  Background: 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 theoretical framework for polarized deuteron DIS with
spectator tagging. Formulate 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 4-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. Analytic and numerical study of the polarized deuteron LF
spectral function and nucleon momentum distributions. Practical procedures for
neutron spin structure extraction from the tagged deuteron spin asymmetries.
  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.
