abstract

Neutral and charged two-pion production in \(p+d\rightarrow {^3{\rm He}}+2\pi \) reactions has been studied at a proton beam energy of 477 MeV. The total cross section for double pion production is 0.22 \(\pm \) 0.03 \(\mu \)b. The ratio of the cross sections for the production of charged pion pairs with isospin \(T=1\) and \(T=0\) was determined to be 1.4 \(\pm \) 0.4.

abstract

Some features of the \(NN\) final state interaction (FSI) and its role in near-threshold meson production in \(NN\) collisions are investigated using realistic \(NN\) potentials. It is shown that the FSI effects should not be factorized from the production amplitude. The issue of the universality of the \(NN\) FSI for the production of mesons with different masses is discussed.

abstract

We present a preliminary report on a new measurement of the radiative \(K_{e3}\) branching ratio and the first study of the photon spectrum in this decay. We find \(BR(K_{e3\gamma }, E_\gamma ^*\gt 30\) GeV, \(\theta _{e\gamma }^*\gt 20^\circ )\)/\(BR(K_{e3}) = 0.911 \pm 0.009\mathrm {(stat.)}^{+0.021}_{-0.010}\mathrm {(syst.)}\%\). Our measurement of the spectrum is consistent with inner bremsstrahlung only as the source of photons at the \(2\sigma \) level.

abstract

The SATURNE charge-exchange program explored the spin-isospin response of nuclei in the \({\mit \Delta }\) region [1]. New results on coherent pion production are presented. They give a precise measure of the spin longitudinal component of this response. This component is particularly interesting since theoretical models predict that attractive residual \({\mit \Delta }\)-hole correlations in the longitudinal channel (pion exchange) softens the response.

abstract

Two-photon collisions at the \(e^+e^-\) colliders allow to investigate the formation and the properties of resonant states in a very clean experimental environment. A remarkable number of new results have been recently obtained giving important contributions to meson spectroscopy and glueball searches. The most recent results from the LEP collider at CERN and CESR at Cornell are reviewed here.

abstract

We show that the Hagedorn temperature is larger for mesons than for baryons.

abstract

The magnetic spectrometer ANKE has been put into operation in spring 1998 at the cooler synchrotron COSY–Jülich. ANKE allows to momentum analyze ejectiles emitted from an internal target at forward angles \(\vartheta \approx 0^\circ \) with high angular acceptance \({\mit \Delta \Omega }\approx 50\) msr. A primary goal of the physics program with ANKE is the investigation of \(K^+\) production in proton–nucleus collisions at energies around and far below the free nucleon–nucleon threshold at \(T=1.58\) GeV. The measurements have been performed with C, Cu and Au targets at various energies in the range \(T=1.0, ..., 2.3\) GeV. The major experimental challenge is that the kaon-to-background ratio is as low as \(\sim 10^{-6}\) at the lowest energy. Data on the target–mass dependence of the production cross sections for kaon momenta \(p_{K^+} = 150, ..., 510\) MeV/c at beam energies of \(T=1.0\) and 2.3 GeV are presented.

abstract

In this work, propagation properties of the \(\rho \) meson in symmetric nuclear matter are studied. We make use of a coupled channel unitary approach to meson-meson scattering, calculated from the lowest order Chiral Perturbation Theory (\(\chi \)PT) lagrangian including explicit resonance fields. Low energy chiral constraints are considered by matching our expressions to those of one loop \(\chi \)PT. To account for the medium corrections, the \(\rho \) couples to \(\pi \pi \) and \(K\bar {K}\) pairs which are properly renormalized in the nuclear medium.

abstract

Using the generating functional method, the \(\sigma \) and \(\omega \) meson propagators in nuclear matter have been determined in relativistic non-linear models. The density dependence of the \(\omega \) meson mass has been considered.

abstract

The production and decay of vector mesons (\(\rho , \omega \)) in \(pA\) reactions at COSY energies is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations, if hadronic in-medium decays like \(\pi ^+\pi ^-\) or \(\pi ^0 \gamma \) might provide complementary information to their dilepton (\(e^+e^-\)) decays. Whereas the \(\pi ^+ \pi ^-\) signal from the \(\rho \)-meson is found to be strongly distorted by pion rescattering, the \(\omega \)-meson Dalitz decay to \(\pi ^0 \gamma \) appears promising even for more heavy nuclei. The perspectives of scalar meson (\(f_0, a_0\)) production in \(pp\) reactions are investigated within a boson-exchange model indicating that the \(f_0\)-meson might hardly be detected in these collisions in the \(K \bar {K}\) or \(\pi \pi \) decay channels whereas the exclusive channel \(pp \rightarrow d a_0^+\) looks very promising.

abstract

Analysis of \(\pi ^{\pm }\)-nucleus scattering has been done using an equivalent local potential to calculate the differential cross sections for pions elastically scattered from \(^{12}\)C, \(^{40}\)Ca and \(^{208}\)Pb above the \({\mit \Delta }\)-resonance in the momentum range of 610 to 895 MeV/\(c\). Our calculations of the local potential, without free parameters, give a reasonable description for the \(\pi ^{\pm }\)-nucleus elastic scattering in the high momentum range. We observe that the EELL parameter effect on the calculations in the high pion momentum is smaller than in the \({\mit \Delta }\)-resonance.

abstract

The hyperon production in elementary proton induced reactions is studied exclusively at the external COSY beam using the time-of-flight spectrometer TOF. For the associated strangeness production process \(pp \rightarrow K^+{\mit \Lambda }\,p\) total and differential cross sections as well as Dalitz plots, mass spectra of the subsystems and the \({\mit \Lambda }\)-polarization are measured. The data show evidence for the influence of \(N^*\) resonances and the \(p{\mit \Lambda }\)-final state interaction on the production process. In the actual step the production of \(\mit \Sigma ^+\) and \({\mit \Sigma }^0\) hyperons in the channels \(pp \rightarrow K^0{\mit \Sigma ^+} p\), \(K^+{\mit \Sigma ^+} n\) and \(K^+{\mit \Sigma }^0 p\) is included. In the future in addition a polarized beam and a polarized target as well as a deuterium target will be used.

abstract

The reaction \(dp\rightarrow dp\pi ^0\) has been measured with the PROMICE/WASA detector setup at CELSIUS with deuteron beam energies between 436 MeV and 558 MeV. The observed energy and angular distributions allow to determine contributions from a quasi-free \(np\rightarrow d\pi ^0\) and a coherent 3-nucleon production mechanism.

abstract

We discuss the near-threshold production of \(\eta \), \(\eta ^{\prime }\), \(\omega \), \(\phi \) and \(a_0^{+}\) mesons in the reactions \(pn\to dM\) and \(pp\to d a_0^+\). The two-step model can reasonably describe the \(\eta \)- meson production. The predictions for the cross sections of \(\eta ^{\prime }\), \(\omega \), \(\phi \) and \(a_0^{+}\) production demonstrate that those reactions can be detected at COSY (Jülich).

abstract

Based on a counting scheme for the application of chiral perturbation theory to pion production in \(NN\) collisions \(p\)-wave production is studied. It is demonstrated that, contrary to the \(s\)-wave production where loops enter at too low an order to allow the scheme predictive power, there are certain \(p\)-wave observables where parameter free predictions are possible. These predictions are consistent with experimental data. We show that the investigation of charged pion production yields a constraint on a contribution to the three-nucleon force with a spin-isospin structure that could resolve the \(A_y\) puzzle in \(nd\) scattering.

abstract

A recent precision measurement of \(m_\pi \) is described. The goal of the experiment is to bring the current mass uncertainty \({\frac {\Delta m_\pi }{ m_\pi }}\) of 3 ppm down to 1 ppm.

abstract

The onshell description of Initial and Final State Interactions (ISI and FSI) of threshold meson production reactions is reviewed. Existing onshell models and their offshell extension are discussed. Unitarity constraints on enhancement factors are formulated. A strategy for the treatment of essential singularities connected to Coulomb-like FSI is given.

abstract

Differential cross sections of the \(pd\rightarrow ^3{\rm H}\,\pi ^+/^3{\rm He}\,\pi ^0\) reactions were measured simultaneously at three beam momenta: \(750\, {\rm MeV}/c\), \(800\, {\rm MeV}/c\) and \(850\, {\rm MeV}/c\). The differential cross section of the \(pd\rightarrow ^3{\rm He}\,\eta \) was measured at \(1675\, {\rm MeV}/c\) beam momentum. All measurements cover a wide angular range in the CM system. The experiments were performed at the COSY accelerator in Jülich, Germany, by means of high purity Germanium Wall detector together with the magnetic spectrometer BIG KARL. Estimated cross sections were compared with the predictions of simple theoretical models. In case of isospin symmetric \(pd\rightarrow ^3{\rm H}\,\pi ^+\)/\(^3{\rm He}\,\pi ^0\) reactions, the average cross section ratio was estimated.

abstract

Preliminary results of the measurement of the reaction \(\pi ^- p \to \eta n\), obtained in 1998 by the Crystal Ball Collaboration, are presented. These new experimental results do not contradict the predictions of the model of pion-nucleon scattering in the \(K\)-matrix approach.

abstract

The interaction of the strange particles (\({\mit \Lambda }\), \(K^+\)) with nucleons is investigated. A comparison of calculated (within CBUU model) and measured \(K^+\) spectra in proton–nucleus reactions is made and shows good agreement. Experimentally determined production cross sections of heavy \({\mit \Lambda }\) hypernuclei in collisions of protons with Bi and U targets, obtained at COSY-Jülich, are compared with calculated values and agreement is obtained if rescattering of the \({\mit \Lambda }\) particles on nucleons is taken into account. The lifetimes of heavy \({\mit \Lambda }\) hypernuclei produced on Bi and U targets were measured. The value obtained for Bi (161\(\pm \)7\(\pm \)14)ps (the most precise value in literature) and the new value of the lifetime obtained for U (152\(\pm \)10\(\pm 25\))ps are consistent with possibility of the violation of \({\mit \Delta }I=1/2\) rule for nonmesonic \({\mit \Lambda }\) decay.

abstract

A new experimental setup for light-ion physics located at the CELSIUS storage ring of The Svedberg Laboratory in Uppsala is now in the commissioning phase. It consists of an internal pellet target station and the WASA detector covering a solid angle close to 4\(\pi \) steradians. Its central part is optimized to measure energies of electrons and photons for energies up to 600 MeV. The setup and the initial physics programme is presented as well as the status of the commissioning.

abstract

The DEAR project is one of the first experiments at the new DA\(\Phi \)NE \(\phi \)-factory at the Laboratori Nazionali di Frascati dell’INFN. The objective of DEAR is a precise determination of the isospin-dependent kaon–nucleon scattering lengths, through a 1% measurement of the \(K_{\alpha } \:\) line shift in kaonic hydrogen and the first observation of the line shift in kaonic deuterium.

abstract

We impose unitarity constraints on the \(S\)-wave isoscalar \(\pi \pi \) amplitudes extracted from the analysis of the \(\pi ^- p \rightarrow \pi ^+ \pi ^- n\) data which have been measured by the CERN–Cracow–Munich collaboration on a transversely polarized target at 17.2 GeV/\(c\) \(\pi ^-\) momentum. Two “steep” solutions contain a narrow \(S\)-wave \(f_{0}(750)\) resonance under the \(\rho (770)\) and exhibit a considerable inelasticity \(\eta \) which is in disagreement with the four pion production data below the \(K\overline {K}\) threshold. We impose \(\eta \equiv 1\) for all data points and examine four sets of solutions for the \(S\)-wave isoscalar phase-shifts. The “down-flat” and “up-flat” solutions easily pass the \(\eta \equiv 1\) constraint but the remaining “down-steep” and “up-steep” are eliminated. We conclude that the 17.2 GeV data cannot be described by a relatively narrow \(f_{0}(750)\).

abstract

A model for the description of photonuclear reactions in the multi-GeV range is described that combines the coherent initial state interactions (shadowing) of the incoming photon with an incoherent coupled channel description of final state interactions of outgoing particles. Particle production itself is treated either, at low energies, through nucleon resonance decays or, at higher energies, through string fragmentation. The initial state nucleons can be far-off-shell due to ground state correlations. Results are shown for semi-inclusive meson production.

abstract

Recent results obtained by the COSY–11 collaboration concerning the production of \(\eta \) and \(\eta ^{\prime }\) mesons in the \(pp \rightarrow ppMeson\) reaction are presented. A comparison of the production amplitude for the \(\pi ^{0}\), \(\eta \) and \(\eta ^{\prime }\) mesons at the same phase space volume allows to conclude that the proton–\(\eta ^{\prime }\) interaction is in the order of, or smaller than, the proton-\(\pi ^{0}\) one. A total cross section determined in a preliminary analysis of the data of elementary kaon and antikaon production via the \(pp \rightarrow pp K^{+}K^{-}\) reaction measured at excess of energy of \(Q\) = 17 MeV is reported.

abstract

We review recent work on various topics related to the modification of kaon properties in nuclei. After a brief exposition of the \(\bar {K}N\) and \(\bar {K}\) nucleus interaction, results from the application to \(K^-\) atoms, renormalization of the \(f_0\) and \(a_0\) scalar resonances in nuclei and \(\phi \) decay in the nucleus are shown.

abstract

With the PROMICE/WASA detector setup at CELSIUS the reaction \(pp \to NN\pi \pi \) has been measured in the energy range from 650 to 775 MeV. These data constitute the first exclusive high-statistics measurements on a pure hydrogen target, supplying both differential and integral cross sections. Most of the differential spectra for \(pp \to pp\pi ^-\pi ^+\) are close to phase space predictions (including \(NN\) final-state interaction) identifying the production via \(N^\ast (1440) \to N(\pi \pi )_{I = L = 0}\) as the dominant process. However, the measured \(M_{\pi ^+\pi ^-}\)-spectrum is strikingly different from phase space simulation pointing to the importance of other processes in this reaction. Possible contributions from dibaryon resonances are discussed.

abstract

The reaction \(pp \rightarrow pp\,\eta \) was measured at excess energies of 15 and 50 MeV at an external beam-line of COSY with the Time of Flight Spectrometer. During one week of beam-time about 5000 events were measured for each excess energy. Both reaction protons of the process \(pp\,\eta \) are detected with an acceptance of nearly 100% and the \(\eta \) is reconstructed by the missing mass technique. In a first step one third of the 15 MeV excess energy events are evaluated and examined with respect to differential values of which preliminary distributions are presented.

abstract

For the first time, a systematic study of differential production cross sections of antikaons in proton nucleus collisions close to threshold has been performed, using the first proton beam at GSI. Preliminary results on \(K^+\) and \(K^-\) production in \(p\)+Au collisions at a beam energy of 2.5 GeV are presented.

abstract

We investigate the process \(\pi N \to \pi \pi N\) in a coupled channel meson exchange model. Preliminary results for \(\pi ^- p \to \pi ^+ \pi ^- n\) total cross sections and angular correlation functions are presented.

abstract

The production and propagation of kaons and antikaons in nucleus-nucleus collisions has been measured with the Kaon Spectrometer at SIS. We present the excitation function of \(K^+\) production in C+C and Au+Au collisions. In Ni+Ni collisions at 1.93 AGeV we measured the phase-space distributions of \(K^-\) and \(K^+\) mesons. The \(K^-\)/\(K^+\) ratio was measured in C+C, Ni+Ni and Au+Au collisions at 1.5 AGeV and was found to be almost independent of the mass of the collision system. Within the framework of present days transport calculations, the measured \(K^+\) and \(K^-\) yields can only be explained if in-medium modifications of the \(K\) meson masses are assumed. In Au+Au collisions at 1 AGeV, we observe a nonisotropic azimuthal emission pattern for \(K^+\) mesons. This effect is can be explained by transport codes when a repulsive in-medium \(K^+N\) potential is taken into account.

abstract

The Jülich model for pion production in nucleon–nucleon collisions is reviewed. First calculations for recently measured spin correlation coefficients are presented. The influence of resonant [via the excitation of the \({\mit \Delta } (1232)\)] and nonresonant \(p\)-wave pion production mechanisms on these observables is examined. In the second part of the talk we discuss the role of pions in the \(\bar {d}-\bar {u}\)-asymmetry of the nucleon sea and its connection with the inclusive production of forward neutrons. We compare predictions for the pion structure function at small \(x\) with the data extracted from deep inelastic forward neutron production.

abstract

Meson production is the most important final state in hadronic reactions. In order to exploit the full physics potential, different production channels for as many as possible mesons have to be investigated, analyzed and interpreted in a consistent way. Besides electromagnetic reactions with real and virtual photons, hadronic interactions, using pions and protons as well as light or heavy nuclei, are used for pseudoscalar \((\pi ,\eta ,\eta ')\), scalar \((a_0, f_0)\), and vector-meson \((\rho , \omega ,\phi )\) production. A number of examples, focusing on the complementarity of the different probes, are given.

abstract

The differential cross sections of light meson production: \(X=\pi ^{0}, \eta \) and \(\omega \) were measured in \(\vec pp \rightarrow ppX\) reaction. A theoretical analysis based on \(s\)-channel contributions in \(2^{+}\) and \(1^{-}\) intermediate states was performed. Narrow peaks were looked for between \(\eta \) and \(\omega \) meson masses, and were extracted at 588, 608, 647, 681 and 700 MeV. The evidence for their existence is discussed in accordance with the corresponding number of standard deviations (SD).

abstract

The isobaric analogue state of the ground state of the halo nucleus \(^6\)He in \(^6\)Li has been studied in a pionic fusion experiment at the CELSIUS storage ring facility in Uppsala, Sweden. The \(^{6}\)Li ions were detected in a zero-degree spectrometer situated in the fourth quadrant of the CELSIUS ring. Differential cross-sections were measured at two energies corresponding to \(E_{\rm c.m.}\)=1.2 and 1.9 MeV above the absolute threshold. The total cross sections are \(\sigma =228\pm 6\ ^{+70}_{-0}\) nb and \(141\pm 12\ ^{+42}_{-0}\) nb respectively.

abstract

We report the first observation of the doubly-radiative decay mode of pionic hydrogen (\(\pi ^- p \rightarrow \gamma \gamma \,n \)) and deuterium (\(\pi ^- d \rightarrow \gamma \gamma X \)) using the RMC pair spectrometer at TRIUMF [1]. The process is interesting in the context of the \(\pi \)-Compton scattering [2] and the \(\chi \)PT predictions of the pion polarizability [3]. We present our preliminary values for the BR\(_{[\pi ^-p\rightarrow \gamma \gamma \,n]}=3.8\times 10^{-5}\) and BR\(_{[\pi ^-d\rightarrow \gamma \gamma X]}=1.6\times 10^{-5}\). Our \(\pi ^-p\) data indicates the dominance of \(\pi \pi \rightarrow \gamma \gamma \) mechanism. The \(\pi ^-d\) data shows no evidence for the \(d^*_1\) dibaryon.

abstract

Cross section measurements of exclusive \(\rho ^{0}\) and \(\phi \) production on a \(^1\)H target in the range of \(0.7\lt Q^2\lt 5.0\) GeV\(^2\) as well as extrapolated photoproduction cross section are presented in comparison with world data. The \(W\)-dependence of the \(\phi \)-leptoproduction cross section is shown to be in agreement with a gluon exchange mechanism. A clear \(W\)-dependence of the cross section ratio of \(\phi \) to \(\rho ^{0}\) leptoproduction is observed over a wide kinematical range of \(W\) at \(2.5\lt Q^2\lt 4\) GeV\(^2\). Fits of the \(Q^2\)-dependences of the total, longitudinal, and transverse cross sections of \(\phi \) and \(\rho ^{0}\) mesons are compared with ones from HERA collider experiments.

abstract

A compilation of the last H1 and ZEUS results for vector meson production in photoproduction on HERA is presented.

abstract

Measurements on \(\chi _{c2}\)-formation are reported by the OPAL and L3 collaborations. Results on \(\eta _c\) formation, in particular a first measurement of the form factor are presented by L3. The DELPHI and L3 experiments give new limits on \(\eta _c'\) formation.

abstract

The Crystal Barrel detector used before at LEAR (CERN) has been modified and installed at the electron stretcher accelerator (ELSA) in Bonn. It will be used to study photo- and electro-production of neutral mesons. One of the major aims of this experiment is the search for missing baryon resonances and to achieve a better understanding of the baryon spectrum. First data from test runs at low energy (250 MeV \(\lt E_\gamma \lt \) 800 MeV) show photoproduction of \(\pi ^0\) and of 2\(\pi ^0\)-pairs.

abstract

The reaction \(\gamma \gamma \to \pi ^+ \pi ^- \pi ^+ \pi ^-\) has been studied with the L3 detector at LEP. The data sample was collected by L3 at centre-of-mass energies from 160 GeV to 202 GeV with a total integrated luminosity of 466.2 \({\rm pb}^{-1}\). The process is dominated by \(\rho ^0 \rho ^0\) production. A spin-parity analysis of the \(\rho ^0 \rho ^0\) system for \(W_{\gamma \gamma } \lt 3\) GeV shows a dominance of \(J^P=2^+\) and helicity 2. A contribution of \(J^P=0^+\) is also observed whereas contributions of negative parity states are found to be negligible.

abstract

A comprehensive formalism is presented to describe the photo- or electroproduction of mesons off the nucleon. Taking the hadrons as extended objects described by form factors, we discuss the issues of gauge invariance and of off-shell form factors (both hadronic and electromagnetic). Specifically, we show how the gauge invariance of the production current is preserved by introducing auxiliary currents which take the place of the exchange currents usually neglected entirely in practical applications. The relation of the present formulation to tree-level-type prescriptions is discussed. Applications to kaon photoproduction are shown.

abstract

The Graal experiment has measured various photoproduction processes off hydrogen in the nucleon resonance region. The simultaneous measurement of cross sections and beam asymmetries in a wide energy and angular range allows to extract with better precision the properties of the nucleon excited states. Results are presented for \(\eta \), \(\pi ^0\) and \(\pi ^+\) photoproduction, and the impact in our knowledge of the barion spectrum is discussed.

abstract

Measurements of the individual multiplicities of \(\pi ^+\), \(\pi ^-\) and \(\pi ^0\) produced in the deep-inelastic scattering of 27.5 GeV positrons on hydrogen have been performed at the HERMES experiment. Moreover, the influence of the nuclear medium on hadron production and quark propagation processes has been studied by semi-inclusive production on deuterium and nitrogen targets.

abstract

We have extended a model for the \(\gamma N\rightarrow \pi \pi N\) reaction to virtual photons and selected the diagrams which have a \({\mit \Delta }\) in the final state. The agreement found with the \(\gamma _v p\rightarrow {\mit \Delta }^0\pi ^+\) and \(\gamma _v p\rightarrow {\mit \Delta }^{++}\pi ^-\) reactions is good. The sensitivity of the results to \(N{\mit \Delta }\) transition form factors is also studied. The present reaction, selecting a particular final state, is an extra test for models of the \(\gamma _{v} N\rightarrow \pi \pi N\) amplitude.

abstract

The production of \(\eta \), \(\eta '\), \(\omega \), \(\phi \), mesons, as well as \({\mit \Lambda }\) and \({\mit \Sigma }\) hyperons has been measured in proton-proton reactions with an incident beam momentum of 3.67 GeV/\(c\) with DISTO spectrometer at SATURNE. The first observation of the \(\phi \) meson near the production threshold and the \(\phi \)/\(\omega \) cross sections ratio has been compared to predictions based on the OZI rule and on one-boson-exchange models. Measured inclusive cross section for \(K^-\) mesons production is about a factor of 20 lower than for inlcusive \(K^+\) production at the same CM energy above threshold. This fact is in strong contrast to cross sections observed in sub-threshold heavy ion collisions. The production of \(\eta \) and \(\eta '\) mesons has also been measured, in an effort to gain information on the \(\eta '\)-nucleon coupling. 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.

abstract

Results from experiments performed with the electromagnetic calorimeter TAPS in combination with the tagged photon beam facility at the MAMI accelerator in Mainz are presented. In this contribution the photoproduction of pion pairs from the proton is discussed. In addition, preliminary results of the \(\pi ^{0}\gamma \) decay of the \({\mit \Delta }\) resonance are reported.

abstract

Corrections to single photon exchange in the \(\pi ^{0}\) Primakoff production on heavy nuclei are considered with an emphasis on multiple Coulomb corrections. Those can be shown to be negligible for small momentum transfers, the region most crucial for Primakoff measurements of \({\mit \Gamma }_{\pi ^{0}}\). For the \(\eta \), which can be produced by the same mechanism, corrections are seen to be of the order of 1 \(\%\).

abstract

We discuss the role of some nonpartonic effects which lead to \(N_p^{\pi ^+} \ne \ N_n^{\pi ^+}\) and \(N_p^{\pi ^-} \ne \ N_n^{\pi ^-}\) and may therefore modify the conclusion on the \(\bar d - \bar u\) asymmetry extracted from semi-inclusive production of pions in DIS. Quantitative estimations for resolved photon and exclusive \(\rho ^0\) are given as examples. The results are discussed in the context of recent HERMES data.

abstract

The \({{K^0_{\rm S}}\,K^\pm \pi ^\mp }\) and \(\eta \pi ^+ \pi ^-\) final states in two-photon collisions are studied with the L3 detector at LEP using data collected at centre of mass energies from 183 to 202 GeV. The mass spectrum of the \(K^0_{\rm S}\, K^\pm \pi ^\mp \) final state shows an enhancement around 1470 MeV, which is identified with the pseudoscalar \(\eta \,{\rm (1440)}\). This state is observed in the \(\gamma \gamma \) collisions for the first time and the value of its two-photon width is obtained. In the \(\eta \pi ^+ \pi ^-\) channel only the \(f_1{\rm (1285)}\) is observed, upper limits for the formation of \(\eta \,{\rm (1440)}\) and \(\eta \,{\rm (1295)}\) are given.

abstract

A summary of all the results achieved so far by the OBELIX Experiment in various measurements of \(\phi \) and \(\omega \) mesons production rates, in different annihilation channels and with data taken under different experimental conditions, is reported.

abstract

Interactions in coupled channels \(\pi \pi \), \(K \bar K\) and an effective \(2\pi 2\pi \) in scalar–isoscalar wave have been analysed. Influence of interchannel couplings on analytical structure of multichannel interaction amplitudes has been studied. Interplay of \(S\)-matrix zeroes and poles and their relation with parameters of scalar resonances has been investigated.

abstract

According to the conventional approach, hadrons can be treated as bound states of quarks. In this formulation one builds creation or annihilation operators of hadrons through the interacting quark operators in the Heisenberg picture \(q(x)\). This approach gives another possibilities to distinguish effects generated by quark–gluon degrees of freedom.

abstract

A study of the fragmentation of \(b\) quarks into \(B\) mesons is presented. The analysis makes use of the statistics collected by the ALEPH experiment around the \(Z\) peak during the years 1991–1995. A semi-exclusive reconstruction of \(B \rightarrow l\nu D^{(*)}\) decays is performed, by combining lepton candidates with fully reconstructed \(D^{(*)}\) mesons, while the neutrino energy is estimated from the missing energy of the event. The mean reduced energy of the weakly decaying and leading \(B\) meson is found to be \(\left \lt x_b^{\rm wd}\right \gt = 0.7198\) \(\pm \) \(0.0045\)(stat) \(\pm \) \(0.0053\)(syst) and \(\left \lt x_b^{L}\right \gt = 0.7394\) \(\pm \) \(0.0045\)(stat) \(\pm \) \(0.0053\)(syst) respectively.

abstract

New experimental data on the spin rotation parameters \(R\) and \(A\) in \(\pi ^\pm p\) elastic scattering at 1.62 GeV/c is presented. The measurements were performed at ITEP accelerator by ITEP-PNPI collaboration. The experimental setup includes polarized proton target and carbon polarimeter based on the multiwire spark chambers. The data obtained confirms our understanding of Barrelet ambiguities of partial wave analysis based on the information about \(A\) and \(R\) parameters at 1.43 GeV/\(c\).

abstract

We review the modern status of QCD theory of diffractive vector meson production with the focus on shrinkage of photons with \(Q^2\) and \((Q^2+m_V^2)\) scaling, \(j\)-plane properties of the QCD pomeron and Regge shrinkage of diffraction cone, \(s\)-channel helicity non-conservation and sensitivity to spin-orbital properties of vector mesons.

abstract

The pion and kaon coupled-channel vector form factors are described by making use of the resonance chiral Lagrangian results together with a suitable unitarization method in order to take care of the final state interactions. A very good reproduction of experimental data is accomplished for the vector form factors up to \(\sqrt {s}\leq 1.2\) GeV and for the \(\pi \pi \) \(P\)-wave phase shifts up to \(\sqrt {s}\leq 1.5\) GeV.

abstract

At the turn of the millennium, a decade of interesting new results will end with major questions being answered. We have a much cleaner picture about the spectrum of light mesons. There are candidates for glueballs and hybrids/multiquarks with a very detailed knowledge about their observable quantities. But there are still many open questions, which will not be answered with the existing data being acquired by the successful experiments of the last decade.

abstract

We discuss a model for the inclusive production of forward neutrons in high energy hadronic and deep inelastic reactions based on Regge phenomenology. The model includes pion and rho exchange as well as a contribution from the decay of \({\mit \Delta }\)-resonances. The \(\pi , \rho \)-exchanges are taken in the reggeized form. A good description of the reaction \(pp \to nX\) is achieved. The application to deep inelastic scattering is discussed.

abstract

We apply the \(^3P_0\) model to the strong decay of a meson into two mesons. We modify the traditional vertex by allowing a momentum and flavor dependent form. Moreover the meson wave functions are obtained using realistic quark–antiquark potentials including finite size for the quarks, instanton effects, spin–orbit and tensor forces. The results are in very good agreement with the data.

abstract

In this paper we present preliminary results of a coherent amplitude analysis of three-pion decays of charm mesons. The analysis includes a greater number of possible resonant states than in previous analyses, and produces masses and Breit–Wigner widths of the isoscalar resonances \({f_{0}}\)(980) and \({f_{0}}\)(1370) with better precision than previous measurements. We also present preliminary results for the mass and width of the low lying \({f_{0}}\)(400), sometimes called the \(\sigma \) meson.

abstract

The current limitations in computer speed mean that in order to compare lattice QCD simulations of hadron structure with data one must extrapolate from quark masses that are 5–10 times too large. In doing so it is vital that the constraints of chiral symmetry are correctly incorporated. We review some recent, exciting developments in our understanding of how to make these extrapolations.

abstract

Electromagnetic form factors of hadrons are studied with recourse to the superconvergent dispersion relation, by which the QCD prediction and the VMD are synthesized. We analyzed experimental data of pion, kaon and nucleon form factors and obtained reasonable agreement with experiments. The experiments on the electromagnetic form factors are expected to provide important data in investigating structure of hadrons.

abstract

The status of exotic mesons is summarized, including both theoretical expectations and experimental candidates. The current experimental candidates are “spin-parity exotics”; since these are most often considered possible hybrid mesons, the theoretical discussion will be mainly concerned with hybrids. The exotic meson candidates discussed are the surprisingly light \(\pi _1(1400)\) and \(\pi _1(1600)\).

abstract

Glueball candidates and \(q\bar {q}\) mesons have been found to be produced with different momentum and angular dependences in the central region of \(pp\) collisions. This talk illustrates this phenomenon and explains the \(\phi \) and \(t\) dependences of mesons with \(J^{PC} = 0^{\pm +},1^{++},2^{\pm +}\). For production of \(0^{++}\) and \(2^{++}\) mesons the analysis reveals a systematic behaviour in the data that appears to distinguish between \(q\bar {q}\) and non-\(q\bar {q}\) or glueball candidates. An explanation is given for the absence of \(0^{-+}\) glueball candidates in central production at present energies and the opportunity for their discovery at RHIC is noted.

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Optical potentials with non-local (gradient) terms are used to describe the spectra of kaonic atoms. The strength of the non-local terms is determined from a many-body calculation of the kaon self energy in nuclear matter. We find that the non-local terms are quantitatively important and the results depend strongly on the way the gradient terms are arranged. Phenomenologically successful description is obtained for \(p\)-wave like optical potentials. It is suggested that the microscopic form of the non-local interaction terms is obtained systematically by means of a semi-classical expansion of the nucleus structure.

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The DIRAC experiment at CERN aims to form \(\pi ^+ \pi ^-\) atomic states and to measure their lifetime with 10% accuracy. This will give a precision of 5% in the determination of \(\pi \pi \) \(S\)-wave scattering lengths, of the same level of Chiral Perturbation Theory calculations. In this way a crucial test on the theory validity would be performed.

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At the GSI Fragment Separator the formation of deeply bound pionic states in Pb has been studied in \(^{208}\)Pb(\(d,^3\)He) and recently also in \(^{206}\)Pb(\(d,^3\)He) reactions. The \(^{208}\)Pb(\(d,^3\)He) experiment is now fully analysed and its result is compared to model predictions. In the \(^{206}\)Pb(\(d,^3\)He) reaction the pionic 1\(s\) state was observed as a well-separated peak in the excitation energy spectrum.

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Lattice gauge calculations require the existence of glueballs. In particular a scalar glueball is firmly predicted at a mass of 1700 MeV. This prediction has led to an intense study of scalar isoscalar interactions and to the discovery of several states. The number of scalar states observed seems to exceed the number of states which can be accommodated in the quark model. None of these states has a decay pattern which is consistent with that of a pure glueball but mixing of scalar \(\bar qq\) states with the scalar glueball provides for a reasonable interpretation of the data. In this paper we scrutinize the evidence for these states and their production characteristics. The \(f_0(1370)\) — a cornerstone of all \(\bar qq\)-glueball mixing scenarios — is shown to be likely of non-\(\bar qq\) nature. The remaining scalar states then do fit into a nonet classification. If this interpretation should be correct there would be no room for resonant scalar gluon-gluon interactions, no room for the scalar glueball.

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We have searched for \(\rho '\) decays in \(\bar {p}N\) annihilation at rest. They may provide information on the nature of the \(\rho '\) states; are they hybrids or \(q\bar {q}\) states? A comparison between data and theoretical calculations gives ambiguous results. Furthermore, we have found hints for a radial excitation of the \(b_1(1235)\) at \(1610\,\)MeV.

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The physics motivation and current status of the Hall D project at JLab is reviewed.

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The \(\eta \pi ^-\) system has been studied in the reaction \(\pi ^-p \rightarrow \eta \pi ^-p\) at 18 GeV/\(c\). A large asymmetry in the angular distribution is observed indicating interference between \(L\)-even and \(L\)-odd partial waves. The data require interference between the \(a_2(1320)\) and an exotic \(J^{PC}=1^{-+}\) resonance \(\pi _1(1400)\) with \(M=(1370 \pm 16^{+50}_{-30})\) MeV/\(c^2\) and \({\mit \Gamma }=(385 \pm 40^{+65}_{-105})\) MeV/\(c^2\). A second exotic \(J^{PC}=1^{-+}\) resonance \(\pi _1(1600)\) has been found in the reaction \(\pi ^-p \rightarrow \pi ^+\pi ^-\pi ^-p\) in the \(\rho \pi \) channel. A mass-dependent fit yields \(M=(1593 \pm 8^{+29}_{-47})\) MeV/\(c^2\) and \({\mit \Gamma }=(168 \pm 20^{+150}_{-12})\) MeV/\(c^2\). Presumably the same resonance is also found in the \(\eta ^{\prime }(959)\pi ^-\) final state with \(M=(1589 \pm 9)\) MeV/\(c^2\) and \({\mit \Gamma }=(380 \pm 22)\) MeV/\(c^2\).

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First measurements of pion photoproduction using circularly polarized photons on longitudinally polarized protons were carried out at MAMI (Mainz) in the energy range \(E_{\gamma }\) = 140–800 MeV. Preliminary results of the helicity dependence of the total inclusive photoabsorption cross section in the full energy range and of the partial reaction channels \(\gamma p\; \rightarrow \; p \pi ^0\) and \(\gamma p \;\rightarrow \; n \pi ^+\) in the \({\mit \Delta }\) region are now available. These data provide new input for multipole analyses and determine the main contribution to the Gerasimov–Drell–Hearn (GDH) integral and the forward spin polarizability \(\gamma _0\).

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After a brief look at \(CP\) violation in kaon decays, a short overview of \(CP\) violation in the \(B\)-meson system and of strategies to determine the angles of the unitarity triangles of the CKM matrix is given. Both general aspects and some recent developments are discussed, including \(B_c^\pm \to D^\pm _s D\) and \(B\to \pi K\) decays, as well as the \(B_d\to \pi ^+\pi ^-\), \(B_s\to K^+K^-\) system.

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The \(\pi ^+ A \rightarrow \pi ^+\pi ^{\pm } A'\) reactions were studied on \(^{2}\)H, \(^{12}\)C, \(^{40}\)Ca and \(^{208}\)Pb nuclei at \(T_{\pi ^{+}}=283\) MeV. Pions were detected in coincidence using the CHAOS spectrometer. The composite ratio \(\cal C_{\pi \pi }^A\) between the \(\pi ^{+}\pi ^{\pm }\) invariant masses on nuclei and on the nucleon is presented. Near the \(2m_{\pi }\) threshold pion pairs couple to \((\pi \pi )_{I=J=0}\) when produced in the \(\pi ^+\rightarrow \pi ^+\pi ^-\) reaction channel. There is a marked near-threshold enhancement of \(\cal C_{\pi ^+\pi ^-}^A\) which is consistent with theoretical predictions addressing the partial restoration of chiral symmetry in nuclear matter. On the other hand, nuclear matter only weakly influences \(\cal C_{\pi ^+\pi ^+}^A\), which displays a flat behaviour throughout the energy range regardless of \(A\).

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We determine the hadronic two-body decay modes of the scalar– isoscalar \(f_0(1370),\) \(f_0(1500)\) and \(f_0(1710)\) states as resulting from the mixture of the lowest lying scalar glueball with the isoscalar states of the ground state \(^3P_0\) quark–antiquark (\(Q\bar Q\)) nonet. In addition we study the production properties of the \(f_0\) states in proton–antiproton (\(p\bar p\)) annihilation reactions of the type \(p\bar p \to \pi ^0 f_0\). The quantitative predictions both for the decay and the production emphasize the role of the gluonic components in the \(f_0\) states and its observable consequences.

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NA48 has searched for rare neutral kaon decays in data collected in 1998 and 1999. Preliminary results for the decays \(K_{\rm L} \rightarrow \pi ^0 \gamma \gamma \) and \(K_{\rm S} , K_{\rm L} \rightarrow \pi ^+ \pi ^- e^+ e^-\) are presented.

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The structure of the light scalar mesons is elucidated by the investigation of the \(S\)-matrix poles and the \(q\bar {q}\) spectral density in a coupled channel model that includes \(\pi \pi \), \(K\bar {K}\) and \(q\bar {q}\) channels. It is shown that the dynamical origin of the \(\sigma \) and \(f_0(980)\) mesons is consistent with the observed spectrum of the scalar states. The \(K\bar {K}\) molecular picture of the \(f_0(980)\) is in good agreement with recent experimental data on the decay \(\varphi \to \gamma \pi \pi \) from Novosibirsk.

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New results (preliminary) are presented on studies of different broken symmetries such as chiral symmetry, SU(3) flavor, \(C\) and \(CP\), obtained with the Crystal Ball multiphoton spectrometer. We have data on \(\pi ^-p\) and \( K^-p\) interactions to various neutral final states up to a beam momentum of 760 MeV/\(c\). We also present results on different neutral \(\eta \)-meson decays. Finally we show data on 2\(\pi ^0\) production by 408 MeV/\(c\) \(\pi ^-\) on complex nuclei to investigate nuclear medium modifications.

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The charge symmetry breaking forward-backward asymmetry of the cross section in \(np\rightarrow d\pi ^0\) is discussed near threshold. Among standard sources of isospin breaking the mixing of the \(\pi \) and \(\eta \) mesons shows up as strongly dominant at these energies. This contrasts elastic \(np\) scattering or \(np\rightarrow d\pi ^0\) in the \(\Delta \) region, where other mechanisms dominate. However, QCD based effective field theory suggests an even more important symmetry breaking mechanism.

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The present status of the knowledge of the \(B_s^0\) meson oscillations is presented. An overview of the different methods to detect \(B_s^0\) oscillations and possibly measure the value of the \(\Delta m_s\) parameter is shown, and the most recent results from different experiments are combined. The overall preliminary lower limit on \(\Delta m_s\) is 14.6 ps\(^{-1}\) for a 95% C.L.

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On the basis of a simultaneous description of the isoscalar \(s\)-wave channelof the \(\pi \pi \) scattering (from the threshold up to 1.9 GeV) and of the \(\pi \pi \to K\overline {K}\) process (from the threshold to \(\sim \) 1.4 GeV) in the model-independent approach, a confirmation of the \(\sigma \)-meson at \(\sim \) 660 MeV and an indication for the glueball nature of the \(f_0(1500)\) state are obtained.

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A review of the most recent measurements of the \(CP\) violating parameter \(\sin 2\beta \) from LEP and CDF is reported. These yield an average value of \(\sin 2\beta =0.91\pm 0.35\), giving a confidence level that \(CP\) violation in the \(B\) system has been observed of almost 99%.

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In this contribution violation of isospin symmetry of the nuclear forces is investigated using the effective field theory of the Standard Model, Chiral Perturbation Theory (\(\chi \)PT). Without going into details of the underlying Nuclear \(\chi \)PT, we concentrate on the construction of a systematic expansion of isospin breaking effects (electromagnetic interaction and quark mass difference) at the level of a chiral Lagrangian. First results especially for the \(^{1}S_{0}\) threshold paramters are shown.

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This presentation discusses some recent developments in our understanding of the lightest quark–antiquark excitations, both of the condensed QCD vacuum and in a nuclear medium. We focus on two selected topics: the thermodynamics of the chiral condensate and in-medium \(s\)-wave interactions of pions with special emphasis on the recently observed deeply bound pionic atom states.

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The NA48 experiment at CERN performed a measurement of direct CP violation in decays of \(K_{\rm L}\) and \(K_{\rm S}\) into \(\pi ^+\pi ^-\) and \(\pi ^0\pi ^0\). Preliminary results based on data taken in 1998 are presented. The new data, combined with published NA48 data from 1997, give the \({\cal CP}\) violating parameter Re\((\varepsilon '/\varepsilon )=(14.0\pm 4.3)\times 10^{-4}\).

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We discuss coupled-channel final-state-interaction effects for \(B\) weak decays into \(\pi \pi \) and \(K\bar {K}\). In particular, it is shown that in the isospin \(I=0\) channel the inelastic final-state rescattering process \((\pi \pi )_{I=0} \rightarrow (K\bar {K})_{I=0}\) strongly affects the phase of the \(B^0 \rightarrow (K\bar {K})_{I=0}\) amplitude.

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The \(\iota (1440)\) also called \(E\)-meson has been discussed as glueball candidate since many years. In \(\bar {p}p \to \pi ^+\pi ^-\pi ^+\pi ^-\eta \) the \(\iota (1440)\) is clearly observed but no evidence was found for the \(\eta (1295)\). The non-observation of the \(\eta (1295)\) in \(\bar {p}p\)-annihilation with the expected yield questions its interpretation as radial excitation of the \(\eta \) and therefore also the nature of the \(\iota (1440)\). Another glueball candidate is the \(f_0\)(1500). The investigation of the different 4\(\pi \)-decay modes of the scalar states may be crucial to clarify if a scalar glueball exists below 2 GeV. In order to do so four different 5\(\pi \)-final states have been investigated. The data are clearly dominated by the scalar isoscalar interaction. At least to scalar states are needed, the \(f_0\)(1370) and the \(f_0\)(1500). The first state decays dominantly into 4\(\pi \), while the 4\(\pi \)-decays of the \(f_0\)(1500) represent about half of its total width.

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This short contribution is a lite MESON2000 conference summary. As appropriate for the 600th anniversary of the Jagellonian University, it begins with a brief summary of the last 600 years of European history and its place in hadron physics. Next a “physicist chirality” order parameter \({\cal PC}\) is introduced. When applied to MESON2000 plenary speakers this order parameter illustrates the separation of hadron physicists into disjoint communities. The individual plenary talks in MESON2000 are next sorted according to the subconference associated with each of the 36 plenary speakers. Finally, I conclude with a previously unreported Feynman story regarding the use of models in hadron physics.