Helicity dependent cross sections for the photoproduction of $\pi^0\pi^{\pm}$ pairs from quasi-free nucleons

TL;DR

This study measures how the production of pion pairs from quasi-free nucleons depends on photon and nucleon spin orientations, providing insights into nucleon resonance decays and testing theoretical models.

Contribution

It presents the first detailed measurement of helicity-dependent cross sections for pion pair photoproduction from quasi-free nucleons, including new polarization observables and comparison with models.

Findings

Measured unpolarized cross sections for reactions on protons and neutrons.

Derived spin-dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$.

Compared results with recent theoretical models.

Abstract

Photoproduction of $\pi^0\pi^{\pm}$-pairs from quasifree nucleons bound in the deuteron has been investigated to study the helicity dependence of this reaction. Measurements with a liquid deuterium target were used to extract the unpolarized cross sections for reactions on protons and neutrons. A deuterated, longitudinally polarized solid-butanol target, together with a circularly polarized photon beam, determined the double polarization observable $E$. From these results the spin-dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$, corresponding to the anti-parallel and parallel spin configurations of the beam photon and target nucleon, have been derived. The measurements were performed at the Mainz MAMI accelerator with tagged, circularly-polarized photon beams produced via bremsstrahlung from longitudinally polarized electron beams. The reaction products were detected with an almost $4\pi$ solid-angle covering calorimeter composed of the Crystal Ball and TAPS detectors, supplemented by plastic scintillation detectors for charged particle identification. The results are sensitive to sequential decays of nucleon resonances via intermediate states and also to the decay of nucleon resonances by emission of charged $\rho$ mesons, and are compared to recent model results.