Understanding instrumental systematics for the CMB-S4 ultra-deep survey

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The CMB Stage-4 experiment is a next-generation array of telescopes that will measure the Cosmic Microwave Background (CMB) to push the boundaries of our knowledge of fundamental physics and astrophysics. One component of CMB Stage-4 is the ultra-deep survey, which will measure the polarization of the CMB at degree angular scales with unprecendented sensitivity. Polarization maps from the ultra-deep survey will be a powerful tool to search for primordial gravitational waves, so long as the maps are free of contamination due to instrumental systematic errors. This award will support development and analysis of detailed simulations targeting systematics for the ultra-deep survey, using lessons learned from the current generation of CMB telescopes. The simulations will inform the design of CMB-S4 and provide a testing ground for development of mitigation techniques. This program will also support a scientific computing bootcamp for incoming graduate students in the University of Cincinnati physics bridge program, making sure that these students have necessary tools for success in graduate school and their future careers.Primordial gravitational waves are a prediction of inflationary theories, which are the leading explanation for the initial conditions of our universe. The most powerful technique to search for these gravitational waves is using the B-mode (curl-type) polarization of the CMB, which is not sourced by density perturbations. However, primordial B-mode polarization must be very faint, so a critical challenge is to control all effects that might leak temperature or E-mode fluctuations into B-modes. It is also necessary to separate primordial CMB signals from lensing-induced secondary anisotropies and Galactic foreground emission; these challenges require combining data across angular scales and observing frequencies at high fidelity. Past and current CMB experiments have demonstrated good control over systematic errors, but the raw sensitivity of CMB-S4 requires correspondingly lower levels of systematics. The CMB-S4 forecasting effort has provided guidelines for the acceptable level of spurious contamination in the maps, but detailed simulations are necessary to connect these limits to specifications on instrument design and calibration. The work supported by this award will establish these connections through a series of targeted simulation sets. The process of generating and analyzing these simulations will also contribute to the development of CMB-S4 data pipelines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.