# BICEP2 / Keck Array XI: Beam Characterization and Temperature-to-Polarization Leakage in the BK15 Dataset

## TL;DR

This paper characterizes the beam shapes and assesses temperature-to-polarization leakage in the BK15 CMB dataset, quantifying its impact on tensor-to-scalar ratio measurements and informing future systematic control improvements.

## Contribution

It provides a detailed analysis of beam mismatch effects and their systematic bias on B-mode polarization measurements in the BK15 dataset.

## Key findings

- Beam mismatch induces a bias of Δr = 0.0027 ± 0.0019 in tensor-to-scalar ratio.
- Analysis of 7,360 beam patterns over five years informs leakage predictions.
- Future improvements can significantly reduce systematic uncertainties.

## Abstract

Precision measurements of cosmic microwave background (CMB) polarization require extreme control of instrumental systematics. In a companion paper we have presented cosmological constraints from observations with the BICEP2 and Keck Array experiments up to and including the 2015 observing season (BK15), resulting in the deepest CMB polarization maps to date and a statistical sensitivity to the tensor-to-scalar ratio of $\sigma(r) = 0.020$. In this work we characterize the beams and constrain potential systematic contamination from main beam shape mismatch at the three BK15 frequencies (95, 150, and 220 GHz). Far-field maps of 7,360 distinct beam patterns taken from 2010-2015 are used to measure differential beam parameters and predict the contribution of temperature-to-polarization leakage to the BK15 B-mode maps. In the multifrequency, multicomponent likelihood analysis that uses BK15, Planck, and WMAP maps to separate sky components, we find that adding this predicted leakage to simulations induces a bias of $\Delta r = 0.0027 \pm 0.0019$. Future results using higher-quality beam maps and improved techniques to detect such leakage in CMB data will substantially reduce this uncertainty, enabling the levels of systematics control needed for BICEP Array and other experiments that plan to definitively probe large-field inflation.