Search for annual modulation of the event rate generated by dark matter in the DarkSide-50 ionization signal

DarkSide-50 is a direct detection experiment at LNGS in Italy searching for dark matter. It utilizes a dual-phase argon time projection chamber. On the basis of the ionization spectrum alone, it has established the most stringent exclusion limit for dark matter candidates with masses below 1.4 GeV/c(2). By exploiting the expected variation of the relative velocity between dark matter and Earth, one can search for dark matter in a model-independent manner. We describe the first search for such an event rate modulation with argon using the DarkSide-50 ionization signal. Likelihood and Lomb-Scargle analyses were used to look for a 1 year period peaking at June 2nd. As a result of years of stable operation of the detector and a thorough knowledge of its response, we were able to obtain the lowest energy threshold, 0.04 keV(ee), ever achieved in this kind of experiments.