  The recent discovery of a Weyl semimetal in TaAs offers the first Weyl
fermion observed in nature and dramatically broadens the classification of
topological phases. However, in TaAs it has proven challenging to study the
rich transport phenomena arising from emergent Weyl fermions. The series
Mo$_x$W$_{1-x}$Te$_2$ are inversion-breaking, layered, tunable semimetals
already under study as a promising platform for new electronics and recently
proposed to host Type II, or strongly Lorentz-violating, Weyl fermions. Here we
report the discovery of a Weyl semimetal in Mo$_x$W$_{1-x}$Te$_2$ at $x =
25\%$. We use pump-probe angle-resolved photoemission spectroscopy (pump-probe
ARPES) to directly observe a topological Fermi arc above the Fermi level,
demonstrating a Weyl semimetal. The excellent agreement with calculation
suggests that Mo$_x$W$_{1-x}$Te$_2$ is the first Type II Weyl semimetal. We
also find that certain Weyl points are at the Fermi level, making
Mo$_x$W$_{1-x}$Te$_2$ a promising platform for transport and optics experiments
on Weyl semimetals.
