On the Observed Diversity of Star Formation Efficiencies in Giant
  Molecular Clouds
  Observations find a median star formation efficiency per free-fall time in
Milky Way Giant Molecular Clouds (GMCs) on the order of $\epsilon_{\rm ff}\sim
1\%$ with dispersions of $\sim0.5\,{\rm dex}$. The origin of this scatter in
$\epsilon_{\rm ff}$ is still debated and difficult to reproduce with analytical
models. We track the formation, evolution and destruction of GMCs in a
hydrodynamical simulation of a Milky Way-like galaxy and by deriving cloud
properties in an observationally motivated way, measure the distribution of
star formation efficiencies which are in excellent agreement with observations.
We find no significant link between $\epsilon_{\rm ff}$ and any measured global
property of GMCs (e.g. gas mass, velocity dispersion). Instead, a wide range of
efficiencies exist in the entire parameter space. From the cloud evolutionary
tracks, we find that each cloud follow a \emph{unique} evolutionary path which
gives rise to wide diversity in all properties. We argue that it is this
diversity in cloud properties, above all else, that results in the dispersion
of $\epsilon_{\rm ff}$.
