Multipole Analysis of a Benchmark Data Set for Pion Photoproduction

Abstract

We have fitted low-and medium-energy benchmark datasets employing methods used in the MAID/SAID and dynamical model analyses. Independent fits from the Mainz, RPI, Yerevan, and Kharkov groups have also been performed over the low-energy region. Results for the multipole amplitudes are compared in order to gauge the model-dependence of such fits, given identical data and a single method for error handling.

Precise measurement of Σ beam asymmetry for positive pion photoproduction on the proton from 800 to 1500 MeV

“…This model, known as MAID2000, in addition to Born terms and vector meson exchange, includes all 3− and 4−star resonances to a CM energy of 1800 MeV, apart from the P 33 (1600) and D 15 (1675). The latest version of these calculations [17], which includes resonance parameters obtained from a fit to the restricted benchmark data base [18], in general reproduces our data up to 1200 MeV, though in some regions discrepancies are still significant.…”

On hadron deformation: A model independent extraction of EMR from pion photoproduction data

“…In Table 3 we present the EMR values of various analyses and models of the last 20 years and make a distinction to the kind of quoted error, statistical, model, and systematic. Excluding the BRAG result [30] which quotes only a model uncertainty and not a statistical error, all other analyses report a statistical+model error combined which is comparable or larger than the ±0.4% value determined in this work. Of the listed analyses, only in the works of references [7,8,12] the analyzed data allowed for full multipole isospin decomposition and in each of them a different approach was used to fix the background.…”

“…4 and 5 where it is evident that the Model Independent AMIAS results (PDFs) are noticeably broader than those resulting from the Model Dependent analysis. Model independent analysis of the "benchmark dataset" [28] has shown that the traditional ansatz [29] of employing several different models and attributing the spread in the solutions as model error [30] although not precise it adequately captures the magnitude of the effect. Figure 9: PDF of the derived EMR(%) from the full dataset of Table 1.…”

“…This model dependence, unavoidably, leads to shifted mean values and underestimated uncertainties in the extracted multipoles and the EMR. It has been argued that in the ∆(1232) region the model error could significantly influence the results [29,30] making it difficult to extract multipoles with the necessary accuracy so as to provide constraints and guidance to the various theoretical models or to address the issue of nucleon deformation.…”

Analysis of pion photoproduction data

“…While the SAID and MAID analyses are qualitatively similar from threshold to 1 GeV, some significant differences exist and these have been mentioned in a recent multi-analysis study which fitted to a benchmark dataset [46]. While some multipoles show significant differences, the photo-decay amplitudes from MAID and SM95 are quite similar, with larger differences between the MAID and SM02 solutions.…”