S3: Accomplishments of the second funding period

The photoinduced production of pseudoscalar mesons is one of the most important sources of information in light baryon spectroscopy. Excited N- and ∆-states manifest themselves as resonances in partial wave amplitudes and, via interference, in 16 single and double spin-observables. The inverse problem of uniquely reconstructing partial wave amplitudes and resonance properties from experimental data is the central task of this project. To reach this goal, the energy and angular dependences of several spin-observables have to be measured with high precision and sophisticated tools for partial wave analyses have to be developed and applied.

  • Photoproduction experiments at MAMI and ELSA
    Until the end of 2016, several experiments with polarized beam and polarized protons and deuterons were successfully performed within the A2 collaboration at MAMI. We have obtained high precision data sets for the γN → ηN and γN → π0N reactions in the energy range accessible with MAMI (W 1.9 GeV). Most of the analyses are completed and the data are published (see publication list). The helicity dependence of π0 production off quasi-free protons and neutrons is presently studied as part of the PhD thesis of F. Cividini. Final results are expected in 2018.
    In order to extend the measurements to higher energies, the reliable dilution cryostat of the Mainz polarized target was combined with the renewed Crystal Barrel experiment at the ELSA accelerator. In spring 2017 this cryostat including the 3He circulation system and the transverse holding coil was transported to ELSA and integrated into the existing Bonn polarized target set-up. A new front part of the outer vacuum container that fits to the Bonn polarizing magnet was constructed, installed, and tested. This work was done in close collaboration with the target groups in Dubna, Bonn and Bochum. During summer 2017, target material was loaded into the cryostat and polarized under typical run conditions. A first beamtime is scheduled for November 2017. Two longer measurements with transversely polarized protons and neutrons are planned for 2018.
    In parallel, the construction of a new focal plane detector for the Mainz tagging spectrometer has started. So far 10 modules have successfully been tested and installed. One module consists of 8 detector channels with 6x6mm2 EJ-200 fibers, which are read out by SensL-SiPMs. A time resolution between 2 detectors of less then σ =100ps was achieved. Until end of November 2017 in total 41 modules (328 channels) will be installed and used in Compton scattering experiments (see S2 project). These modules will cover 8-85% of the full photon energy range.
  • Partial wave analysis
    In this funding period we focused on the interpretation of the γN → ηN reaction using all available data and following a 3-fold strategy:
  1. New η-MAID isobar models have been developed. They include explicit resonances on top of a non-resonant background, which consists of Born-terms and t-channel meson exchanges. In order to obtain a better understanding of t-channel background processes we analyzed high energy data above W = 4 GeV with different Regge-models (see publication list). A detailed paper about ηMAID fits to data in the resonance region in under preparation.
  2. Isobar models as ηMAID violate fixed-t dispersion relations, which can be derived for the relevant invariant amplitudes. As part of his PhD thesis, K. Nikonov is working on modifications of the MAID model using fixed-t dispersion relations. Starting with the imaginary parts of the amplitudes the real parts can be calculated from dispersion relations. By fitting the experimental data using this procedure, modified amplitudes are obtained. Technical issues as the implementation of fast integration algorithms and the extrapolation of amplitudes to unphysical regions below the η production threshold have been solved. Publishable results are expected by mid of next year.
  3. The least biased determination of multipole amplitudes is possible in so-called single-energy or energy-independent analyses. Here, a truncated partial wave expansion of the measured angular distributions is performed at each individual energy bin without assuming any model for the energy-dependence. However, it can be shown that fully unconstrained single-energy analyses lead to so-called continuum ambiguities. These are related to angular and energy dependent phases which, in principle, cannot be determined from experimental data alone. Therefore, such analyses are typically constrained by some model. In order to avoid this, we have developed a new method to apply model-independent constraints from fixed-t analyticity. In an iterative procedure we performed fixed-t amplitude analyses using a so-called Pietarinen expansion method and single-energy partial wave analyses with a mutual support and constraint of each others. This work was performed in close collaboration with groups in Tuzla and Zagreb. The results are submitted for publication and are presently under review.