A possible interpretation of the \pi_1(1400) exotic meson

I will discuss the current theoretical and experimental status of exotic mesons and a possible interpretation of the light, exotic meson as a final state interaction effect.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Mk Glueball and nonstandard multi-quark/gluon states - 13.60.Le Meson productionA.P. Szczepaniak: Present address: Department of Physics, Indiana University, Bloomington, IN 47405, USA     

Determination of vector meson properties by matching resonance saturation to a constituent quark model

The properties of mesonic resonances can be calculated in terms of the low-energy coefficients of chiral perturbation theory ( PT) by extending unitarized PT to higher energies. On the other hand, these low-energy coefficients can be calculated in two different models, namely i) by assuming resonance saturation and ii) within a constituent quark model. By matching the expressions of the two models combined with the results of unitarized PT and the Weinberg sum rules, the properties of vector and axial-vector mesons can be calculated in the combined large-N _c and chiral limit.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Fe Phenomenological quark models: Chiral Lagrangians - 14.40.Cs Properties of specific particles: Other mesons with S = C = 0, mass < 2.5 GeV     

Hadrons with charmed quarks in matter

We investigate the decay width of excited charmonium states at finite nuclear density with simultaneous modification of both D- and -mesons in nuclear matter. The strongest effect is found for the -meson. The medium modification can be detected by dilepton spectroscopy as substantial broadening and anomalous absorption.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.25.Gv Decays of , , and other quarkonia - 14.40.Lb Charmed mesons - 14.65.Dw Charmed quarks - 24.85. + p Quarks, gluons, and QCD in nuclei and nuclear processes     

Meson spectrum in a relativistic harmonic model with instanton-induced interaction

On the basis of the phenomenological relativistic harmonic model for quarks, we have obtained the ground-state masses of the light pseudo-scalar and vector mesons. The full Hamiltonian used in the investigation has Lorentz scalar + vector confinement potential, along with one-gluon-exchange potential (OGEP) and the instanton-induced quark-antiquark interaction. A good agreement is obtained with the experimental masses. The respective role of instanton-induced interaction and OGEP for the determination of the meson masses is discussed.Received: 18 October 2002, Revised: 13 January 2003, Published online: 23 December 2003PACS: 12.39.Ki Relativistic quark model - 12.39.Pn Potential models - 14.40.Aq , K, and mesonsS. Pepin: Present address: rue de Sluse 13, B-4000 Liége, Belgium     

Transport analysis of in-medium hadron effects in pA and AA collisions

The production and decay of vector mesons ( ) in pA and AA reactions is studied with particular emphasis on their in-medium spectral functions. It is explored within transport calculations if hadronic in-medium decays like or might provide complementary information to their dilepton (e ⁺ e ^-) decays. Whereas the signal from the -meson is found to be strongly distorted by pion rescattering, the -meson Dalitz decay to appears promising even for more heavy nuclei in and pA reactions. Furthermore, the influence of nucleon and kaon/antikaon potentials on the yields and spectra in pA collisions is calculated and compared to the recent data from the ANKE Collaboration.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 13.75.Jz Kaon-baryon interactions - 14.40.Aq , K, and mesons - 25.40.-h Nucleon-induced reactionsW. Cassing: In collaboration with E.L. Bratkovskaya, M. Büscher, Ye.S. Golubeva, V. Grishina, V. Hejny, V. Metag, J. Messchendorp, B. Kamys, L.A. Kondratyuk, P. Kulessa, Z. Rudy, S. Schadmand, A.A. Sibirtsev, H. Ströher     

Mott effect and J/\psi dissociation at the quark-hadron phase transition

We investigate the in-medium modification of pseudoscalar and vector mesons in a QCD-motivated chiral quark model by solving the Dyson-Schwinger equations for quarks and mesons at finite temperature for a wide mass range of meson masses, from light ( , ) to open-charm (D, D ^*) states. At the chiral/deconfinement phase transition, the quark-antiquark bound states enter the continuum of unbound states and become broad resonances (hadronic Mott effect). We calculate the in-medium cross-sections for charmonium dissociation due to collisions with light hadrons in a chiral Lagrangian approach, and show that the D- and D ^*-meson spectral broadening lowers the threshold for charmonium dissociation by - and -mesons. This leads to a step-like enhancement in the reaction rate. We suggest that this mechanism for enhanced charmonium dissociation may be the physical mechanism underlying the anomalous suppression observed by NA50.Received: 30 September 2002, Published online: 22 October 2003PACS: 05.20.Dd Kinetic theory - 12.38.Mh Quark-gluon plasma - 14.40.-n Mesons - 25.75.Nq Quark deconfinement, quark-gluon plasma production, and phase transitions     

Eta physics at threshold

The production of - and -mesons in elementary nucleon-nucleon collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE. The high-quality proton beam with low emittance and small momentum spread permits to study the creation of these mesons very close to the kinematical threshold, where --due to the rapid growth of the phase space volume-- the total cross-section increases by orders of magnitude over a few MeV range of the excess energy. The magnitude and energy dependence of the total cross-section as well as the occupation distribution of the phase space serve as observables for investigating the mechanisms underlying the production processes and the interaction of mesons with nucleons. The precise data on the and creation via the reactions allowed to settle the general features of the - and -meson production and revealed the sensitivity of the mentioned observables to the nucleon-nucleon-meson final-state interaction. The particular production properties, like for example, the determination of the dominating exchange processes which lead to the excitation of the S ₁₁ nucleon isobar in the case of creation, must be established by confrontation with other observables. The present status of this investigation with an emphasis on the results of the COSY-11 Collaboration is briefly presented. The available data are interpreted in view of the production mechanism and the meson-nucleon interaction.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 13.85.Lg Total cross sections - 25.40.-h Nucleon-induced reactions     

Cross-sections and spin observables in proton-proton elastic scattering: Results from EDDA at COSY

At the Cooler-Synchrotron COSY/Jülich polarized and unpolarized elastic proton-proton scattering has been investigated with the EDDA-Experiment in the energy range ( -2.5 GeV). By taking scattering data during the acceleration of the beam with a large-acceptance ( - ) detector, precise excitation functions for differential cross-section and analyzing power have been measured in small energy steps with consistent normalization with respect to luminosity and polarization. These data have helped to improve the determination of phase-shifts at higher energies and impose tight quantitative upper bounds on possible resonant contributions to pp elastic scattering, as they might arise from exotic 6-quark configurations. Recently, with polarized beam and target, the spin-correlation parameters , , and have been determined at 10 energies between 0.8 and 2.5 GeV. The observable has been measured the first time above 800 MeV and our results are in sharp contrast to phase-shift predictions at higher energies.Received: 30 September 2002, Published online: 22 October 2003PACS: 24.70. + s Polarization phenomena in reactions - 25.40.Cm Elastic proton scattering - 11.80.Et Partial-wave analysis - 13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.)     

Chromofields of strings and baryons

We calculate color electric fields of quark/antiquark ( ) and 3-quark (qqq) systems within the Chromodielectric Model (CDM). We explicitly evaluate the string tension of flux tubes in the -system and analyze their profile. To reproduce results of lattice calculations we use a bag pressure from which an effective strong-coupling constant follows. With these parameters we get a Y-shaped configuration for large qqq-systems.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.10.Lm Field theory: Nonlinear or nonlocal theories and models - 11.15.Kc Gauge field theories: Classical and semiclassical techniques - 12.39.Ba Phenomenological quark models: Bag model     

Exclusive and semi-inclusive strangeness and charm production in \pi N and NN reactions

Using the Quark-Gluon Strings Model (QGSM) combined with Regge phenomenology we consider the reactions and which are dominated by the contributions of the K ^* and D ^* Regge trajectories, respectively. The spin structure of the amplitudes is described by introducing Reggeized Born terms. It is found that the existing data for the reaction are in reasonable agreement with the model predictions. To describe the absolute values of the cross-sections it is necessary to introduce also suppression factors which can be related to absorption corrections. Furthermore, assuming the SU(4) symmetry to hold for Regge residues and the universality of absorption corrections we calculate the cross-section of the reaction . Employing the latter results from reactions we then estimate the contributions of the pion exchange mechanism to the cross-sections of the reactions and and compare them with the contributions of the K and D exchanges. We find that the NN reactions are dominated not by pion exchange but by K and D exchanges, respectively. Moreover, assuming the SU(4) symmetry to hold approximately for the coupling constants = we analyze also the production of leading -hyperons in the reaction . It is shown that the non-perturbative mechanism should give an essential contribution to the yield for .Received: 30 September 2002, Published online: 22 October 2003PACS: 13.85.Fb Inelastic scattering: two-particle final states - 13.85.Hd Inelastic scattering: many-particle final states - 14.20.Lq Charmed baryons - 14.40.Lb Charmed mesons     

Experimental review of charmonium spectroscopy

Charmonium spectroscopy is one of the best means of making precision study of QCD, the strong-interaction component of the Standard Model of particle physics. Recent progress in the study of charmonium, the need for additional precision data, and the opportunities that the new facilities offer, are discussed.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.Qk Quantum chromodynamics: Experimental tests - 12.39.Pn Potential models - 13.25.Gv Decays of , , and other quarkonia - 14.40.Gx Mesons with S = C = B = 0, GeV (including quarkonia)     

Aspects of a 0 0-f 0 mixing in the reaction pn \to dX

Possibilities to study the a ⁰ ₀-f ₀ mixing amplitude in meson production reactions in nucleon-nucleon collisions are discussed. It is argued that, even without knowing the production operator, it is possible to gain valuable insights from these reactions.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 14.40.Cs Other mesons with S = C = 0, mass < 2.5 GeV     

Cluster structure of <Superscript>16</Superscript>O

A folding potential describing the -scattering on ¹⁶O over a broad energy range 25.8-146.0 MeV is constructed on the basis of -like cluster and unclustered-nucleon configurations of ¹⁶O. The resulting potential does not need any renormalization to fit the angular distribution of elastic cross-sections. The effects of the repulsive part of - and -nucleon interactions are investigated. The analysis suggests that both the - repulsive potential and the unclustered nucleonic configuration in the target are important to describe the scattering data over a broad range of incident energies. The root-mean-square radius for the ¹⁶O nucleus is deduced.Received: 5 December 2002, Published online: 9 October 2003PACS: 25.55.Ci Elastic and inelastic scattering - 24.10.Ht Optical and diffraction models - 21.60.Gx Cluster models     

Exclusive \eta production in pp reactions

Angular distributions for the exclusive reaction observed via the decay channel have been measured at GeV, 2.50 GeV and 2.85 GeV (excess energies Q = 324 MeV, 412 MeV and 554 MeV). The polar angle of the shows an anisotropy with respect to the beam direction for the lowest energy, which vanishes for the higher energies. The anisotropy of the pp polar angle increases slightly with the beam energy.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.75.Cs Nucleon-nucleon interactions (including antinucleons, deuterons, etc.) - 14.40.Aq , K, and mesons - 25.40.Ve Other reactions above meson production thresholds (energies > 400 MeV)Y. Bedfer: Present address: DAPNIA/SPhN, CEA Saclay, France.A. Brenschede: Present address: Syngenio AG, StuttgartF. Brochard: Present address: LPHNHE, Ecole Polytechnique 91128 Palaiseau, France.Seonho Choi: Present address: Temple University, Philadelphia, PA, USA.R. Dressler: Present address: Paul Scherrer Institut, Villigen, CH-5232.J. Foryciarz: Present address: Motorola Polska Software Center, Kraków, Poland.H.-W. Pfaff: Present address: d-fine GmbH, Eschborn.     

High-resolution in-beam particle-gamma coincidence spectroscopy

A short summary of the present status of prompt particle decays in the mass region is presented. It includes recent results from a GAMMASPHERE experiment, which aimed at the combined high-resolution spectroscopy of light charged particles and -rays to allow for more detailed studies of known decays and the identification of new cases of discrete prompt proton and -particle emission from highly and superdeformed states.Received: 10 October 2002, Published online: 17 February 2004PACS: 23.50. + z Decay by proton emission - 23.60. + e decay - 27.40. + z - 27.50. + e      

S = -1 meson-baryon scattering in coupled-channel unitarized Chiral Perturbation Theory

The s-wave meson-baryon scattering amplitude is analyzed for the strangeness S = -1 and isospin I = 0 sector in a Bethe-Salpeter coupled-channel formalism incorporating Chiral Symmetry. Four two-body channels have been considered: , , , . The needed two-particle irreducible matrix amplitude is taken from lowest-order Chiral Perturbation Theory in a relativistic formalism. Off-shell behaviour is parameterized in terms of low-energy constants, which outnumber those assumed in previous works and provide a better fit to the data. The position of the complex poles in the second Riemann sheet of the scattering amplitude determines masses and widths of the and resonances which compare well with accepted numbers.Received: 30 September 2002, Published online: 22 October 2003PACS: 11.10.St Bound and unstable states; Bethe-Salpeter equations - 11.30.Rd Chiral symmetries - 11.80.Et Partial-wave analysis - 13.75.Lb Meson-meson interactionsE. Ruiz Arriola: Spokesperson     

Search for the \phi1860 Pentaquark at COMPASS

Narrow and resonances produced by quasi-real photons have been searched for by the COMPASS experiment at CERN. The study was stimulated by the recent observation of an exotic baryonic state decaying into , at a mass of 1862 MeV, interpreted as a pentaquark. While the ordinary hyperon states and are clearly seen, no exotic baryon is observed in the data taken in 2002 and 2003.Received: 15 March 2005, Published online: 31 May 2005PACS: 13.60.Hb, 13.60.Rj, 14.20.Jn, 14.80.-j     

Associated strangeness production at threshold

The associated strangeness dissociation at threshold has been studied at the COSY-11 facility measuring the hyperon and the K ⁺ K ^- meson pair production. Measurements of the near-threshold and production via the reaction (S. Sewerinet al., Phys. Rev. Lett. 83, 682 (1999)) at COSY-11 have shown that the cross-section ratio exceeds the value at high excess energies ( MeV (A. Baldiniet al., Total Cross-Sections for Reactions of High-Energy Particles, Landolt-Börnstein, New Series, Vol. I/12 (Springer, Berlin, 1988))) by an order of magnitude. For a better understanding additional data have been taken between 13 MeV and 60 MeV excess energy. The near-threshold production of the charged kaon-antikaon pair is related to the discussion about the nature of the scalar states in the 1 GeV/c ² mass range, i.e. the f ₀(980) and a ₀(980) (O. Krehl, R. Rapp, J. Speth, Phys. Lett. B 390, 23 (1997)). The interpretation as a molecule is strongly dependent on the K- interaction which can be studied via the production channel. A first total cross-section value on the reaction at an excess energy of 17 MeV (C. Quentmeieret al., Phys. Lett. B 515, 276 (2001)), i.e. below the production threshold, was measured.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.75.-n Hadron-induced low- and intermediate-energy reactions and scattering (energy GeV) - 14.20.Jn Hyperons - 14.40.Aq , K, and mesons - 25.40.Ep Inelastic proton scattering     

\pi\pi correlations in \gamma + A reactions

Preliminary differential cross-sections of the reactions and with A = ¹ H, ¹² C, and ^nat Pb are presented. A significant nuclear-mass dependence of the invariant-mass distribution is found in the channel. The dependence is not observed in the channel. The in-medium observation in the channel is consistent with an in-medium modification of the interaction in the I = J = 0 channel, changing width and pole position of a resonant state.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Le Meson production - 21.65. + f Nuclear matterJ.G. Messchendorp: For the TAPS and A2 Collaborations     

Phase diagram of the three-dimensional NJL model

With the exception of confinement the three-dimensional Nambu-Jona-Lasinio (NJL₃) model incorporates many of the essential properties of QCD. We discuss the critical properties of the model at non-zero temperature T and/or non-zero chemical potential . We show that the universality class of the thermal transition is that of the d = 2 classical spin model with the same symmetry. We provide evidence for the existence of a tricritical point in the -plane. We also discuss numerical results by Handset al. which showed that the system is critical for and the diquark condensate is zero.Received: 30 September 2002, Published online: 22 October 2003PACS: 71.10.Fd Lattice fermion models (Hubbard model, etc.) - 05.70.Fh Phase transitions: general studies