= BABAR time-dependent CP violation experiment
{c}
BABAR used asymmetric <Electron>-<Positron> collisions near the <Upsilon meson> $4S$ resonance to prepare neutral <B meson> pairs. It reconstructed one decay into <charmonium> plus a <kaon>, and used <flavour tagging> on the other <B meson> with charged <leptons> or <kaons>. <Silicon> tracking reconstructed <decay vertices> for the decay-time difference. Self-tagging decays calibrated mistags and time resolution; charged decays provided a control. The tagged time distributions established <CP violation>, limited by backgrounds, mistags and vertex alignment.<Observation of CP violation in the B0 meson system by BABAR Collaboration>[PDF pp. 4–6, Eq. (1), controls, tagging, vertices and systematics]{ref}
The <Lorentz boost> converts vertex separation to $\Delta t\simeq\Delta z/(\beta_{\rm boost}\gamma_{\rm boost}c)$ at leading order. Unequal beam energies thus make a time difference measurable as a distance.<Nobel Lecture: CP violation and flavor mixing by Makoto Kobayashi>[p. 1023, Section IV, Fig. 5]{ref}
\Image[/b-factory-flavour-tag-time-asymmetry.png]
{title=From flavour tag and decay vertices to a toy CP asymmetry}
{description=Functional <B meson> readout above, an original illustrative <time-dependent neutral-B CP asymmetry> below. The curves are a teaching model, not measured events or a detector drawing.}
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