Search for Electronic Recoil Event Rate Modulation with 4 Years of
  XENON100 Data
  We report on a search for electronic recoil event rate modulation signatures
in the XENON100 data accumulated over a period of 4 years, from January 2010 to
January 2014. A profile likelihood method, which incorporates the stability of
the XENON100 detector and the known electronic recoil background model, is used
to quantify the significance of periodicity in the time distribution of events.
There is a weak modulation signature at a period of $431^{+16}_{-14}$ days in
the low energy region of $(2.0-5.8)$ keV in the single scatter event sample,
with a global significance of $1.9\,\sigma$, however no other more significant
modulation is observed. The expected annual modulation of a dark matter signal
is not compatible with this result. Single scatter events in the low energy
region are thus used to exclude the DAMA/LIBRA annual modulation as being due
to dark matter electron interactions via axial vector coupling at
$5.7\,\sigma$.
