  Evanescent operators are a special class of operators that vanish classically
in four-dimensional spacetime, while in general dimensions they are non-zero
and are expected to have non-trivial physical effects at the quantum loop level
in dimensional regularization. In this paper we initiate the study of
evanescent operators in pure Yang-Mills theory. We develop a systematic method
for classifying and constructing the $d$-dimensional Lorentz invariant
evanescent operators, which start to appear at mass dimension ten. We also
compute one-loop form factors for the dimension-ten operators via the
$d$-dimensional unitarity method and obtain their one-loop anomalous
dimensions. These operators are necessary ingredients in the study of high
dimensional operators in effective field theories involving a Yang-Mills
sector.
