arXiv subjects

W. L. K. Wu

Publications and source records attributed to W. L. K. Wu.

At least 91 records · Page 5Linked to original sources

arXiv2020

Characterizing the Sensitivity of 40 GHz TES Bolometers for BICEP Array

The BICEP/Keck (BK) experiment aims to detect the imprint of primordial gravitational waves in the Cosmic Microwave Background polarization, which would be direct evidence of the inflation theory. While the tensor-to-scalar ratio has been constrained to be r_0.05 < 0.06 at 95% c.l., further improvements on this upper limit are hindered by polarized Galactic foreground emissions and removal of gravitational lensing polarization. The 30/40 GHz receiver of the BICEP Array (BA) will deploy at the end of 2019 and will constrain the synchrotron foreground with unprecedented accuracy within the BK sky patch. We will show the design of the 30/40 GHz detectors and test results summarizing its performance. The low optical and atmospheric loading at these frequencies requires our TES detectors to have low saturation power in order to be photon-noise dominated. To realize the low thermal conductivity required from a 250 mK base temperature, we developed new bolometer leg designs. We will present the relevant measured detector parameters: G, Tc, Rn, Psat , and spectral bands, and noise spectra. We achieved a per bolometer NEP including all noise components of 2.07E-17 W/sqrt(Hz), including an anticipated photon noise level 1.54E-17 W/sqrt(Hz).

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arXiv2020

Constraints on Cosmological Parameters from the 500 deg$^2$ SPTpol Lensing Power Spectrum

We present cosmological constraints based on the cosmic microwave background (CMB) lensing potential power spectrum measurement from the recent 500 deg$^2$ SPTpol survey, the most precise CMB lensing measurement from the ground to date. We fit a flat $Λ$CDM model to the reconstructed lensing power spectrum alone and in addition with other data sets: baryon acoustic oscillations (BAO) as well as primary CMB spectra from Planck and SPTpol. The cosmological constraints based on SPTpol and Planck lensing band powers are in good agreement when analysed alone and in combination with Planck full-sky primary CMB data. With weak priors on the baryon density and other parameters, the CMB lensing data alone provide a 4\% constraint on $σ_8Ω_m^{0.25} = 0.0593 \pm 0.025$.. Jointly fitting with BAO data, we find $σ_8=0.779 \pm 0.023$, $Ω_m = 0.368^{+0.032}_{-0.037}$, and $H_0 = 72.0^{+2.1}_{-2.5}\,\text{km}\,\text{s}^{-1}\,\text{Mpc}^{-1} $, up to $2\,σ$ away from the central values preferred by Planck lensing + BAO. However, we recover good agreement between SPTpol and Planck when restricting the analysis to similar scales. We also consider single-parameter extensions to the flat $Λ$CDM model. The SPTpol lensing spectrum constrains the spatial curvature to be $Ω_K = -0.0007 \pm 0.0025$ and the sum of the neutrino masses to be $\sum m_ν < 0.23$ eV at 95\% C.L. (with Planck primary CMB and BAO data), in good agreement with the Planck lensing results. With the differences in the $S/N$ of the lensing modes and the angular scales covered in the lensing spectra, this analysis represents an important independent check on the full-sky Planck lensing measurement.

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arXiv2020

Microwave multiplexing on the Keck Array

We describe an on-sky demonstration of a microwave-multiplexing readout system in one of the receivers of the Keck Array, a polarimetry experiment observing the cosmic microwave background at the South Pole. During the austral summer of 2018-2019, we replaced the time-division multiplexing readout system with microwave-multiplexing components including superconducting microwave resonators coupled to radio-frequency superconducting quantum interference devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification between room temperature and the cold stages, and a SLAC Microresonator Radio Frequency system for the warm electronics. In the range 5-6 GHz, a single coaxial cable reads out 528 channels. The readout system is coupled to transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole phased-array antennas. Observations began in April 2019, and we report here on an initial characterization of the system performance.

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arXiv2020

Fractional Polarisation of Extragalactic Sources in the 500-square-degree SPTpol Survey

We study the polarisation properties of extragalactic sources at 95 and 150 GHz in the SPTpol 500 deg$^2$ survey. We estimate the polarised power by stacking maps at known source positions, and correct for noise bias by subtracting the mean polarised power at random positions in the maps. We show that the method is unbiased using a set of simulated maps with similar noise properties to the real SPTpol maps. We find a flux-weighted mean-squared polarisation fraction $\langle p^2 \rangle= [8.9\pm1.1] \times 10^{-4}$ at 95 GHz and $[6.9\pm1.1] \times 10^{-4}$ at 150~GHz for the full sample. This is consistent with the values obtained for a sub-sample of active galactic nuclei. For dusty sources, we find 95 per cent upper limits of $\langle p^2 \rangle_{\rm 95}<16.9 \times 10^{-3}$ and $\langle p^2 \rangle_{\rm 150}<2.6 \times 10^{-3}$. We find no evidence that the polarisation fraction depends on the source flux or observing frequency. The 1-$σ$ upper limit on measured mean squared polarisation fraction at 150 GHz implies that extragalactic foregrounds will be subdominant to the CMB E and B mode polarisation power spectra out to at least $\ell\lesssim5700$ ($\ell\lesssim4700$) and $\ell\lesssim5300$ ($\ell\lesssim3600$), respectively at 95 (150) GHz.

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arXiv2020

Galaxy Clusters Selected via the Sunyaev-Zel'dovich Effect in the SPTpol 100-Square-Degree Survey

We present a catalog of galaxy cluster candidates detected in 100 square degrees surveyed with the SPTpol receiver on the South Pole Telescope. The catalog contains 89 candidates detected with a signal-to-noise ratio greater than 4.6. The candidates are selected using the Sunyaev-Zel'dovich effect at 95 and 150 GHz. Using both space- and ground-based optical and infrared telescopes, we have confirmed 81 candidates as galaxy clusters. We use these follow-up images and archival images to estimate photometric redshifts for 66 galaxy clusters and spectroscopic observations to obtain redshifts for 13 systems. An additional 2 galaxy clusters are confirmed using the overdensity of near-infrared galaxies only, and are presented without redshifts. We find that 15 candidates (18% of the total sample) are at redshift of $z \geq 1.0$, with a maximum confirmed redshift of $z_{\rm{max}} = 1.38 \pm 0.10$. We expect this catalog to contain every galaxy cluster with $M_{500c} > 2.6 \times 10^{14} M_\odot h^{-1}_{70}$ and $z > 0.25$ in the survey area. The mass threshold is approximately constant above $z = 0.25$, and the complete catalog has a median mass of approximately $ M_{500c} = 2.7 \times 10^{14} M_\odot h^{-1}_{70}$. Compared to previous SPT works, the increased depth of the millimeter-wave data (11.2 and 6.5 $μ$K-arcmin at 95 and 150 GHz, respectively) makes it possible to find more galaxy clusters at high redshift and lower mass.

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arXiv2019

Particle Physics with the Cosmic Microwave Background with SPT-3G

The cosmic microwave background (CMB) encodes information about the content and evolution of the universe. The presence of light, weakly interacting particles impacts the expansion history of the early universe, which alters the temperature and polarization anisotropies of the CMB. In this way, current measurements of the CMB place interesting constraints on the neutrino energy density and mass, as well as on the abundance of other possible light relativistic particle species. We present the status of an on-going 1500 sq. deg. survey with the SPT-3G receiver, a new mm-wavelength camera on the 10-m diameter South Pole Telescope (SPT). The SPT-3G camera consists of 16,000 superconducting transition edge sensors, a 10x increase over the previous generation camera, which allows it to map the CMB with an unprecedented combination of sensitivity and angular resolution. We highlight projected constraints on the abundance of sterile neutrinos and the sum of the neutrino masses for the SPT-3G survey, which could help determine the neutrino mass hierarchy.

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arXiv2019

Response to NITRD, NCO, NSF Request for Information on "Update to the 2016 National Artificial Intelligence Research and Development Strategic Plan"

We present a response to the 2018 Request for Information (RFI) from the NITRD, NCO, NSF regarding the "Update to the 2016 National Artificial Intelligence Research and Development Strategic Plan." Through this document, we provide a response to the question of whether and how the National Artificial Intelligence Research and Development Strategic Plan (NAIRDSP) should be updated from the perspective of Fermilab, America's premier national laboratory for High Energy Physics (HEP). We believe the NAIRDSP should be extended in light of the rapid pace of development and innovation in the field of Artificial Intelligence (AI) since 2016, and present our recommendations below. AI has profoundly impacted many areas of human life, promising to dramatically reshape society --- e.g., economy, education, science --- in the coming years. We are still early in this process. It is critical to invest now in this technology to ensure it is safe and deployed ethically. Science and society both have a strong need for accuracy, efficiency, transparency, and accountability in algorithms, making investments in scientific AI particularly valuable. Thus far the US has been a leader in AI technologies, and we believe as a national Laboratory it is crucial to help maintain and extend this leadership. Moreover, investments in AI will be important for maintaining US leadership in the physical sciences.

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arXiv2019

A Measurement of the Cosmic Microwave Background Lensing Potential and Power Spectrum from 500 deg$^2$ of SPTpol Temperature and Polarization Data

We present a measurement of the cosmic microwave background (CMB) lensing potential using 500 deg$^2$ of 150 GHz data from the SPTpol receiver on the South Pole Telescope. The lensing potential is reconstructed with signal-to-noise per mode greater than unity at lensing multipoles $L \lesssim 250$, using a quadratic estimator on a combination of CMB temperature and polarization maps. We report measurements of the lensing potential power spectrum in the multipole range of $100< L < 2000$ from sets of temperature-only, polarization-only, and minimum-variance estimators. We measure the lensing amplitude by taking the ratio of the measured spectrum to the expected spectrum from the best-fit $Λ$CDM model to the $\textit{Planck}$ 2015 TT+lowP+lensing dataset. For the minimum-variance estimator, we find $A_{\rm{MV}} = 0.944 \pm 0.058{\rm (Stat.)}\pm0.025{\rm (Sys.)}$; restricting to only polarization data, we find $A_{\rm{POL}} = 0.906 \pm 0.090 {\rm (Stat.)} \pm 0.040 {\rm (Sys.)}$. Considering statistical uncertainties alone, this is the most precise polarization-only lensing amplitude constraint to date (10.1 $σ$), and is more precise than our temperature-only constraint. We perform null tests and consistency checks and find no evidence for significant contamination.

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arXiv2019

Consistency of cosmic microwave background temperature measurements in three frequency bands in the 2500-square-degree SPT-SZ survey

We present an internal consistency test of South Pole Telescope (SPT) measurements of the cosmic microwave background (CMB) temperature anisotropy using three-band data from the SPT-SZ survey. These measurements are made from observations of ~2500 deg^2 of sky in three frequency bands centered at 95, 150, and 220 GHz. We combine the information from these three bands into six semi-independent estimates of the CMB power spectrum (three single-frequency power spectra and three cross-frequency spectra) over the multipole range 650 < l < 3000. We subtract an estimate of foreground power from each power spectrum and evaluate the consistency among the resulting CMB-only spectra. We determine that the six foreground-cleaned power spectra are consistent with the null hypothesis, in which the six cleaned spectra contain only CMB power and noise. A fit of the data to this model results in a chi-squared value of 236.3 for 235 degrees of freedom, and the probability to exceed this chi-squared value is 46%.

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arXiv2019

BICEP2 / Keck Array IX: New Bounds on Anisotropies of CMB Polarization Rotation and Implications for Axion-Like Particles and Primordial Magnetic Fields

We present the strongest constraints to date on anisotropies of cosmic microwave background (CMB) polarization rotation derived from 150 GHz data taken by the BICEP2/Keck Array CMB experiments up to and including the 2014 observing season (BK14). The definition of the polarization angle in BK14 maps has gone through self-calibration in which the overall angle is adjusted to minimize the observed TB and EB power spectra. After this procedure, the QU maps lose sensitivity to a uniform polarization rotation but are still sensitive to anisotropies of polarization rotation. This analysis places constraints on the anisotropies of polarization rotation, which could be generated by CMB photons interacting with axionlike pseudoscalar fields or Faraday rotation induced by primordial magnetic fields. The sensitivity of BK14 maps ($\sim 3μ$K-arcmin) makes it possible to reconstruct anisotropies of the polarization rotation angle and measure their angular power spectrum much more precisely than previous attempts. Our data are found to be consistent with no polarization rotation anisotropies, improving the upper bound on the amplitude of the rotation angle spectrum by roughly an order of magnitude compared to the previous best constraints. Our results lead to an order of magnitude better constraint on the coupling constant of the Chern-Simons electromagnetic term $g_{aγ}\leq 7.2\times 10^{-2}/H_I$ (95% confidence) than the constraint derived from the B-mode spectrum, where $H_I$ is the inflationary Hubble scale. This constraint leads to a limit on the decay constant of $10^{-6}\lesssim f_a/M_{\rm pl}$ at mass range of $10^{-33}< m_a< 10^{-28}$ eV for $r=0.01$, assuming $g_{aγ}\simα/(2πf_a)$ with $α$ denoting the fine structure constant. The upper bound on the amplitude of the primordial magnetic fields is 30nG (95% confidence) from the polarization rotation anisotropies.

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arXiv2018

Maps of the Southern Millimeter-wave Sky from Combined 2500 deg$^2$ SPT-SZ and Planck Temperature Data

We present three maps of the millimeter-wave sky created by combining data from the South Pole Telescope (SPT) and the Planck satellite. We use data from the SPT-SZ survey, a survey of 2540 deg$^2$ of the the sky with arcminute resolution in three bands centered at 95, 150, and 220 GHz, and the full-mission Planck temperature data in the 100, 143, and 217 GHz bands. A linear combination of the SPT-SZ and Planck data is computed in spherical harmonic space, with weights derived from the noise of both instruments. This weighting scheme results in Planck data providing most of the large-angular-scale information in the combined maps, with the smaller-scale information coming from SPT-SZ data. A number of tests have been done on the maps. We find their angular power spectra to agree very well with theoretically predicted spectra and previously published results.

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arXiv2018

Measurements of Degree-Scale B-mode Polarization with the BICEP/Keck Experiments at South Pole

The BICEP and Keck Array experiments are a suite of small-aperture refracting telescopes observing the microwave sky from the South Pole. They target the degree-scale B-mode polarization signal imprinted in the Cosmic Microwave Background (CMB) by primordial gravitational waves. Such a measurement would shed light on the physics of the very early universe. While BICEP2 observed for the first time a B-mode signal at 150 GHz, higher frequencies from the Planck satellite showed that it could be entirely due to the polarized emission from Galactic dust, though uncertainty remained high. Keck Array has been observing the same region of the sky for several years, with an increased detector count, producing the deepest polarized CMB maps to date. New detectors at 95 GHz were installed in 2014, and at 220 GHz in 2015. These observations enable a better constraint of galactic foreground emissions, as presented here. In 2015, BICEP2 was replaced by BICEP3, a 10 times higher throughput telescope observing at 95 GHz, while Keck Array is now focusing on higher frequencies. In the near future, BICEP Array will replace Keck Array, and will allow unprecedented sensitivity to the gravitational wave signal. High resolution observations from the South Pole Telescope (SPT) will also be used to remove the lensing contribution to B-modes.

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arXiv2018

BICEP2 / Keck Array x: Constraints on Primordial Gravitational Waves using Planck, WMAP, and New BICEP2/Keck Observations through the 2015 Season

We present results from an analysis of all data taken by the BICEP2/Keck CMB polarization experiments up to and including the 2015 observing season. This includes the first Keck Array observations at 220 GHz and additional observations at 95 & 150 GHz. The $Q/U$ maps reach depths of 5.2, 2.9 and 26 $μ$K$_{cmb}$ arcmin at 95, 150 and 220 GHz respectively over an effective area of $\approx 400$ square degrees. The 220 GHz maps achieve a signal-to-noise on polarized dust emission approximately equal to that of Planck at 353 GHz. We take auto- and cross-spectra between these maps and publicly available WMAP and Planck maps at frequencies from 23 to 353 GHz. We evaluate the joint likelihood of the spectra versus a multicomponent model of lensed-$Λ$CDM+$r$+dust+synchrotron+noise. The foreground model has seven parameters, and we impose priors on some of these using external information from Planck and WMAP derived from larger regions of sky. The model is shown to be an adequate description of the data at the current noise levels. The likelihood analysis yields the constraint $r_{0.05}<0.07$ at 95% confidence, which tightens to $r_{0.05}<0.06$ in conjunction with Planck temperature measurements and other data. The lensing signal is detected at $8.8 σ$ significance. Running maximum likelihood search on simulations we obtain unbiased results and find that $σ(r)=0.020$. These are the strongest constraints to date on primordial gravitational waves.

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arXiv2018

Dark Energy Survey Year 1 Results: Methodology and Projections for Joint Analysis of Galaxy Clustering, Galaxy Lensing, and CMB Lensing Two-point Functions

Optical imaging surveys measure both the galaxy density and the gravitational lensing-induced shear fields across the sky. Recently, the Dark Energy Survey (DES) collaboration used a joint fit to two-point correlations between these observables to place tight constraints on cosmology (DES Collaboration et al. 2017). In this work, we develop the methodology to extend the DES year one joint probes analysis to include cross-correlations of the optical survey observables with gravitational lensing of the cosmic microwave background (CMB) as measured by the South Pole Telescope (SPT) and Planck. Using simulated analyses, we show how the resulting set of five two-point functions increases the robustness of the cosmological constraints to systematic errors in galaxy lensing shear calibration. Additionally, we show that contamination of the SPT+Planck CMB lensing map by the thermal Sunyaev-Zel'dovich effect is a potentially large source of systematic error for two-point function analyses, but show that it can be reduced to acceptable levels in our analysis by masking clusters of galaxies and imposing angular scale cuts on the two-point functions. The methodology developed here will be applied to the analysis of data from the DES, the SPT, and Planck in a companion work.

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arXiv2018

Year two instrument status of the SPT-3G cosmic microwave background receiver

The South Pole Telescope (SPT) is a millimeter-wavelength telescope designed for high-precision measurements of the cosmic microwave background (CMB). The SPT measures both the temperature and polarization of the CMB with a large aperture, resulting in high resolution maps sensitive to signals across a wide range of angular scales on the sky. With these data, the SPT has the potential to make a broad range of cosmological measurements. These include constraining the effect of massive neutrinos on large-scale structure formation as well as cleaning galactic and cosmological foregrounds from CMB polarization data in future searches for inflationary gravitational waves. The SPT began observing in January 2017 with a new receiver (SPT-3G) containing $\sim$16,000 polarization-sensitive transition-edge sensor bolometers. Several key technology developments have enabled this large-format focal plane, including advances in detectors, readout electronics, and large millimeter-wavelength optics. We discuss the implementation of these technologies in the SPT-3G receiver as well as the challenges they presented. In late 2017 the implementations of all three of these technologies were modified to optimize total performance. Here, we present the current instrument status of the SPT-3G receiver.

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arXiv2018

BICEP Array cryostat and mount design

Bicep Array is a cosmic microwave background (CMB) polarization experiment that will begin observing at the South Pole in early 2019. This experiment replaces the five Bicep2 style receivers that compose the Keck Array with four larger Bicep3 style receivers observing at six frequencies from 30 to 270GHz. The 95GHz and 150GHz receivers will continue to push the already deep Bicep/Keck CMB maps while the 30/40GHz and 220/270GHz receivers will constrain the synchrotron and galactic dust foregrounds respectively. Here we report on the design and performance of the Bicep Array instruments focusing on the mount and cryostat systems.

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arXiv2018

Ultra-Thin Large-Aperture Vacuum Windows for Millimeter Wavelengths Receivers

Targeting faint polarization patterns arising from Primordial Gravitational Waves in the Cosmic Microwave Background requires excellent observational sensitivity. Optical elements in small aperture experiments such as Bicep3 and Keck Array are designed to optimize throughput and minimize losses from transmission, reflection and scattering at millimeter wavelengths. As aperture size increases, cryostat vacuum windows must withstand larger forces from atmospheric pressure and the solution has often led to a thicker window at the expense of larger transmission loss. We have identified a new candidate material for the fabrication of vacuum windows: with a tensile strength two orders of magnitude larger than previously used materials, woven high-modulus polyethylene could allow for dramatically thinner windows, and therefore significantly reduced losses and higher sensitivity. In these proceedings we investigate the suitability of high-modulus polyethylene windows for ground-based CMB experiments, such as current and future receivers in the Bicep/Keck Array program. This includes characterizing their optical transmission as well as their mechanical behavior under atmospheric pressure. We find that such ultra-thin materials are promising candidates to improve the performance of large-aperture instruments at millimeter wavelengths, and outline a plan for further tests ahead of a possible upcoming field deployment of such a science-grade window.

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arXiv2018

BICEP Array: a multi-frequency degree-scale CMB polarimeter

BICEP Array is the newest multi-frequency instrument in the BICEP/Keck Array program. It is comprised of four 550 mm aperture refractive telescopes observing the polarization of the cosmic microwave background (CMB) at 30/40, 95, 150 and 220/270 GHz with over 30,000 detectors. We present an overview of the receiver, detailing the optics, thermal, mechanical, and magnetic shielding design. BICEP Array follows BICEP3's modular focal plane concept, and upgrades to 6" wafer to reduce fabrication with higher detector count per module. The first receiver at 30/40 GHz is expected to start observing at the South Pole during the 2019-20 season. By the end of the planned BICEP Array program, we project $σ(r) \sim 0.003$, assuming current modeling of polarized Galactic foreground and depending on the level of delensing that can be achieved with higher resolution maps from the South Pole Telescope.

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