Narrow Results

Near threshold production has been studied in several hadronic reactions at COSY. Data on pp→ppK⁺K^- and pd→³HeK⁺K^- are available at several excess energies. ϕ mesons are clearly observed whereas the contribution from the scalar resonances a₀/f₀(980) is unclear. Data on were measured at two excess energies and for the lower energy the total cross section as well as differential distributions have been extracted. The main contribution to this reaction is identified as the channel. Furthermore, the differential distributions may give information on the FSI and scattering length. The data at the higher energy are still under analysis, and a preliminary value for the total cross section is given. production in pn reactions is also investigated in a recently performed measurement.

An overview is given of selected recent HERMES results involving meson production. Topics including single and double spin asymmetries in semi-inclusive and exclusive meson production, hard exclusive production of pion pairs and quark fragmentation in nuclei will be presented.

The diffractive production of vector mesons ep→eVMY, with VM=ρ⁰, ω, ϕ, J/ψ, ψ′ or ϒ and with Y being either the scattered proton or a low mass hadronic system, has been extensively investigated at HERA.

HERA offers a unique opportunity to study the dependences of diffractive processes on different scales: the mass of the vector meson, m_VM, the centre-of-mass energy of the γp system, W, the photon virtuality, Q² and the four-momentum transfer squared at the proton vertex, |t|. Strong interactions can be investigated in the transition from the hard to the soft regime, where the confinement of quarks and gluons occurs.

We present here a model-independent irreducible tensor approach to meson production in NN collisions, which leads to a spin-structure of the T-matrix valid at all energies together with exact partial-wave expansions for the reaction amplitudes. We then make use of these expressions to estimate the values of the partial-wave amplitudes for the reaction , using the IUCF data of Meyer et al.

The Crystal Ball Spectrometer was used at Brookhaven National Laboratory in a series of experiments which study all neutral final states of π^-p and K^-p induced reactions. We report about the experimental program and new results for the radiative capture reactions π^-p → γn and K^-p → γΛ, charge exchange π^-p → π°n, two π° production π^-p → π° π°n, and η production π^-p → ηn reactions. Data have also been obtained on the decays of N*, Δ, Λ, and Σ resonances. Threshold η production has been studied in detail for both π^-p and K^-p. Sequential resonance decays have been investigated by studying the 2π° production mechanism both in the fundamental interaction and in nuclei.

An overview is given of selected recent HERMES results involving hadron production. Topics including transversity, hard exclusive production of pion pairs, observation of the Θ⁺ pentaquark state and quark fragmentation in nuclei will be presented.

The COSY-11 collaboration has conducted experiments aiming at the determination of the excitation function and phase-space population of the pp→ppη reaction close to the kinematical threshold. The precise data obtained with the stochastically cooled proton beam of the cooler synchrotron COSY and the high resolution zero-degree magnetic spectrometer allowed for the observation of the significant deviations – in the shape of the excitation function and two-particle invariant masses – from the predictions based on the assumption that the reaction phase space is homogenously populated. Comparison of the shape of the excitation function for the pp→ppη and pp→ppη′ reaction allows to distinquish in the model independent way an influence originating from the proton-proton and proton-η interaction. For the comparison the full data set from experiments performed at COSY and other laboratories is used.

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.

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.

One major focus of the physics studied at the COoler SYnchrotron COSY of the Forschungszentrum Jülich is the production of mesons and hyperons in the hadron-hadron scattering with the aim to investigate relevant production processes, interactions of the participating particles as well as symmetries and symmetry breaking. Due to the excellent experimental conditions at COSY also single- and double-polarization measurements can be performed with high reaction rates. This contribution gives an overview about the physics program which is currently under investigation at the three big COSY installations ANKE, COSY-TOF and WASA-at-COSY.

We have measured excitation function for dp → pppπ^- reaction near the η production threshold. We observe an enhancement of the counting rate above the threshold which can originate from the production of the η meson in the reaction dp → ³He η and its subsequent absorption on neutron in the ³He nucleus leading to creation of the pπ^- pair.

With the HADES spectrometer at GSI we have studied dilepton production in various collision systems from elementary N+N, over p+A, up to the medium-heavy Ar+KCl system. We have confirmed the puzzling results of the former DLS collaboration at the Bevalac. While we have traced the origin of the excess pair yield in C+C collisions to elementary p+p and n+p processes, we find a significant contribution from the dense phase of the collision in larger Ar+KCl system. From recently obtained e⁺e^- pair spectra in p+p and p+Nb interactions at 3.5 GeV kinetic beam energy the inclusive production cross sections for neutral pions, η , ω and ρ mesons are extracted for the first time at this beam energy. Furthermore, the production mechanisms of the vector mesons, which are not known at these energies, are investigated. The direct comparison of p+p and p+Nb data allows us to investigate in-medium mass modifications of vector mesons at nuclear ground state density.

On the other hand, exclusive production of ω and η mesons in p+p reactions at 3.5 GeV were studied via their π⁺π^-π⁰ decay channel. Production cross sections and angular distributions for both states were determined.

Meson electro-production is used in Hall C at Jefferson Lab to probe nucleon, baryon and nuclear structure. The experimental program in Hall C includes studies of semi-inclusive pion production, p, d(e, e′π^±)X, where low energy factorization has been observed, suggesting that these reactions can be used to probe nucleon structure, including transverse momentum distributions of quarks, at energies available at JLab after the upcoming 12 GeV upgrade.

During the last few years photoproduction of mesons off light target nuclei (²H, ³He, ⁷Li) has been systematically studied at the tagged photon beams of the Bonn ELSA accelerator and the Mainz MAMI accelerator. The experiments used the combined Crystal Barrel/TAPS setup in Bonn and the Crystal Ball/TAPS setup in Mainz. Most measurements investigated meson photoproduction off the neutron for many different reaction channels in order to establish the iso-spin structure of electromagnetic resonance couplings. This project used mostly deuteron targets. The measurements with the He and Li targets aimed at the search for η-mesic nuclei and hadron in-medium properties.

With an effective Lagrangian approach, we give a full analysis on the NN → NNππ and reactions by exploring the roles of various resonances with mass up to 1.72 GeV. We find large contributions from Δ, N*(1440), Δ(1600) and Δ(1620) resonances. Our calculations also indicate sizeable contributions from nucleon poles for the energies close to the threshold. A good description to the existing data of different isospin channels of NN → NNππ and for beam energies up to 2.2 GeV is reached. Our results provide important implications to the ABC effect and guildlines to the future experimental projects at COSY, HADES and HIRFL-CSR. We point out that the at FAIR could be an essential place for studying the properties of baryon resonances and the data with baryon and anti-baryon in final states are worth analyzing.

With the advent of chiral perturbation theory, the low-energy effective field theory of QCD, high accuracy calculations for hadronic reactions have become possible. We review the recent developments in the reaction NN → NNπ in chiral EFT.

We study charge symmetry breaking (CSB) in the reaction pn → dπ⁰. CSB manifests itself in a forward-backward asymmetry of the differential cross section measured recently at TRIUMF. A complete calculation of CSB effects at leading order in chiral perturbation theory is performed. A new leading-order operator is included. This allows us to extract the strong contribution to the neutron-proton mass difference from the analysis. The value obtained is consistent with the result of Gasser and Leutwyler based on the Cottingham sum rule and with an extraction from lattice QCD.

One of the objectives of the physics programme of the WASA-at-COSY facility is to study the isospin violating η hadronic decays into π⁺π^-π⁰ systems driven by the term of QCD Lagrangian which depends on the d and u quark mass difference. These studies can be made in terms of the Dalitz plot parameters describing the density population which is proportional to the square of the amplitude |A(x, y)|². This contribution describes the current status of the analysis of the η → π⁺π^-π⁰ decay in the pd →³ Heη and as well in the pp → ppη reaction with WASA-at-COSY.

For the very first time, the correlation femtoscopy method is applied to a kinematically complete measurement of meson production in the collisions of hadrons. The shape of the two-proton correlation function derived for the pp → ppη reaction differs from that for the pp → pp(pions) and both do not show a peak structure opposite to results determined for inclusive measurements of heavy ion collisions.

The cross sections for exclusive D⁺D^- meson pair production in nucleus - nucleus collisions are calculated and differential distributions are shown. Realistic (Fourier transform of the charge density) charge form factors of nuclei are used. Absorption effects are included. Predictions for the STAR and PHENIX collaboration measurements at RHIC are presented.