First measurement of polarisation transfer $C^n_{x'}$ in deuteron photodisintegration

TL;DR

This paper reports the first measurement of the neutron polarisation transfer $C^n_{x'}$ in deuterium photodisintegration over a broad energy range, revealing structures unexplained by conventional models, thus providing new insights into nuclear dynamics.

Contribution

It presents the first experimental data on $C^n_{x'}$ in deuteron photodisintegration and compares these results with theoretical models, highlighting discrepancies near the $d^*(2380)$ resonance.

Findings

Measured $C^n_{x'}$ over 370-700 MeV photon energy range.

Observed structures in $C^n_{x'}$ near $d^*(2380)$ not explained by existing models.

Data constrains future theoretical developments in nuclear reaction mechanisms.

Abstract

A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C^n_{x'}$) in deuterium photodisintegration has been carried out. This quantity is determined over the photon energy range 370~--~700~MeV and for neutron centre-of-mass breakup angles $\sim45-120^{\circ}$. The polarisation of the final state neutrons was determined by an ancillary large-acceptance nucleon polarimeter, surrounding a cryogenic liquid deuterium target within the Crystal Ball detector at MAMI. The polarimeter characterised $(n,p)$ charge exchange of the ejected neutrons to determine their polarisation. The new $C^n_{x'}$ data are also compared to a theoretical model based on nucleonic and nucleon resonance degrees of freedom constrained by the current world-database of deuterium photodisintegration measurements. Structures in $C^n_{x'}$ observed in the region of the $d^*(2380)$ could not be explained by conventional models of deuteron photodisintegration.