Enhanced production of ${\mathrm{\Lambda }}_b^0$ baryons in high-multiplicity $pp$ collisions at $\sqrt{s}=13$ TeV

The production rate of ${\mathrm{\Lambda }}_b^0$ 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 ${\mathrm{\Lambda }}_b^0$ 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 $\sim 2$ with increasing multiplicity. At relatively low transverse momentum, the ratio of ${\mathrm{\Lambda }}_b^0$ 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.