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Dramatic implications of unitarity for meson spectroscopy
Authors: George Rupp, Eef van Beveren
Ref.: J. High Energy Phys. (2019)
Abstract: An unambiguous definition of meson resonance masses requires a description of the associated phase shifts in terms of a manifestly unitary S-matrix and its complex poles. However, the commonly used Breit-Wigner (BW) parametrisations can lead to appreciable deviations. We demonstrate this for a simple elastic resonance, viz. ρ(770), whose pole and BW masses turn out to differ by almost 5 MeV. In the case of the very broad f0(500) and K⋆0(700) scalar mesons, the discrepancies are shown to become much larger, while also putting question marks at the listed PDG BW masses and widths. Furthermore, some results are reviewed of a manifestly unitary model for meson spectroscopy, which highlight the potentially huge deviations from static model predictions. Finally, a related unitary model for production amplitudes is shown to explain several meson enhancements as non-resonant threshold effects, with profound implications for spectroscopy.
