  We report the first results of DarkSide-50, a direct search for dark matter
operating in the underground Laboratori Nazionali del Gran Sasso (LNGS) and
searching for the rare nuclear recoils possibly induced by weakly interacting
massive particles (WIMPs). The dark matter detector is a Liquid Argon Time
Projection Chamber with a (46.4+-0.7) kg active mass, operated inside a 30 t
organic liquid scintillator neutron veto, which is in turn installed at the
center of a 1 kt water Cherenkov veto for the residual flux of cosmic rays. We
report here the null results of a dark matter search for a (1422+-67) kg d
exposure with an atmospheric argon fill. This is the most sensitive dark matter
search performed with an argon target, corresponding to a 90% CL upper limit on
the WIMP-nucleon spin-independent cross section of 6.1x10^-44 cm^2 for a WIMP
mass of 100 GeV/c^2.
