  The ratio of the cross section for $p\bar{p}$ interactions producing a $Z$
boson and at least one $b$ quark jet to the inclusive $Z+{\rm jet}$ cross
section is measured using $4.2\ {\rm fb}^{-1}$ of $p\bar{p}$ collisions
collected with the \dzero\ detector at the Fermilab Tevatron collider at
$\sqrt{s}=1.96$ TeV. The $Z\rightarrow\ell^+\ell^-$ candidate events with at
least one $b$ jet are discriminated from $Z+$ charm and light jet(s) events by
a novel technique that exploits the properties of the tracks associated to the
jet. The measured ratio is $0.0193\pm0.0027$ for events having a jet with
transverse momentum $\pt > 20 \GeV$ and pseudorapidity $|\eta| \leq 2.5$, which
is the most precise to date and is consistent with theoretical predictions.
