Mapmaking, Null Tests, and Systematics with SPIDER-2 Data

Abstract

The goal of many modern cosmic microwave background (CMB) experiments is to probe the physics of the early universe. In particular, many experiments aim to measure or constrain the tensor-to-scalar ratio, r, whose detection would provide strong evidence for primordial gravitational waves generated during inflation. SPIDER is a balloon-borne CMB experiment designed to search for this signal through observations of large-scale polarization anisotropies. An important stage of the analysis is the use of null tests to assess the self consistency of the data and diagnose potential systematic contamination. In this thesis, a preliminary version of the SPIDER-2 null-test pipeline is implemented and applied to three data splits: a left/right scan split, an inner/outer focal-plane radius split, and a checkerboard split. The X1 150 GHz data pass all null tests. The 90 GHz FPUs, X2 and X6, also pass all null tests, with the exception of the X6 inner/outer focal-plane split, which exhibits a statistically significant failure driven by excess power at low multipoles. Map-level analysis reveals a periodic systematic feature in the X6 data, particularly in the Q polarization maps. While the physical origin of this feature remains unclear, it is localized to a subset of detectors and to specific time intervals, providing a clear direction for future timestream-level investigation. The 280 GHz FPUs (Y3–Y5) pass the left/right null test but fail both the inner/outer and checkerboard splits. These failures are also driven by excess low multipole power in the null spectra. In contrast to the X6 inner/outer radius split failure, no clear corresponding features are identified in map space, suggesting a more diffuse or complex systematic origin. These results demonstrate the importance of null tests as a diagnostic tool in CMB data analysis and provide guidance for a continued investigation of systematic effects in SPIDER-2.