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| Polarbear collaboration | |
|---|---|
| Name | Polarbear collaboration |
| Established | 2005 |
| Members | ~100 (institutions worldwide) |
| Headquarters | Atacama Desert, Chile (telescope site) |
| Instruments | Polarbear, Polarbear-2, Simons Array receivers |
| Research field | Cosmic Microwave Background polarization |
Polarbear collaboration Polarbear collaboration is an international experimental consortium focused on measurements of the polarization of the Cosmic Microwave Background using ground-based millimeter-wavelength telescopes sited in the Atacama Desert and supported by partner institutions and funding agencies across North America, East Asia, and Europe. The collaboration brings together instrument builders, observational cosmologists, and data analysts to probe inflationary cosmology, gravitational lensing, and neutrino physics through high-resolution polarization maps. Polarbear has strong links to complementary efforts including the Atacama Cosmology Telescope, South Pole Telescope, and the Planck mission.
Polarbear collaboration operates a phased program of instruments—Polarbear (first-generation), Polarbear-2, and deployments comprising the Simons Array—to achieve sensitive measurements of E-mode and B-mode polarization of the Cosmic Microwave Background across multiple angular scales. The project emphasizes control of systematic errors, novel cryogenic detector technology such as transition-edge sensors, and multifrequency observations to separate foregrounds like Galactic dust and synchrotron radiation from primordial signals. Its science drivers intersect with constraints on tensor-to-scalar ratio (r), the sum of neutrino masses, and tests of ΛCDM extensions.
The collaboration emerged in the mid-2000s from a convergence of groups at University of California, Berkeley, Lawrence Berkeley National Laboratory, Princeton University, Stanford University, and institutions in Japan and Chile. Early design and site testing drew on expertise from the High Energy Physics Division at CERN-adjacent instrumentation communities and lessons from experiments such as BOOMERanG, BICEP/Keck, and QUaD. Polarbear's first light and initial science campaigns in the late 2000s and early 2010s were enabled by partnerships with the National Science Foundation (United States), Japan Society for the Promotion of Science, and the Simons Foundation, alongside on-site cooperation with the Comisión Nacional de Investigación Científica y Tecnológica of Chile.
Primary goals include detection or upper limits on primordial B-mode polarization from inflationary gravitational waves, precision measurements of lensing B-modes induced by large-scale structure traced by surveys like Dark Energy Survey and Hyper Suprime-Cam, and characterization of polarized foregrounds relevant to LiteBIRD and future CMB-S4 missions. Instrumentation centers on polarization-sensitive focal planes employing arrays of superconducting detectors such as TES bolometers read out with SQUID multiplexing and silicon lenslet coupling. Optics and cryogenic systems were developed with contributions from NIST, Jet Propulsion Laboratory, and university laboratories; receiver bands typically include 95 GHz, 150 GHz, and higher-frequency channels to target dust contamination.
Participating institutions span universities and laboratories: University of California, Berkeley, Lawrence Berkeley National Laboratory, Princeton University, Stanford University, University of Tokyo, KEK, Nobeyama Radio Observatory, Nagoya University, McGill University, University of British Columbia, University of Toronto, Carnegie Mellon University, University of Pennsylvania, Harvard University, Massachusetts Institute of Technology, California Institute of Technology, NASA Jet Propulsion Laboratory, National Institute of Standards and Technology, Kavli Institute for Particle Astrophysics and Cosmology, Simons Foundation, Canadian Institute for Theoretical Astrophysics, Max Planck Institute for Astrophysics, École Normale Supérieure, University of Chicago, Yale University, Columbia University, Rutgers University, University of Minnesota, University of California, San Diego, University of Colorado Boulder, University of California, Santa Barbara, Brown University, Cornell University, Imperial College London, University of Oxford, University of Cambridge, ETH Zurich, University of Geneva, University of Bonn, University of Kyoto, RIKEN, Fermilab, SLAC National Accelerator Laboratory.
Polarbear produced pioneering measurements of small-scale B-mode polarization, providing high-resolution detections of lensing B-modes and constraints on primordial tensor modes reported in leading journals such as Physical Review Letters, Astrophysical Journal, and Journal of Cosmology and Astroparticle Physics. Key publications include cross-correlation analyses with lensing maps from Herschel Space Observatory and galaxy surveys like Sloan Digital Sky Survey and Dark Energy Survey, reports on polarized foreground characterization relevant to Planck results, and method papers on instrument calibration and beam systematics. The collaboration's measurements contributed to joint parameter constraints with Planck and BICEP/Keck teams refining limits on the tensor-to-scalar ratio (r) and the sum of neutrino masses.
Data pipelines integrate time-ordered data cleaning, atmospheric noise mitigation using techniques developed alongside the Atacama Large Millimeter/submillimeter Array, map-making algorithms tailored to polarization such as maximum-likelihood estimators, and component separation routines comparable to those used by Planck and WMAP. Simulations employ end-to-end pipelines with beam, pointing, and detector non-idealities modeled in software frameworks used across collaborations including HEALPix, CAMB, and CLASS. Statistical inference leverages Monte Carlo Markov Chain tools like CosmoMC and likelihood frameworks shared with CMB-S4 planning efforts.
Polarbear collaboration has engaged in public outreach and student training through partnerships with institutions such as University of California, Berkeley, Princeton University, University of Tokyo, and the Simons Foundation, hosting workshops, summer schools, and data challenges used by graduate students and postdoctoral researchers. The project fostered technology transfer of superconducting detector techniques to observatories including Atacama Cosmology Telescope and hardware training that benefits future missions like LiteBIRD and community initiatives such as CMB-S4. The collaboration's public talks and media coverage involved institutional press offices at Lawrence Berkeley National Laboratory and participating universities.