First measurement of polarisation transfer $C_{x^{\prime }}^n$ in deuteron photodisintegration

Abstract

A first measurement of the polarisation transfer from a circularly-polarised photon to the final state neutron ($C_{x^{\prime }}^n$) 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 $\sim 45-120^{\circ }$. 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 $C_{x^{\prime }}^n$ 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 $C_{x^{\prime }}^n$ observed in the region of the $d^{\ast }(2380)$ could not be explained by conventional models of deuteron photodisintegration.