  It is a long-standing debate as to whether or not the annual modulation in
the event rate observed by the DAMA sodium iodide experiment is caused by the
interaction of dark matter particles. To resolve this issue, several groups
have been working to develop new experiments with the aim of reproducing or
refuting DAMA's results using the same sodium iodide target medium. The
COSINE-100 experiment is one of these that is currently operating with 106 kg
of low-background sodium iodide crystals at the Yangyang underground
laboratory. Analysis of the initial 59.5 days of COSINE-100 data showed that
the annual modulation signal reported by DAMA is inconsistent with explanation
using spin-independent interaction of weakly interacting massive particles
(WIMPs), a favored candidate of dark matter particles, with sodium or iodine
nuclei in the context of the standard halo model. However, this first result
left open interpretations using certain alternative dark matter models, dark
matter halo distributions, and detector responses that could allow room for
consistency between DAMA and COSINE-100. Here we present new results from over
1.7 years of COSINE-100 operation with improved event selection and energy
threshold reduced from 2 keV to 1 keV. We find an order of magnitude
improvement in sensitivity, sufficient for the first time to strongly constrain
these alternative scenarios, as well as to further strengthen the previously
observed inconsistency with the WIMP-nucleon spin-independent interaction
hypothesis.
