Helicity asymmetries in double pion photoproduction on the proton

Abstract

Based on a prior model on double pion photoproduction on the proton, successfully tested in total cross sections and invariant mass distributions, we make a theoretical study of the angular dependence of helicity asymmetries from the interaction of circularly polarized photons with unpolarized protons. We show that this observable is sensitive to details of the internal mechanisms and, thus, represents a complementary test of the theoretical model.

Photoproduction of mesons off nuclei

“…Fig. 18) are close to the data, but also in this case the Valencia model [175], which is in good agreement with all other observables, is out of phase with the data. [%] Fig.…”

“…Several model predictions [172,173,174,175] indicated that polarization observables are extremely sensitive to the reaction mechanisms, but it came as a surprise when the first measurement of the beam helicity asymmetry of the γp → pπ + π − reaction at the CLAS facility [21] produced results that could not be reproduced by any reaction models. This observable can be measured for three-body final states with a circularly polarized photon beam on an unpolarized target and is defined by:…”

“…Right hand side: results for the free proton compared to models. Curves: green: fit to data, black: Bonn-Gatchina model [20,19], red: Fix and Arenhövel [173], blue: Roca [175].…”

The Beam-Helicity Asymmetry for Two Pseudoscalar Mesons in Photoproduction and a Partial Wave Analysis for Excited Hyperons

“…In order to get a complete model of the reaction, 15 independent measurements are needed to obtain the contributing amplitudes and their phases [52]. In addition, models like the ones in references [53][54][55], whose cross section predictions are accurate, fail at modeling polarization observables. This is due to a lack of understanding of the interactions involved so a better understanding of the reaction models is needed.…”

“…5.48 as the assumed probability distribution for the hyperon decay angular distribution and V as the parameters for the model, it follows that 54) where N is the number of measurements. Reference [63] asserts that the parameter estimation by maximizing the likelihood function is at least asymptotically unbiased and efficient.…”