The Simons Observatory: Combining cross-spectral foreground cleaning with multitracer <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> </mml:math> -mode delensing for improved constraints on inflation

Abstract

The Simons Observatory (SO), due to start full science operations in early 2025, aims to set tight constraints on inflationary physics by inferring the tensor-to-scalar ratio <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>r</a:mi> </a:math> from measurements of cosmic microwave background (CMB) polarization <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>B</c:mi> </c:math> -modes. Its nominal design including three small-aperture telescopes (SATs) targets a precision <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>σ</e:mi> <e:mo stretchy="false">(</e:mo> <e:mi>r</e:mi> <e:mo>=</e:mo> <e:mn>0</e:mn> <e:mo stretchy="false">)</e:mo> <e:mo>≤</e:mo> <e:mn>0.003</e:mn> </e:math> without delensing. Achieving this goal and further reducing uncertainties requires a thorough understanding and mitigation of other large-scale <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>B</i:mi> </i:math> -mode sources such as Galactic foregrounds and weak gravitational lensing. We present an analysis pipeline aiming to estimate <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mi>r</k:mi> </k:math> by including delensing within a cross-spectral likelihood, and demonstrate it for the first time on SO-like simulations accounting for various levels of foreground complexity, inhomogeneous noise and partial sky coverage. As introduced in an earlier SO delensing paper, lensing <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mi>B</m:mi> </m:math> -modes are synthesized using internal CMB lensing reconstructions as well as -like cosmic infrared background maps and LSST-like galaxy density maps. We then extend SO’s power-spectrum-based foreground-cleaning algorithm to include all auto- and cross-spectra between the lensing template and the SAT <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mi>B</o:mi> </o:math> -modes in the likelihood function. This allows us to constrain <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mi>r</q:mi> </q:math> and the parameters of our foreground model simultaneously. Within this framework, we demonstrate the equivalence of map-based and cross-spectral delensing and use it to motivate an optimized pixel-weighting scheme for power spectrum estimation. We start by validating our pipeline in the simplistic case of uniform foreground spectral energy distributions. In the absence of primordial <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"> <s:mi>B</s:mi> </s:math> -modes, we find that the <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"> <u:mn>1</u:mn> <u:mi>σ</u:mi> </u:math> statistical uncertainty on <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi>r</w:mi> </w:math> , <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:mi>σ</y:mi> <y:mo stretchy="false">(</y:mo> <y:mi>r</y:mi> <y:mo stretchy="false">)</y:mo> </y:math> , decreases by 37% as a result of delensing. Tensor modes at the level of <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"> <cb:mi>r</cb:mi> <cb:mo>=</cb:mo> <cb:mn>0.01</cb:mn> </cb:math> are successfully detected by our pipeline. Even when using more realistic foreground models including spatial variations in the dust and synchrotron spectral properties, we obtain unbiased estimates of <eb:math xmlns:eb="http://www.w3.org/1998/Math/MathML" display="inline"> <eb:mi>r</eb:mi> </eb:math> both with and without delensing by employing the moment-expansion method. In this case, uncertainties are increased due to the higher number of model parameters, and delensing-related improvements range between 27% and 31%. These results constitute the first realistic assessment of the delensing performance at SO’s nominal sensitivity level. Published by the American Physical Society 2024