A Measurement of the Largest-Scale CMB E-mode Polarization with CLASS

TL;DR

This paper reports large-scale CMB E-mode polarization measurements from the CLASS experiment, demonstrating effective systematic error mitigation, novel bias correction techniques, and a ground-based detection of cosmic reionization with results consistent with Planck.

Contribution

The paper introduces a new pixel-space transfer matrix method for bias correction and demonstrates a ground-based measurement of reionization optical depth using large-scale polarization data.

Findings

Achieved polarization sensitivity of 78 μK·arcmin, comparable to Planck.

Detected cosmic reionization at 99.4% significance.

Measured reionization optical depth τ=0.053^{+0.018}_{-0.019}.

Abstract

We present measurements of large-scale cosmic microwave background (CMB) E-mode polarization from the Cosmology Large Angular Scale Surveyor (CLASS) 90 GHz data. Using 115 det-yr of observations collected through 2024 with a variable-delay polarization modulator, we achieved a polarization sensitivity of $78\,\mathrm{\mu K\,arcmin}$, 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 non-linearity, being characterized as less than $3\%$. Through cross-correlation with Planck, we detected the cosmic reionization at $99.4\%$ significance and measured the reionization optical depth $\tau=0.053^{+0.018}_{-0.019}$, marking the first ground-based attempt at such a measurement. At intermediate angular scales ($\ell>30$), our results, both independently and in cross-correlation with Planck, remain fully consistent with Planck's measurements.