Analysis of Temperature-to-Polarization Leakage in BICEP3 and Keck CMB
  Data from 2016 to 2018
  The BICEP/Keck Array experiment is a series of small-aperture refracting
telescopes observing degree-scale Cosmic Microwave Background polarization from
the South Pole in search of a primordial $B$-mode signature. As a pair
differencing experiment, an important systematic that must be controlled is the
differential beam response between the co-located, orthogonally polarized
detectors. We use high-fidelity, in-situ measurements of the beam response to
estimate the temperature-to-polarization (T $\rightarrow$ P) leakage in our
latest data including observations from 2016 through 2018. This includes three
years of BICEP3 observing at 95 GHz, and multifrequency data from Keck Array.
Here we present band-averaged far-field beam maps, differential beam mismatch,
and residual beam power (after filtering out the leading difference modes via
deprojection) for these receivers. We show preliminary results of "beam map
simulations," which use these beam maps to observe a simulated temperature (no
$Q/U$) sky to estimate T $\rightarrow$ P leakage in our real data.
