  The Standard Model of particle physics cannot explain the dominance of matter
over anti-matter in our Universe. In many model extensions this is a very
natural consequence of neutrinos being their own anti-particles (Majorana
particles) which implies that a lepton number violating radioactive decay named
neutrinoless double beta ($0\nu\beta\beta$) decay should exist. The detection
of this extremely rare hypothetical process requires utmost suppression of any
kind of backgrounds.
  The GERDA collaboration searches for $0\nu\beta\beta$ decay of $^{76}$Ge
($^{76}\rm{Ge} \rightarrow\,^{76}\rm{Se} + 2e^-$) by operating bare detectors
made from germanium with enriched $^{76}$Ge fraction in liquid argon. Here, we
report on first data of GERDA Phase II. A background level of $\approx10^{-3}$
cts/(keV$\cdot$kg$\cdot$yr) has been achieved which is the world-best if
weighted by the narrow energy-signal region of germanium detectors. Combining
Phase I and II data we find no signal and deduce a new lower limit for the
half-life of $5.3\cdot10^{25}$ yr at 90 % C.L. Our sensitivity of
$4.0\cdot10^{25}$ yr is competitive with the one of experiments with
significantly larger isotope mass.
  GERDA is the first $0\nu\beta\beta$ experiment that will be background-free
up to its design exposure. This progress relies on a novel active veto system,
the superior germanium detector energy resolution and the improved background
recognition of our new detectors. The unique discovery potential of an
essentially background-free search for $0\nu\beta\beta$ decay motivates a
larger germanium experiment with higher sensitivity.
