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Because the Levi-Civita connection preserves the spacetime volume form, only derivatives of $X$ remain in ${\mathcal{L}}_{X}T$. Lower the raised indices in $T_{ab}{}^{cd}={ϵ}_{ab}{}^{cd}$, contract successively with the supplied products of two Levi-Civita tensors, and separate ${\nabla }_a{X}_b$ into antisymmetric, trace and symmetric trace-free parts. The antisymmetric part cancels automatically, while ${\mathcal{L}}_{X}T=0$ is equivalent to vanishing of the symmetric trace-free part:Thereforeso $\boxed{\alpha =1/2}$. This is the four-dimensional Conformal Killing equation, and $X$ is a Conformal Killing vector field.