Dark Energy Survey Year 3 Results: Calibration of Lens Sample Redshift
  Distributions using Clustering Redshifts with BOSS/eBOSS
  We present clustering redshift measurements for Dark Energy Survey (DES) lens
sample galaxies to be used in weak gravitational lensing and galaxy clustering
studies. To perform this measurement, we cross-correlate with spectroscopic
galaxies from the Baryon Acoustic Oscillation Survey (BOSS) and its extension,
eBOSS. We validate our methodology in simulations, including a new technique to
calibrate systematic errors due to the galaxy clustering bias, finding our
method to be generally unbiased in calibrating the mean redshift. We apply our
method to the data, and estimate the redshift distribution for eleven different
photometrically-selected bins. We find general agreement between clustering
redshift and photometric redshift estimates, with differences on the inferred
mean redshift to be below $|\Delta z|=0.01$ in most of the bins. We also test a
method to calibrate a width parameter for redshift distributions, which we
found necessary to use for some of our samples. Our typical uncertainties on
the mean redshift ranged from 0.003 to 0.008, while our uncertainties on the
width ranged from 4 to 9\%. We discuss how these results calibrate the
photometric redshift distributions used in companion DES Year 3 Results papers.
