We present a method to measure time-dependent $CP$ violation in $B^0$ decays produced in $\mathrm{\Upsilon }(4S)\to B^0{\overline{B}}^0$ events at $B$ factories without reconstructing the signal decay vertex. The method exploits the sensitivity of the tag ${\overline{B}}^0$ decay time to $CP$ violation in the $B^0$ signal. It relies on a compact $e^{+}{e}^{-}$ interaction region and excellent vertex resolution, which enable precise measurement of the displacement between production and decay vertices of the tag meson. We study an application to $B^0\to {\pi }^0{\pi }^0$ decays using a simplified simulation that approximates the conditions of the Belle II experiment at the SuperKEKB collider. Our approach achieves a sensitivity on mixing-induced $CP$ violation that would require 20 times larger samples with standard approaches. When incorporated into the $B\to \pi \pi$ isospin analysis, the expected results would reduce the degeneracy of ${\varphi }_2$ solutions and significantly increase precision. This work opens a path to previously inaccessible $CP$-violation studies enhancing and accelerating the reach of Belle II and future flavor physics programs.

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• Cabibbo–Kobayashi–Maskawa matrix
• Particle mixing & oscillations
• Bottom mesons