= Solution
In a proton-proton collision, associate the reconstructed neutral <B meson> with the accompanying opposite-side bottom <hadron>. A direct semileptonic decay $b\to c\ell^-\bar\nu$ tags a $b$ on that side and hence an initial $\bar b$, or $B^0=d\bar b$, on the signal side. The opposite <charged lepton> tags the conjugate assignment. Charged <kaons> from the opposite-side decay chain and same-side fragmentation tracks can supply additional statistical <flavour tagging> information; the common CP final state alone does not reveal the production <quark flavor>[flavour].
A magnetic spectrometer with precise silicon vertex tracking, charged-particle identification and <muon> or <electron> identification is suitable. A forward geometry can exploit the boosted bottom-hadron pairs at the <LHC>. It reconstructs displaced decay vertices, measures the signed tagging tracks, and infers the signal proper time from $t\simeq Lm_B/|\mathbf p_B|$. The supplied lifetime corresponds to a rest-frame mean flight length $c\tau\simeq0.45\ \mathrm{mm}$, enlarged by the <Lorentz boost>[boost].
Even a perfect <particle detector>[detector] cannot remove physical mistags. An opposite-side neutral bottom meson can oscillate before its semileptonic decay, reversing the <lepton> charge relative to its production <quark flavor>[flavour]. A cascade $b\to c\to\ell^+$ produces the charge opposite to a direct $b\to\ell^-$ decay. Additional bottom pairs or unrelated fragmentation <particles> can also make the chosen association incorrect. For mistag probability $w$, the measured asymmetry is diluted by $D=1-2w$; this is why the tagging calibration is part of a CP measurement.
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