Searches for Dark Matter Annual Modulation With COSINE-100

The COSINE-100 dark matter detection experiment is an NaI-based dark matter detection
experiment with the primary goal of providing a model-independent test of the
long-standing but controversial result of a dark matter-induced annual modulation signal,
reported by the DAMA collaboration. As a model-independent test of the DAMA result,
the target material and search method at COSINE-100 is the same as that in the DAMA
experiments, that is searching for an annual modulation signal in the rate of signal-like
events in NaI crystals. Results of searches with the first 2.8 years of COSINE-100 data
have already been published, but with an insufficiently large exposure to conclusively rule
on the DAMA result.
Presented in this thesis are the results of a new search for a dark matter-induced
annual modulation signal at COSINE-100, using the first 4.7 years of data collected. This
is the first time these data have been analysed. An improved analysis procedure is used
in this search and this procedure is also described. These improvements from the method
used in previous searches include an updated method of accounting for detector deadtime,
folding this parameter into the fit model, and also from a updated background
model in light of issues found with the treatment of surface 210Pb in previous analyses.
Using this new procedure, best-fit annual modulation amplitudes of 0.0040 ± 0.0034 dru
(0.0047±0.0041 dru) in the 1-6 keV (2-6 keV) energy regions in a fixed-phase analysis are
found. These values are smaller than those reported in previous COSINE-100 analyses, but
still statistically consistent with both the DAMA and no-modulation hypotheses. Similar
results are found in the floated-phase case. Also presented are improvements in the process
of validating the analysis procedure using simulated data, as well as a description of the
long-term monitoring and resulting corrections of the liquid scintillator veto at COSINE-
100, and an investigation into an intermittent type of PMT noise in the detector.