Analysis of Temperature-to-Polarization Leakage in BICEP3 and Keck CMB Data from 2016 to 2018

TL;DR

This paper investigates temperature-to-polarization leakage in BICEP/Keck CMB data from 2016-2018, using high-fidelity beam measurements and simulations to understand and mitigate systematic errors in polarization measurements.

Contribution

It provides detailed analysis of beam mismatch and T→P leakage using in-situ measurements and simulations for the BICEP3 and Keck experiments.

Findings

Quantified differential beam mismatch and residual beam power.

Presented band-averaged far-field beam maps.

Estimated T→P leakage through beam map simulations.

Abstract

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.