cold and dense antikaonic nuclear systems, we review the results of very first experiments which give indications of the existence of such exotic clusters of matter. Then ongoing efforts to substantiate the early findings are presented and future experimental approaches which will allow a very detailed study of the decay modes, the sizes and density distributions of these kaonic nuclear clusters are discussed including their relevance for possible phase transitions in cold dense matter.

The planned upgrade of DAΦNE at LNF with a peak luminosity in the order of L = 10³³cm^-2s^-1 will be the ideal machine to search for and prove the existence of antikaon-mediated deeply bound nuclear cluster. Antikaon-mediated deeply bound nuclear clusters will allow the study of most important problems in hadron physics today, namely, how the hadron masses and hadron interactions change in the nuclear medium and what is the structure of cold dense hadronic matter. AMADEUS will use the KLOE detector with an additional dedicated target and detector system around the interaction zone to perform exclusive measurements – all particles in the formation and decay processes of deeply bound nuclear clusters will be detected. These measurements will prove beyond doubt the anticipated formation production of antikaon-mediated deeply bound nuclear clusters.

The photoproduction of vector mesons on various nuclei has been studied using the Cebaf Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory, and here we present preliminary results. All three vector mesons, ρ, ω, and ϕ, are observed via their decay to e⁺e^-, in order to reduce the effects of final state interactions in the nucleus. Of particular interest is possible in-medium effects on the properties of the ρ meson. The ρ spectral function is extracted from the data on various nuclei, carbon, iron, and titanium, and compared to the spectrum from liquid deuterium, which is relatively free of nuclear effects. We observe no effects on the mass of the ρ meson, however, there is some widening of the resonance in titanium and iron, which is consistent with expected collisional broadening.

The High Acceptance Di-Electron Spectrometer HADES¹ has been constructed at the SIS accelerator (GSI Darmstadt) to investigate electron-positron pairs produced in proton, pion and heavy ion induced reactions. The main goal of these studies is to explore properties of hadrons in nuclear matter. The apparatus and the experimental results from C+C at 2.0 AGeV and 1.0 AGeV and p+p at 2.2 GeV compared with Monte-Carlo events from a generator based on known cross-sections and branching ratios are presented.

Invariant mass spectra of low-mass vector mesons in the e⁺e^- channel are measured in 12 GeV p + C and p + Cu reactions at the KEK Proton Synchrotron. Significant excess is observed at the low-mass side of ω meson peak over the known hadronic sources both in the C and Cu target data. Also below the ϕ meson peak, an excess is found in the Cu target data for slowly moving mesons whose βγ is less than 1.25. They can be interpreted as signatures of the spectral modification of vector mesons in cold nuclear matter.

A diverse experimental program for the study of the photoproduction of mesons off nuclei has been carried out - and is still running - at the Mainz MAMI and Bonn ELSA electron accelerators with the TAPS, Crystal Barrel, and Crystal Ball calorimeters. It is motivated as a detailed study of the in-medium properties of hadrons and the meson - nucleus interactions. Typical examples for the in-medium behavior of vector mesons (ω), scalar mesons (σ), and nucleon resonances (P₃₃(1232), S₁₁(1535), D₁₅(1520)) are discussed. Special attention is paid to meson - nucleus final state interactions.

In this talk various aspects of in-medium behavior of hadrons are discussed with an emphasis on observable effects. It is stressed that final state interactions can have a major effect on observables and thus have to be considered as part of the theory. This is demonstrated with examples from photo-nucleus and neutrino-nucleus interactions.

We study pseudoscalar and scalar mesons using a practical and symmetry preserving truncation of QCD's Dyson-Schwinger equations. We investigate and compare properties of ground and radially excited meson states. In addition to exact results for radial meson excitations we also present results for meson masses and decay constants from the chiral limit up to the charm-quark mass, e.g., the mass of the χ_c0(2P) meson.

A new resonance X(1835) in the J/psi radiative decays, a couple of significant enhancements near threshold, and clear evidences for a broad 1^-- resonant structure of K⁺K^-, the σ and κ mesons in the J/psi hadronic decays from BES are reported. Favored interpretations of these observations are discussed.

Recent BABAR results for decays of B-mesons to combinations of non-charm mesons are presented. This includes B decays to two vector mesons, B → η′(π, K, ρ) modes, and a comprehensive Dalitz Plot analysis of B → KKK decays.

The Belle detector at the KEKB asymmetric e⁺e^- collider provides an excellent experimental environment for a wide variety of measurements including searches for yet undiscovered particles. The paper reviews recent discoveries of new hadron states and measurements of their properties.

Measurements of Charmonium production at the electron proton collider HERA are presented. In inelastic production the charmonium is formed from pairs that are produced in photon gluon fusion. The measurements of the cross sections and helicity distributions are compared to calculations performed in the framework of non-relativistic quantum chromodynamics (NRQCD), which includes both color-singlet and color-octet contributions.

Recent results related to open charm and charmonium spectroscopy from data collected by the Babar experiment in e⁺e^- collisions near are reviewed. New estimates of the , D_s1(2460)⁺, and D_s1(2536)⁺ meson properties and the discovery of a new meson at 2.86 GeV/c² decaying to DK are presented. New results on the two charmonium candidates X(3872) and Y(4260) are discussed also.

FOCUS is a photoproduction experiment designed to reconstruct charm particles and to study their decays. Data was collected at Fermilab during the 1996-97 fixed-target run. We present recent results on semileptonic meson decays and a search for pentaquarks.

A summary of the present experimental status of meson physics is presented. The presentation includes the new results presented at the MESON06 workshop, as well as other recent experimental developments in the field.

This is a summary of theoretical plenary contributions to the biennial hadron physics conference Meson2006, which was the ninth in this series. The topics covered in the meeting include low energy pion-pion and pion-nucleon interactions, photoproduction and hadronic production of light mesons and baryons, in-medium effects, recent developments in charmed mesons, charmonia and B mesons, the status of exotica, and some related topics such as final state interactions. In this contribution we review and summarize the plenary talks presented by theorists at the meeting, and emphasize some of the main points of their presentations. Where appropriate we will add brief comments on some aspects of QCD spectroscopy. Finally, following tradition, we conclude with a new Feynman story.

Simple–minded thoughts about the cross sections for the reactions pp → ppK⁺K^- and pp → ppK⁰K⁰ are presented, which certainly do not account for the complex coupled channel problem but rather provide some ideas into the mutual reaction dynamics.

The production of pairs has been investigated at ANKE (COSY-Jülich) in the reaction at excitation energies Q of 47.4 and 104.7 MeV. Partial wave analyses of the invariant mass and angular spectra show the dominance of the a₀-channel ( configuration). In the non-isospin selective pn → dK⁺K^- reaction the scalar production is strongly suppressed with respect to ϕ vector mesons. A measurement of the isoscalar production in the isospin selective reaction dd → α K⁺K^- has been performed in April 2006.

At CELSIUS-WASA the two-pion production in proton-proton collisisons has been measured exclusively from threshold up to the energy regime, where both of the collision partners are expected to be excited to the Δ state. The measurements constitute the first kinematically complete data samples of solid statistics in this energy range. Most of the data have been obtained for the π⁺π^- and π⁰π⁰channels. Whereas at near-threshold energies the differential distributions can be succesfully explained by chiral dynamics and Roper excitation, respectively, the data for the π⁺π^- channel in the ΔΔ region can be described only, if the special configuration (ΔΔ)_0⁺ is assumed. The data for the π⁰π⁰ channel moreover exhibit a low-mass enhancement in the π⁰π⁰ invariant mass spectrum, which is reminiscent of the ABC-effect found in double-pionic fusion to light nuclei.

In order to understand experimental data for K⁺ production in proton-deuteron interactions and draw conclusions about the ratio between K⁺ production in pn and pp, one needs to develop an approach for the description of all relevant pp results. We perform an analysis of the pp data available in COSY energy range for three reaction channels. It can be concluded that Λ and Σ⁰ production channels can be well described by the three-body phase space incorporating the hyperon-nucleon final-state interactions under the assumption of a constant matrix element. Experimental data on the pp → K⁺ nΣ⁺ reaction channel are very poor and more information is needed to understand a strange behavior of the energy dependence of the total cross section. Using ANKE inclusive K⁺ data at 2.85 and 2.95 GeV/c, preliminary values for the total cross-section of Σ⁺ production in pp interactions has been extracted applying a simple phase space approach. This gives a result roughly one order of magnitude less than that of the recently published exclusive measurement.

Information about the production mechanism of the η meson in proton-proton collisions can be inferred by confronting the experimental studies on the analysing power for the reaction with the theoretical predictions of this observable. The determined analysing powers for Q=10 MeV and Q=36 MeV are consistent with zero. Results show that the predictions of pure pseudoscalar meson exchange model fairly describe the experimental data, while the predictions of pure vector meson exchange dominance model disagree with the data at the level of 4.3 σ.

The measurement of the η meson production in reaction has been performed at the Big Karl magnetic spectograph at COSY synchrotron in Jülich. The vector and tensor polarized deuetron beam at the excess energy of Q=16 MeV was used. The preliminary results for the unpolarized cross section are presented here.

Preliminary results of recent measurements of the near threshold η meson production in the dp → dpη reaction are presented. The experiment was performed at the COSY-Juelich accelerator with the use of the COSY-11 detection system. Data were taken for three values of deuteron beam momenta corresponding to excess energies of 3.2, 6.1 and 9.2 MeV. The energy dependence of the total cross section confirms a strong effect of the final state interaction.

The HADES spectrometer installed at GSI Darmstadt is devoted to study the production of di-electron pairs from proton, pion and nucleus induced reactions at 1-2 AGeV. In pp collisions at 2.2 GeV we have focused mainly on exclusive reconstruction of the η meson decays in the hadronic (η → π⁺π^-π⁰) and the electromagnetic channels (η → e⁺e^-γ). We present analysis techniques and discuss first results on η production, with the main focus on comparisons of reconstructed distributions to results obtained by other experiments and theoretical predictions.

A theoretical model for the recently measured γp → π⁰ηp and related reactions is presented. These reactions are an interesting testing ground for existing γN → ππN models in an extension to SU(3). The chiral unitary schemes in which the N*(1535) and Δ*(1700) appear dynamically generated predict the dominant resonance couplings in the two-meson production processes.

The invariant cross sections for direct photon production in hadron-hadron collisions are calculated for several initial energies (SPS, ISR, , RHIC, Tevatron, LHC) including initial parton transverse momenta within the formalism of unintegrated parton distributions (UPDF). Kwieciński UPDFs provide very good description of all world data, especially at SPS and ISR energies. Inclusion of the QCD evolution effects and especially their effect on initial parton transverse momenta allowed to solve the long-standing problem of understanding the low energy and low transverse momentum data.

The neglected 35 MeV/c² particle mass quantization hypothesis has recently been reassessed for all known meson states. The rule is found to be statistically relevant, once the states are grouped by quark composition and J^PC, with slightly different mass units for each group. In certain groups the mass unit is spin-dependent. Also the mass units are linearly quantized, with highly structured correlation patterns. The baryon masses are organized along similar lines. These results support an indication that hadrons might be shell-structured.

We present a theoretical framework that accounts for the new D_J and D_sJ mesons measured in the open-charm sector. These resonances are properly described if considered as a mixture of conventional P–wave quark-antiquark states and four-quark components. The narrowest states are basically P–wave quark-antiquark mesons, while the dominantly four-quark states are shifted above the corresponding two-meson threshold. We study the electromagnetic decay widths as basic tools to scrutiny their nature.

We discuss and compare different approaches to include gluon transverse momenta for heavy quark-antiquark pair and meson production. The results are illustrated with the help of different unintegrated gluon distributions (UGDF) from the literature. We compare results obtained with on-shell and off-shell matrix elements and kinematics. The results are compared with recent experimental results of the CDF collaboration.

We present a generalization of the hyperspherical harmonic formalism to study systems made of quarks and antiquarks of the same flavor. This generalization is based on the symmetrization of the N–body wave function with respect to the symmetric group using the Barnea and Novoselsky algorithm. Our analysis shows that four-quark systems with non-exotic 2⁺⁺ quantum numbers may be bound independently of the quark mass. 0^+- and 1^+- states become attractive only for larger quarks masses.

The lifetimes of the long-lived threshold-state elementary particles exhibit a clear-cut spacing in powers of the fine structure constant α. The uncertainly principle leads to a reciprocal 1/α ≃ 137 quantization of the mass-widths, and hence the masses, of these particles, which is evident in the experimental data. This empirical α-dependence includes all types of particle states — leptons, quarks, hadrons, and gauge bosons — and reveals that they share a common mass structure. The masses of the threshold-state particles (where the various quark combinations first appear) are accurately reproduced as multiples of three basic α-generated mass quanta: the first-order boson and fermion α-masses m_b ≃ 70 MeV and m_f ≃ 105 MeV, and the second-order fermion α-mass (which is produced at the Tevatron). By adding distinctively-patterned multiples of these masses to the electron ground state, we reproduce the isospin-averaged masses of the mu and tau leptons, the Standard Model constituent quarks (including the t), the proton, the B_c and Upsilon mesons, and the WZ gauge boson doublet, to better than 1% accuracy.

The WASA detector built and operated at the storage ring CELSIUS in Uppsala, was during 2005 moved to the COoler SYnchrotron COSY in Jülich where it is now being reassembled. The main physics concerns decays of η and η′ mesons and isospin violation in deuteron-deuteron interactions. Several modifications and upgrades are planned to adapt the WASA detector to the COSY environment and beam energy. The first commissioning run is scheduled to start August 2006. Here we report on the status of the installation work and upgrades planned for the WASA detector at COSY.

Fundamental questions concerning the strong interaction are addressed by the PANDA experiment at the future FAIR facility at GSI-Darmstadt. PANDA will be a general purpose detector of modular design optimized to measure the interaction of an antiproton beam with proton and heavy nuclear targets. The physics program at PANDA covers a wide range of topics including gluonic excitations, spectroscopy of strange and charm mesons, Drell-Yan and CP violating processes. This contribution focuses on the part of the physics program dealing with antiproton - proton annihilation.

The PANDA experiment at FAIR comprises the possibility to study antiproton annihilations on nuclear targets. Such reactions are ideally suited to investigate the in-medium-potential of hadrons – charmed mesons, charmonium, antikaons, antibaryons – making use of the PANDA detector system to detect the final state particles. The paper discusses connected experimental results, predictions i.a. for the so far unexplored region of the charm quark mass, and the intended experimental procedure.

Inclusive cross section for pion production in proton - proton collisions are calculated based on unintegrated parton distribution functions (uPDFs). In addition to purely gluonic terms the present approach includes also quark degrees of freedom. Phenomenological fragmentation functions from the literature are used. The new mechanisms are responsible for π⁺ - π^- asymmetry. In contrast to standard collinear approach, application of 2 → 1 k_t - factorization approach can be extended towards much lower transverse momenta, both at mid and forward rapidity region. The results of the calculation are compared with SPS and RHIC data.

The inclusive production of high-p_⊥ particles (pions) in the beam fragmentation regions of high–energy hadronic collisions is driven by the breakup of valence constituents of the beam hadrons into their two–body Fock–state components,and their subsequent fragmentation. We briefly discuss an approach, that allows the consistent inclusion of intrinsic and radiatively generated transverse momenta of initial state partons, and describe an extension of our approach to nuclear targets.

We study the production amplitude for the reaction NN → NNπ up to next-to-leading order in chiral perturbation theory. We show that the irreducible chiral loops at this order exactly cancel those terms that arise from the off-shell parts of the πN rescattering vertex. This cancellation is required for formal consistency of the whole scheme. The net effect of the inclusion of all next-to-leading order loops is to enhance the leading rescattering amplitude by a factor of 4/3 compared to phenomenological studies, bringing its contribution to the cross section for pp → dπ⁺ close to the experimental value.

An experiment to search for the bound state of η meson in nuclei has been performed at COSY, Juelich using the ENSTAR detector and Big Karl magnetic spectrometer. Reaction (p,³He) on ²⁷Al target has been used to search for the η-mesic nucleus ²⁵Mg_η in the missing mass distribution from the measured ³He particles. A preliminary figure of 1.5 nb for the upper limit of the formation cross section of η-nucleus bound state has been determined.

The production of e⁺e^- - pairs has been measured for C + C collisions at a beam energy of 1 AGeV. Preliminary invariant pair mass spectra are constructed utilizing a probabilistic analysis method for the identification of single e⁺/e^- tracks. The results are compared to predictions based on sources with known production cross sections, and to earlier measurements.

We have searched for an enhancement in the excitation function of the yield of pions in the pion production reaction by protons and deutrons at bombarding energies near 350 MeV/nucleon. The measurements was carried out on the JINR Nuclotron internal beam and target in March and June 2004 Nuclotron Runs for Ag, Cu, Al and C targets.

Study of strange particles produced at (sub)threshold energies in nucleus-nucleus collisions can deliver insights into fundamental questions about the in-medium properties of hadrons in dense baryonic matter. Many theoretical calculations for the production and propagation of strangeness at SIS energies lead to various predictions concerning the existence and the magnitude of in-medium effects for strange particles in nuclear matter. FOPI measured the flow of strange particles in Ni + Ni collisions at 1.93A GeV and the results are compared with transport models which are favorable to in-medium effect.

The WASA detector system requires upgrades of its components to be efficient at the higher beam energies available at COSY. One of the proposed items of the WASA upgrade is to add a Cherenkov Detector. The current status of its development is presented.

The reaction pn → dπ⁰π⁰ constitutes the most fundamental system for the study of the ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of the ππ system in double-pionic fusion to nuclear bound states. The first exclusive measurements of this reaction have now been carried out at CELSIUS-WASA. They exhibit a striking low-mass enhancement in the ππ invariant mass spectrum, prove it to be a σ-channel phenomenon and falsify conventional theoretical models, which predict an associated high-mass enhancement not present in the new exclusive data. In the kinematically complete data sample we observe a ΔΔ excitation, where the interaction between the two Δ obviously plays a crucial role for the generation of the ππ low-mass enhancement.

One of the experimental programs at the TOF spectrometer located at the COSY-accelerator (Forschungszentrum Jülich, Germany) is the study of ω-meson production in proton proton collisions (pp → ppω). Recently, a measurement was performed with a polarized beam at an excess energy of ∊ = 129 MeV, which offers the possibility to analyze polarization observables of this reaction channel for the first time. The analyzing power (A_y) of the pp → ppω-reaction was determined to be compatible with zero.

The ABC effect - a puzzling threshold enhancement in the ππ invariant mass spectrum of double-pionic fusion to nuclear bound states - has been investigated since 40 years by inclusive measurements without providing a conclusive interpretation. First exclusive measurements to this topic have now been carried out at CELSIUS-WASA. They confirm a huge ππ threshold enhancement, prove it to be of scalar-isoscalar nature, i.e., a σ-channel phenomenon and reveal the theoretically predicted high-mass enhancement in inclusive spectra to be of πππ rather than of ππ nature - falsifying thus corresponding model predictions for the ABC effect. From the kinematically complete data samples we infer that the interaction between the two Δ particles created in this process is of crucial importance.

The deeply bound KNN and KNNN states are discussed and variational estimates of the binding energies are given. The basic KN amplitudes involve the deep subthreshold energy region, which is not under direct control. Large uncertainties are due to the KN interaction range. The dominant attraction comes from the S-wave Λ(1405). In the KNNN systems, an additional strong contribution comes from the P-wave Σ(1385) resonance.

Dynamical effects for deeply bound nuclear states are explored within a relativistic mean field (RMF) model. Varying the strength of - nucleus interaction, we cover a wide range of binding energies in order to evaluate the corresponding widths. A lower limit is placed on the width expected for binding energy in the range of . Substantial polarization of the core nucleus is found in light nuclei. We discuss the results of the FINUDA experiment at DAΦNE which presented evidence for deeply bound K^- pp states in Li and ¹²C.

The decay modes J/ψ → γπ⁰, γη, and γη′ are analyzed using a data sample of 58 million J/ψ decays collected with the BESII detector at the BEPC. The branching fractions are determined to be , Br(J/ψ → γη) = (11.23 ± 0.89) × 10^-4, and Br(J/ψ → γη′) = (5.55 ± 0.44) × 10^-3, where the errors are combined statistical and systematic errors. The ratio of partial widths and the singlet-octet pseudoscalar mixing angle of η-η′ system is determined. The η′ → γπ⁺π^- decay dynamics is studied using 22,000 reconstructed J/ψ → γη′ → γγπ⁺π^- events, the π⁺π^- mass spectrum is fitted using different theoretical descriptions. The preliminary results show that only the ρ(770) is not enough to describe the η′ → γπ⁺π^- decay process and a non-ρ(770) contribution is needed.

Within the dispersion relation approach we give the double spectral representation for space-like and time-like B → f₀(980) and D → f₀(980) transition form factors in the whole q² range. The spectral densities, being the input of the dispersion relations, are obtained from a triangle diagram of relativistic constituent quarks.

Analysis of charged and neutral B meson decays into π⁺π^-K, K⁺K^-K and is performed using a unitary representation of the ππ and final state interactions. Comparison of the theoretical model with the experimental data of the Belle and BaBar Collaborations indicates that charming penguin contributions are necessary to describe the B → f₀(980) K and B → ρ(770)⁰ K decays.

In approach, based on analyticity and unitarity, experimental data on the isoscalar S- and D-waves of processes ππ → ππ, , ηη, ηη′ have been analyzed for studying the status and QCD nature of the f₀- and f₂-mesons below 2.3 GeV. Assignment of scalar and tensor mesons to lower nonets is proposed.

We describe the present status of the pion distribution amplitude as it originated from two sources: (i) a nonperturbative approach, based on QCD sum rules with nonlocal condensates and (ii) a NLO QCD analysis of the CLEO data on F^γγ*^π(Q²), supplemented by the recent high-precision lattice calculations of the second moment of the pion distribution amplitude.

Comparative studies of hadron-induced interactions and heavy ion collisions have been performed at beam energies of 158 GeV/nucleon, corresponding to . They indicate that the heavy ion reaction is a mixture of various processes, including multiple nucleon collisions, isospin effects, and final state Coulomb interactions. The latter interactions result in surprising phenomena, like the presence of large and strongly varying structures in the shape of double-differential particle spectra. These phenomena depend on the initial conditions of the reaction and therefore can provide new information on the space and time evolution of the non-perturbative meson production process.

Previous work is extended from SU(2) to SU(3) and we present results for the mass spectrum of the [70,ℓ⁺] (ℓ = 0,2) nonstrange and strange baryons in the 1/N_c expansion. We show that the dominant term is the spin-spin interaction and its contribution vanishes at large excitations.

I discuss double-diffractive (double-elastic) production of the η′ and η_c mesons in the pp → pXp reaction within the formalism of unintegrated gluon distribution functions (UGDF). The contribution of γ*γ* → η′ fusion is estimated. The distributions in the Feynman x_F (or rapidity), transferred four-momenta squared between initial and final protons (t₁, t₂) and azimuthal angle difference between outgoing protons (Φ) are calculated and discussed. The results are compared with the WA102 data. Predictions at higher energies are presented.

The temporal pseudoscalar meson correlation function in a QCD plasma is investigated in a range of temperatures exceeding T_c and first time for a finite momenta which is of the experimental interest. The imaginary time formalism is employed for the finite temperature calculations. The behavior of the meson spectral function and of the temporal correlator is studied in the HTL approximation, where one replaces the free thermal quark propagators with the HTL resumed ones.

The influence of the nuclear environment on the hadronization of charged pions, kaons and (anti)protons in semi-inclusive deep inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. Identified hadron multiplicities have been measured for helium, neon and krypton relative to thate of deuterium as a function of ν, z and . A large increase in the multiplicity ratios with was observed, clarifying the role of FSI in the nuclear transverse momentum broadening. In addition, double-hadron production in the nuclear medium was investigated. Nuclear effects on the double hadron-production ratio provided an additional tool for studying modifications of hadronization in nuclear matter.