  We characterize Galactic dust filaments by correlating BICEP/Keck and Planck
data with polarization templates based on neutral hydrogen (H I) observations.
Dust polarization is important for both our understanding of astrophysical
processes in the interstellar medium (ISM) and the search for primordial
gravitational waves in the cosmic microwave background (CMB). In the diffuse
ISM, H I is strongly correlated with the dust and partly organized into
filaments that are aligned with the local magnetic field. We analyze the deep
BICEP/Keck data at 95, 150, and 220 GHz, over the low-column-density region of
sky where BICEP/Keck has set the best limits on primordial gravitational waves.
We separate the H I emission into distinct velocity components and detect dust
polarization correlated with the local Galactic H I but not with the H I
associated with Magellanic Stream I. We present a robust, multifrequency
detection of polarized dust emission correlated with the filamentary H I
morphology template down to 95 GHz. For assessing its utility for foreground
cleaning, we report that the H I morphology template correlates in B modes at a
$\sim$10-65$\%$ level over the multipole range $20 < \ell < 200$ with the
BICEP/Keck maps, which contain contributions from dust, CMB, and noise
components. We measure the spectral index of the filamentary dust component
spectral energy distribution to be $\beta = 1.54 \pm 0.13$. We find no evidence
for decorrelation in this region between the filaments and the rest of the dust
field or from the inclusion of dust associated with the intermediate velocity H
I. Finally, we explore the morphological parameter space in the H I-based
filamentary model.
