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

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.

arXiv comment18 pages, 9 figures, 4 tables. Minor correction to Figure 2 and caption

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• BICEP / Keck XIII: Improved Constraints on Primordial Gravitational Waves using Planck, WMAP, and BICEP/Keck Observations through the 2018 Observing Season
• CMB-S4: Forecasting Constraints on Primordial Gravitational Waves
• BICEP / Keck XVII: Line of Sight Distortion Analysis: Estimates of Gravitational Lensing, Anisotropic Cosmic Birefringence, Patchy Reionization, and Systematic Errors
• Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: 40 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
• Instrumental systematics biases in CMB lensing reconstruction: a simulation-based assessment
• Optical characterization of the Keck Array and BICEP3 CMB Polarimeters from 2016 to 2019
• Forecasts of CMB lensing reconstruction of AliCPT-1 from the foreground cleaned polarization data
• Optical Design and Characterization of 40-GHz Detector and Module for the BICEP Array
• Cosmology Large Angular Scale Surveyor (CLASS): 90 GHz Telescope Pointing, Beam Profile, Window Function, and Polarization Performance
• The Simons Observatory: Beam characterization for the Small Aperture Telescopes
• PROTOCALC, A W-band polarized calibrator for CMB Telescopes: application to Simons Observatory and CLASS
• Cosmological Non-Gaussianity from Neutrino Seesaw Mechanism
• Design and Performance of 30/40 GHz Diplexed Focal Plane for BICEP Array
• Deprojecting beam systematics for next-generation CMB B-mode searches
• The Simons Observatory: laboratory beam characterization for the first small aperture telescope
• Receiver development for BICEP Array, a next-generation CMB polarimeter at the South Pole
• Mitigating point-source contamination in CMB polarization: a Generalized Point Spread Function fitting approach