BICEP/Keck XVIII: Measurement of BICEP3 polarization angles and consequences for constraining cosmic birefringence and inflation

• Main Authors: Collaboration, BICEP/Keck, :, Ade, P. A. R, Ahmed, Z, Amiri, M, Barkats, D, Thakur, R. Basu, Bischoff, C. A, Beck, D, Bock, J. J, Boenish, H, Buza, V, CheshireIV, J. R, Connors, J, Cornelison, J, Crumrine, M, Cukierman, A. J, Denison, E, Duband, L, Eiben, M, Elwood, B. D, Fatigoni, S, Filippini, J. P, Fortes, A, Gao, M, Giannakopoulos, C, Goeckner-Wald, N, Goldfinger, D. C, Grayson, J. A, Grimes, P. K, Hall, G, Halal, G, Halpern, M, Hand, E, Harrison, S. A, Henderson, S, Hubmayr, J, Hui, H, Irwin, K. D, Kang, J. H, Karkare, K. S, Kefeli, S, Kovac, J. M, Kuo, C, Lau, K, Lautzenhiser, M, Lennox, A, Liu, T, Megerian, K. G, Minutolo, L, Moncelsi, L, Nakato, Y, Nguyen, H. T, O'brient, R, Patel, A, Petroff, M. A, Polish, A. R, Prouve, T, Pryke, C, Reintsema, C. D, Romand, T, Salatino, M, Schillaci, A, Schmitt, B, Singari, B, Sjoberg, K, Soliman, A, Germaine, T. St, Steiger, A, Steinbach, B, Sudiwala, R, Thompson, K. L, Tsai, C, Tucker, C, Turner, A. D, Vergès, C, Vieregg, A. G, Wandui, A, Weber, A. C, Willmert, J, Wu, W. L. K, Yang, H, Yu, C, Zeng, L, Zhang, C, Zhang, S
• Format: Journal Article
• Language: English
• Published: 15-10-2024
• Subjects: Physics - Cosmology and Nongalactic Astrophysics
• DOI: 10.48550/arxiv.2410.12089

We use a custom-made calibrator to measure individual detectors' polarization
angles of BICEP3, a small aperture telescope observing the cosmic microwave
background (CMB) at 95GHz from the South Pole. We describe our calibration
strategy and the statistical and systematic uncertainties associated with the
measurement. We reach an unprecedented precision for such measurement on a CMB
experiment, with a repeatability for each detector pair of $0.02\deg$. We show
that the relative angles measured using this method are in excellent agreement
with those extracted from CMB data. Because the absolute measurement is
currently limited by a systematic uncertainty, we do not derive cosmic
birefringence constraints from BICEP3 data in this work. Rather, we forecast
the sensitivity of BICEP3 sky maps for such analysis. We investigate the
relative contributions of instrument noise, lensing, and dust, as well as
astrophysical and instrumental systematics. We also explore the constraining
power of different angle estimators, depending on analysis choices. We
establish that the BICEP3 2-year dataset (2017--2018) has an on-sky sensitivity
to the cosmic birefringence angle of $\sigma = 0.078\deg$, which could be
improved to $\sigma = 0.055\deg$ by adding all of the existing BICEP3 data
(through 2023). Furthermore, we emphasize the possibility of using the BICEP3
sky patch as a polarization calibration source for CMB experiments, which with
the present data could reach a precision of $0.035\deg$. Finally, in the
context of inflation searches, we investigate the impact of
detector-to-detector variations in polarization angles as they may bias the
tensor-to-scalar ratio r. We show that while the effect is expected to remain
subdominant to other sources of systematic uncertainty, it can be reliably
calibrated using polarization angle measurements such as the ones we present in
this paper.