A Measurement of the Largest-scale CMB <i>E</i>-mode Polarization with CLASS
Yunyang Li, Joseph R. Eimer, John W. Appel, C. L. Bennett, Michael K. Brewer, Sarah Marie Bruno, Ricardo Bustos, Yan Yan Chan, David T. Chuss, Joseph Cleary, Sumit Dahal, Rahul Datta, Jullianna Denes Couto, Kevin L. Denis, Rolando Dünner, Thomas Essinger-Hileman, Kathleen Harrington, Kyle Helson, Johannes Hubmayr, Jeffrey Iuliano, John Karakla, Tobias A. Marriage, N. J. Miller, Carolina Morales Pérez, Lucas Parker, Matthew A. Petroff, R. Reeves, Karwan Rostem, Caleigh Ryan, Rui Shi, Koji Shukawa, Deniz A. N. Valle, Duncan J. Watts, J. L. Weiland, Edward J. Wollack, Zhilei Xu, Lingzhen Zeng
Abstract
Abstract We present measurements of large-scale cosmic microwave background E -mode polarization from the Cosmology Large Angular Scale Surveyor 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>82</mml:mn> <mml:mspace width="0.25em"/> <mml:mi>μ</mml:mi> <mml:mi mathvariant="normal">K</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">arcmin</mml:mi> </mml:math> , comparable to Planck at similar frequencies (100 and 143 GHz ). The analysis demonstrates effective mitigation of systematic errors and addresses challenges to large-angular-scale power recovery posed by time-domain filtering in maximum-likelihood map-making. A novel implementation of the pixel-space transfer matrix is introduced, which enables efficient filtering simulations and bias correction in the power spectrum using the quadratic cross-spectrum estimator. Overall, we achieved an unbiased time-domain filtering correction to recover the largest angular scale polarization, with the only power deficit, arising from map-making nonlinearity, being characterized as <3%. Through cross-correlation with Planck, we detected the cosmic reionization at 99.4% significance and measured the reionization optical depth <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>τ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.05</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.019</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.018</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , marking the first ground-based attempt at such a measurement. At intermediate angular scales ( ℓ > 30), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck’s measurements.