RECENT PROGRESS IN CHARMONIUM PHYSICS: ROLE OF HEAVY MESON LOOPS

In recent years a large number of new states in the charm sector was discovered above the first inelastic threshold that not at all match to the predictions of the quark model, which on the other hand was extremely successful below that threshold. Since most of those states are located close to a two–hadron threshold, many authors proposed a molecular nature for them. In this talk we will collect the evidences and discuss how the picture can be tested further. In addition, if hadron loops induce non-perturbative effects above the inelastic threshold, one should also expect them to have some impact on the properties of standard charmonia. In the last part of this write-up an effective field theory is introduced that allows one to investigate this issue systematically.

NEW PARAMETERIZATION OF THE RESONANT PRODUCTION AMPLITUDES NEAR AN INELASTIC THRESHOLD

New formulae for the resonant scattering and the production amplitudes near an inelastic threshold are derived. It is shown that the Flatté formula, frequently used in experimental analyses, is not sufficiently accurate. Its application to data analysis can lead to a substantial distortion of the effective mass spectra and of the resonance pole positions. A unitary parameterization, satisfying a generalized Watson theorem for the production amplitudes, is proposed. It can be easily applied to study production processes, multichannel meson-meson interactions and the resonance properties, including among others the scalar resonances a0(980) and f0(980).

Hyperspherical close-coupling calculation of D-wave positronium formation and excitation cross sections in positron–hydrogen scattering

The hyperspherical close-coupling method is used to calculate the elastic, positronium-formation, and excitation cross sections for positron collisions with atomic hydrogen at energies below the H (n = 4) threshold for the J = 2 partial wave. The resonances below each inelastic threshold were also analyzed. The adiabatic hyperspherical potential curves are used to identify the nature of these resonances.