# Neutron spin structure from polarized deuteron DIS with proton tagging

## TL;DR

This paper proposes a method to measure neutron spin structure using polarized deuteron deep-inelastic scattering with proton tagging, enabling model-independent extraction of neutron information at future colliders.

## Contribution

It introduces a theoretical framework for analyzing polarized tagged DIS with proton detection, allowing control over nuclear effects and extraction of free neutron spin structure.

## Key findings

- Calculated spin asymmetries depend on proton momentum and deuteron polarization.
- Proposed method can isolate pure S-wave configurations in the deuteron.
- Model-independent neutron spin structure extraction via pole extrapolation.

## Abstract

Polarized electron-deuteron deep-inelastic scattering (DIS) with detection of the spectator proton ("tagged DIS") enables measurements of neutron spin structure with maximal control of nuclear effects. We calculate the longitudinal spin asymmetries in polarized tagged DIS using methods of light-front nuclear structure and study their dependence on the measured proton momentum. Asymmetries can be formed with all three deuteron spin states ($\pm 1, 0$) or the two maximum-spin states only ($\pm 1$, involving tensor polarization). The proton momentum dependence can be used to select pure S-wave configurations in the deuteron and eliminate D-wave depolarization (transverse momenta $p_{pT} \lesssim$ 100 MeV). Free neutron spin structure can be extracted model-independently through pole extrapolation of the tagged asymmetries. Such measurements could be performed at a future electron-ion collider (EIC) with polarized deuteron beams and forward proton detectors.