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Programm und Abstracts der Sitzung HK 38

Photoproduction of r⁰ mesons and D⁺⁺ baryons in the reaction gp ® p p⁺ p^-

•Karl-Heinz Glander für die SAPHIR-Kollaboration
Physikalisches Institut, Universität Bonn

Photoproduction of r⁰ mesons and D⁺⁺ baryons at photon energies from its threshold to 2.6 GeV have been studied with the SAPHIR detector at the electron stretcher ELSA. The total and differential cross sections were obtained. The angular decay distributions of the r⁰ meson show the s-channel helicity conservation, which is kept at higher photon energies, to be broken slightly near threshold. One-boson-exchange models can describe approximately the behavior of total and differential cross sections of r⁰ photoproduction near threshold.

Two-Pion Decay of Resonances and the Reaction pN®ppN

•Sonja Schneider und Siegfried Krewald
Institut für Kernphysik, Forschungszentrum Jülich, D-52425 Jülich

The understanding of the spectrum and the decay properties of baryons is essential to our understanding of QCD in the non-perturbative regime. We focus on the investigation of the two-pion decay of the low-lying N^* resonances, especially the Roper Resonance P₁₁(1440), in the reaction pN ® ppN.

To describe pion-induced two-pion production on the proton, we start from a resonance exchange model similar to the model of Oset and Vicente-Vacas[1] with contributions from the excitation of the D isobar and the N^* resonances P₁₁(1440), D₁₃(1520) and S₁₁(1535).

A calculation at Born level already leads to reasonable agreement with the available data. Only the angular distributions are not well reproduced - for p⁺p® p⁺p⁺n and p^-p®p⁰p⁰ n they even have the opposite curvature. The simple tree-level model can be improved by the inclusion of initial and final state interaction. Such effects can be included for the pole contributions in a relatively simple way. The results we obtain with this improved model will be discussed.

[1] E. Oset and M.J. Vicente-Vacas, Nucl. Phys. A446, 584 (2000).

Close to threshold strangeness production in pp collisions

•Peter Winter für die COSY-11-Kollaboration
Institut für Kernphysik, Forschungszentrum Jülich

Exclusive data on the pp® pp K⁺K^- reaction near the production threshold have been taken at the internal experiment COSY-11 [1]. These studies are motivated not only by the possibility to investigate the proton kaon interaction but also by the discussion on the nature of the scalar resonances f₀(980) and a₀(980). A first total cross section s = 1.80±0.27^+0.28_-0.35 for the excess energy Q = 17 MeV has been determined [2].

In contrary to the h, w and h¢ production, the cross section values obtained at COSY-11 [2] and DISTO [3] are in good agreement with one-meson exchange calculations neglecting final state interaction (FSI) effects [4] as well as with a fit of a four body phase space including the strong pp FSI. Since the strong pp interaction could be compensated by the not well established pK^- FSI or due to an additional degree of freedom caused by the four-body final state [4], the investigation of the close to threshold region was extended at the excess energies Q = 10 and 17 MeV where strong FSI effects should be more pronounced. The data analysis will be presented and discussed.

[1] S. Brauksiepe et al., Nucl. Instr. & Meth. A 376 (1996) 397.

[2] C. Quentmeier et al., Phys. Lett. B 515 (2001) 276.

[3] F. Balestra et al., Phys. Rev. C 63 (2001) 024004.

[4] A. Sibirtsev, Publ. of Research Centre Jülich: Matter and Materials 11 (2002) 239.

Two-Pion Production at CELSIUS-WASA and the Quest for the Nature of s Meson and Roper Resonance^*

•T. Skorodko, M. Bashkanov, H. Clement, E. Doroshkevich, M. Kaskulov, R. Meier und G.J. Wagner für die CELSIUS-WASA-Kollaboration
Physikalisches Institut, Universität Tübingen

The two-pion production in proton-proton collisions has been measured at CELSIUS in the energy range T_p = 775 - 1360 MeV using the WASA 4p detector with hydrogen pellet target. The WASA detector containing magnetic field, tracking detectors and calorimeter, allows the detection of both charged and neutral particles. The channels ppp⁺p^-, ppp⁰p⁰ and dp⁺p⁰ have been measured exclusively with 4 overconstraints. Our first exclusive ppp⁺p^- data taken at PROMICE/WASA near threshold have been shown [1] to be in agreement with the process proceeding via the Roper resonance as predicted by calculations of the Valencia group [2]. Alternatively, the data are equally well described by pp rescattering leading to a dynamic formation of the s meson. With increasing incident energies the agreement of the Valencia prediction with the new data in ppp⁺p^- and ppp⁰p⁰ channels gets worse and finally at variance for T_p > 1 GeV. Rather the data in these channels get consistent with the formation of a (DD)_0⁺ configuration at the highest energies. In the dp⁺p⁰ channel the data show the onset of r-production.

[1] Brodowski et al. PRL 88 192301 (2002); PRC 67 052202 (2003)

[2] L. Alvarez-Ruso et al., Nucl. Phys. A633, 519 (1998) ^*supported by the BMBF (06 TU 201), DFG (Europ. Graduiertenkolleg) and Landesforschungsschwerpunkt (Quasiteilchen)

Indication of anomalous exchange in exclusive charmonium photoproduction.

•Alexander Sibirtsev¹, Siegfried Krewald¹ und Anthony W. Thomas^2
1IKP-TH, FZJ, Juelich
²Adelaide University

We find that available data on J/Y photoproduction at |t| ³ 1 GeV are energy independent over the range 22 £ Ös £ 170 GeV and show a very soft t dependence. These results are in striking contradiction with perturbative QCD. Through a systematic analysis of the data we detect a new trajectory, which manifests itself at threshold and is energy independent. The trajectory couples to the axial form factor of the nucleon and can reproduce the J/Y exclusive photoproduction data at large -t.

It would be of great interest to study J/Y photoproduction at low energies, which should be possible with the operation of HALL D at Jefferson Lab and to clarify whether the forward cross section approaches a value of order 20 nb·GeV^-2 starting from threshold.

The contribution from the new trajectory can be investigated experimentally in elastic antiproton-proton scattering at large t that can be done at HESR. The contribution from the new trajectory, which scales as s⁰ should be well distinguished from the hard scattering processes that scale as s^-10. HESR is an unique machine that provides access to large t within large range of s and let us to study the interplay between perturbative and non-perturbative QCD.

Baryon resonances in pion- and photo-production reactions

•Vitaly Shklyar und Ulrich Mosel
Institut für Theoretische Physik, Universität Giessen, 35392 Giessen, Germany

The interest in studies of baryon resonance spectra has grown significantly last year. Despite the great efforts made in the past, the properties of many observed baryon resonances cited in PDG [1] are not fairly settled yet. For the consistent identification of the resonances and their properties, the advent of multi-channel, unitary models is inevitable. With this aim in mind we develop an effective Lagrangian coupled-channel approach which preserves unitarity and allows for a simultaneous analysis of gN, pN, 2pN, hN, K L, K S, and wN final states. The obtained results are presented and discussed.

Work supported by FZ Jülich and GSI.

[1] K. Hagiwara et al., Phys. Rev. D66 (2002) 010001, http://pdg.lbl.gov.

Coupled channels calculations for photo-induced K-L and K-S production

•Alexander Usov und Olaf Scholten
KVI, 9747 AA Groningen, The Netherlands

We calculated K-L and K-S photoproduction in gauge-invariant coupled channels framwork. Different recipe's for restoring gauge invariance are compared, such as Ohta's minimal-substitution method[1] and Davidson - Workman's recipe[2]. The difference between these procedures is analyzed in terms of a four-point 'contact'-term. We find that the calculated cross sections strongly depend on the choice for this contact term. A recent microscopic calculation[3] suggest that the structure of the contact term depends strongly on the mass of the intermediate particles used to dress the vertex. A consistent calculation of vertex dressing, in line with [4] is thus called for.

[1] K. Ohta, Phys. Rev. C40, 1335 (1989); S. Kondratyuk and O. Scholten, Nucl. Phys. A677, 396 (2000)

[2] R.M. Davidson and R. Workman, Phys. Rev. C63, 025210 (2001); and Phys. Rev. C63, 058201 (2001)

[3] A.Yu. Korchin and O. Scholten, Phys. Rev. C68, 045206 (2003)

[4] S. Kondratyuk and O. Scholten, Phys. Rev. C64, 024005 (2001)

Photo-induced F-meson production in a coupled channels framework.

•Olaf Scholten
KVI, 9747 AA Groningen, The Netherlands

Results are presented of a coupled channels calculation of photo-induced F-meson production off the proton. In addition to p-N also the coupling to the h-N, K-L and K-S channels is taken into account with realistic coupling constants in a gauge-invariant approach. The approach is in some respects similar to that of ref.[1]

Coupled channels effects are shown to have large effects on the cross sections at larger scattering angles. Much of the structure seen in various polarization observables appears to be due to the interplay between the direct reaction component and coupled channels effects.

[1] A.Yu. Korchin, O. Scholten and R.G.E. Timmermans, Phys. Lett. B438, 1 (1998); G. Penner and U. Mosel, Phys. Rev. C 66, 055212 (2002).

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