= Solution
In vacuum the electron-flavour survival <probability> depends only on $|U_{ei}|^2$ and the <mass-squared differences>. Complex conjugation of the <lepton mixing matrix> therefore leaves it unchanged:
$$
\boxed{P(\nu_e\to\nu_e)=P(\bar\nu_e\to\bar\nu_e),\qquad
\frac{\partial P_{ee}}{\partial\delta}=0.}
$$
Thus a reactor disappearance measurement cannot search for the intrinsic <CP violation> phase in the standard three-<neutrino> model. Sensitivity to that phase calls for suitable appearance channels and, usually, comparison of <neutrino> and <antineutrino> beams. Matter can produce differences between propagation of <neutrinos> and <antineutrinos>; these are not by themselves intrinsic <CP violation>, and are small for the proposed short reactor baseline.
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