Neutron polarisation transfer, $C_{x'}^n$, in $\pi^+$ photoproduction off the proton

TL;DR

This paper presents the first measurement of the $C_{x'}$ double-polarisation observable in $ ext{π}^+$ photoproduction off the proton, offering new experimental data to refine nucleon resonance models.

Contribution

It introduces the first measurement of the $C_{x'}$ observable in this reaction, providing crucial constraints for theoretical models of nucleon resonances.

Findings

Measured $C_{x'}$ for photon energies 800-1400 MeV

Provides new constraints on nucleon resonance models

First polarization observable measurement for this reaction

Abstract

We report a first measurement of the double-polarisation observable, $C_{x'}$, in $\pi^+$ photoproduction off the proton. The $C_{x'}$ double-polarisation observable represents the transfer of polarisation from a circularly polarised photon beam to the recoiling neutron. The MAMI circularly polarised photon beam impinged on a liquid deuterium target cell, with reaction products detected in the Crystal Ball calorimeter. Ancillary apparatus surrounding the target provided tracking, particle identification and determination of recoil nucleon polarisation. The $C_{x'}$ observable is determined for photon energies 800-1400 MeV, providing new constraints on models aiming to elucidate the spectrum and properties of nucleon resonances. This is the first determination of any polarisation observable from the beam-recoil group of observables for this reaction, providing a valuable constraint and systematic check of the current solutions of partial wave analysis based theoretical models.