An isobar model for η photo- and electroproduction on the nucleon

Abstract

Eta photo-and electroproduction on the nucleon is studied using an isobar model. The model contains Born terms, and contributions from vector meson exchanges and nucleon resonances. Our results are compared with recent eta photoproduction data for differential and total cross sections, beam asymmetry, and target asymmetry, as well as electroproduction data. Besides the dominant S 11 (1535) resonance, we show that the second S 11 resonance, S 11 (1650), is also necessary to be included in order to extract S 11 (1535) resonance parameters properly. In addition, the beam asymmetry data allow us to extract very small (< 0.1%) N * → ηN decay branching ratios of the D 13 (1520) and F 15 (1680) resonances because of the overwhelming swave dominance. This model (ETA-MAID) is implemented as a part of the MAID program 1 .

Production of η and η′ mesons on nucleons and nuclei

“…Main figure shows data from [30] (TAPS 95), [31] (GRAAL 02), [32] (CLAS 02), [33] (Crystal Barrel 05), [34] (LNS 06), [35] (GRAAL 07), [36] (CLAS 09), [37] (Crystal Barrel 09), and [38] (Crystal Ball 10). The insert shows the average of the data, together with model results from [39] (MAID 1), [40] (MAID 2), [38] (SAID), and [41,42] (BnGa). Upper right corner: Total cross section for γn → nη for quasi-free photoproduction off neutrons bound in the deuteron or 3 He nuclei (see text) from [43,44,45].…”

“…Upper right corner: Total cross section for γn → nη for quasi-free photoproduction off neutrons bound in the deuteron or 3 He nuclei (see text) from [43,44,45]. Model curves from MAID 1 (solid) [39] and BnGa [46]. Insert: cross sections for quasi-free reactions off protons from same references compared to average freeproton cross section.…”

“…Before we discuss the experimental results, we first give a short summary of existing modelling approaches, which are quite diverse. For the reaction off free protons, they include the SAID partial wave analysis [38] (and references therein), effective Lagrangian models of different types such as the MAID isobar model (MAID 1) [39] and the Reggeized MAID isobar model (MAID 2) [40], different coupled channel models such as the Giessen model [57], the Bonn-Gatchina (BnGa) model [41,42], and even an approach based on the constituent quark model [58]. The γn → nη reaction was also analyzed in the framework of the MAID models [39,40] and with the BnGa coupled-channel analysis [46].…”

“…The largest systematic discrepancy occurs in the fits of the angular distributions for the A 4 coefficient 2). Left column: average of all experimental data compared to MAID 1 [39], SAID [38], and BnGa [41,42] (notation as in Fig. 1).…”

“…4, black open squares) compared to the results for quasi-free γd → ηp(n) [44,50] (blue dots). Model curves are from MAID 1 [39] (dashed), BnGa [41,42] (solid), and SAID [38] (dotted). Lower row: coefficients for quasi-free γd → ηp(n) [44,50] compared to model results for MAID 1 [39] (dashed) and BnGa [46] (solid).…”