Background: Deep-inelastic scattering (DIS) on the polarized deuteron with detection of a proton in the nuclear breakup region (spectator tagging) represents a unique method for extracting the neutron spin structure functions and studying nuclear modifications. The tagged proton momentum controls the nuclear configuration during the DIS process and enables a differential analysis of nuclear effects. Such measurements could be performed with the future electron-ion collider (EIC) and forward proton detectors if deuteron beam polarization could be achieved.

Purpose: Develop a theoretical framework for polarized deuteron DIS with spectator tagging. Formulate practical procedures for neutron spin structure extraction.

Methods: A covariant spin density matrix formalism is used to describe general deuteron polarization in collider experiments (vector/tensor, pure/mixed). Light-front (LF) quantum mechanics is employed to factorize nuclear and nucleonic structure in the DIS process. A four-dimensional representation of LF spin structure is used to construct the polarized deuteron LF wave function and efficiently evaluate the spin sums. Free neutron structure is extracted using the impulse approximation and analyticity in the tagged proton momentum (pole extrapolation).

Results: General expressions of the polarized tagged DIS observables in collider experiments are presented. The polarized deuteron LF spectral function and nucleon momentum distributions are characterized in analytic and numerical form. Practical procedures for neutron spin structure extraction from the tagged deuteron spin asymmetries are proposed.

Conclusions: Spectator tagging provides new tools for precise neutron spin structure measurements. $D$-wave depolarization and nuclear binding effects can be eliminated through the tagged proton momentum dependence. The methods can be extended to tensor-polarized observables, spin-orbit effects, and diffractive processes.

• Received 17 July 2020
• Revised 5 September 2020
• Accepted 22 September 2020

DOI:https://doi.org/10.1103/PhysRevC.102.065204

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Published by the American Physical Society