  The production rate of $\Lambda_{b}^{0}$ baryons relative to $B^{0}$ mesons
in $pp$ collisions at a center-of-mass energy $\sqrt{s} = 13$ TeV is measured
by the LHCb experiment. The ratio of $\Lambda_{b}^{0}$ to $B^{0}$ production
cross-sections shows a significant dependence on both the transverse momentum
and the measured charged-particle multiplicity. At low multiplicity, the ratio
measured at LHCb is consistent with the value measured in $e^{+}e^{-}$
collisions, and increases by a factor of $\sim2$ with increasing multiplicity.
At relatively low transverse momentum, the ratio of $\Lambda_{b}^{0}$ to
$B^{0}$ cross-sections is higher than what is measured in $e^{+}e^{-}$
collisions, but converges with the $e^{+}e^{-}$ ratio as the momentum
increases. These results imply that the evolution of heavy $b$ quarks into
final-state hadrons is influenced by the density of the hadronic environment
produced in the collision. Comparisons with several models and implications for
the mechanisms enforcing quark confinement are discussed.
