Task 4.1 - Resonance parameter determination


    Coordinator:  Michael Ostrick

    Contributors: Paolo Pedroni , Derek Glazier, Bryan McKinnon , Ulrike Thoma , Sebastian Neubert, Elena Santopinto, Alfred Švarc, Claude Semay, Cintia Willemyns , Lorenzo Cimino



    General description:


    The formation of light-quark baryon resonances is studied in meson nucleon scattering and photon-induced meson production. Due to the huge experimental effort at several photon-beam facilities, in particular ELSA, GRAAL, TJNAF, and MAMI, precision data for many spin-dependent observables in photo- and electroproduction of light mesons are available or are expected in the near future. More recently, the production of light baryon resonances in charmonium decay is investigated using the high-statistics data sets obtained in the BESIII experiment. Furthermore, the study of baryons with heavy charm quarks is booming due to new high-energy data from LHCb.

    Common to all these approaches is the necessity to apply sophisticated theoretical tools in order to extract partial wave amplitudes and finally resonance poles from the experimental data. Here is a strong overlap and synergy with the research and developments in the other Tasks. The process is complex and ambiguities arise e.g. due to the finite accuracy of experimental data, unknown quantum mechanical phases, and non-resonant background. Sophisticated single- and coupled channel models with idealizations and approximations at different levels or single-energy analyses in combination with techniques to expand the amplitudes into the complex energy plane are applied in order to obtain resonance poles.

    The goal of this project is to refine these methods and to extend them to new experimental data. Using different approaches with different strengths and drawbacks in a combined effort will allow us to obtain baryon resonance pole parameters with a significantly reduced model dependence and reliable error estimates.




    Progress [1-18 months]:


    In the reporting period progress was made in both key aspects of this project, publishing new experimental data and refining partial wave analyses.

    New measurements of spin dependent π, η, ππ, and πη photoproduction from ELSA and MAMI were published [Br19 – Af21].

    One major focus of different partial wave analysis groups was the γNηN reaction, where all previous analyses showed significant differences in the underlying partial wave and resonance structure. A new EtaMAID model was published [Ti18] and a refit including new data within the multi-channel Bonn-Gatchina (BnGa) PWA [Mu20] revealed that the large difference in the N η-branching ratio between the N(1535)1/2− and the N(1650)1/2− almost disappeared. Such a difference was found in earlier analyses and led to quite different interpretation of the two resonances. Much interest was attracted by a cusp structure at the η' threshold, observed in the recent cross section from MAMI. New measurements of the beam asymmetry at ELSA were able to confirm this structure [Af21] and a refit of the BnGa PWA was necessary to describe this region properly. EtaMAID and BnGa agree that this structure originates from the coupling of N(1895)1/2− to both channels, and pη', and a similar description of the N(1895)1/2− was obtained. This is in contrast to the JüBo model, which does not require the N (1895)1/2− at all.

    Beyond these model-dependent studies, a model-independent single-energy analysis with analytical constraints was published [Os19]. Furthermore, a novel self-consistent two-step approach based on constraining a single energy PWA by results of an analysis of transversity amplitudes was proposed and applied to η photoproduction [Sv20].





    • [Br19] W. J. Briscoe et al. [A2], “Cross section for γn → π0 n at the Mainz A2 experiment,” Phys. Rev. C 100 (2019) no.6, 065205 (doi:10.1103/PhysRevC.100.065205)
    • [So20] V. Sokhoyan et al. [A2], “Measurement of the beam-helicity asymmetry in photoproduction of π0 η pairs on carbon, aluminum,
      and lead,” Phys. Lett. B 802 (2020), 135243 (doi:10.1016/j.physletb.2020.135243)
    • [Di20] M. Dieterle et al., “Helicity-dependent cross sections for the photoproduction of π0 pairs from nucleons,” Phys. Rev. Lett. 125 (2020) no.6, 062001 (doi:10.1103/PhysRevLett.125.062001)
    • [Mu20] J.Müller et al. [CBELSA/TAPS Collab.], “New data on ~γ ~ p → ηp with polarized photons and protons and their implications for N → N η decays”, Phys. Lett. B 803 (2020) 135323.
    • [Af21] F. Afzal et al. [CBELSA/TAPS] ], “Precise beam asymmetry data for γp → pη in the pη’ -threshold region”, accepted for publication in PRL
    • [Ti18] L. Tiator et al., “Eta and Etaprime Photoproduction on the Nucleon with the Isobar Model EtaMAID2018,” EPJ Web Conf. 199 (2019), 01019 (doi:10.1051/epjconf/201919901019) and
      Eur. Phys. J. A 54 (2018) no.12, 210 (doi:10.1140/epja/i2018-12643-x)
    • [Os19] H. Osmanović et al., “Single-energy partial wave analysis for π0 photoproduction on the proton with fixed-t analyticity imposed,” Phys. Rev. C 100 (2019) no.5, 055203 (doi:10.1103/PhysRevC.100.055203)
    • [Sv20] A. Švarc, Y. Wunderlich and L. Tiator, “Amplitude- and truncated partial-wave analyses combined: A novel, almost theory-independent single-channel method for extracting photoproduction multipoles directly from measured data,” [arXiv:2008.01355 [nucl-th]]



    This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 824093.

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