Signatures of the $d^*(2380)$ hexaquark in d($\gamma$,$p\vec{n}$)

TL;DR

This study measures recoil neutron polarisation in deuterium photodisintegration at energies where quark substructure influences the process, revealing a high polarisation structure potentially linked to the $d^*(2380)$ hexaquark, challenging existing theories.

Contribution

It provides the first measurement of neutron polarisation at these energies and suggests a connection to the $d^*(2380)$ hexaquark, offering new insights into photodisintegration mechanisms.

Findings

High neutron polarisation observed around 570 MeV photon energy.

The polarisation structure is inconsistent with current nucleon resonance models.

Possible indication of $d^*(2380)$ hexaquark involvement.

Abstract

We report a measurement of the spin polarisation of the recoiling neutron in deuterium photodisintegration, utilising a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300~--~700~MeV provides the first measurement of recoil neutron polarisation at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarisation in a narrow structure centred around $E_{\gamma}\sim$~570~MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behaviour could be related to the excitation of the $d^*(2380)$ hexaquark.