Enhanced Production of ${\mathrm{\Lambda }}_b^0$ Baryons in High-Multiplicity $pp$ Collisions at $\sqrt{s}=13\mathrm{TeV}$

The production rate of ${\mathrm{\Lambda }}_b^0$ baryons relative to $B^0$ mesons in $pp$ collisions at a center-of-mass energy $\sqrt{s}=13\mathrm{TeV}$ 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.

Figure 1

Invariant mass spectra in the transverse momentum interval $10<p_{\mathrm{T}}<12\mathrm{GeV}/c$ for the (a) $J/\psi p{K}^{-}$ and (b) $J/\psi {\pi }^{-}{K}^{+}$ final states, integrated over multiplicity. Fit projections including signal and background components are shown.

Figure 2

Ratio of ${\mathrm{\Lambda }}_b^0$ to $B^0$ production cross-sections as a function of $p_{\mathrm{T}}$ (blue circles). The error bars represent the quadrature sum of the statistical and systematic uncertainties and the boxes depict the global uncertainties due to branching fractions. The data are compared to previous $pp$ measurements using semileptonic decays (gray circles) [31], and data from $p\mathrm{Pb}$ collisions (squares) [57]. Calculations from a statistical hadronization model (SHM) [58], and the pythia8 [42] and EPOS4HQ [59] event generators are also shown.

Figure 3

Ratio of ${\mathrm{\Lambda }}_b^0$ to $B^0$ cross-sections as a function of the total track multiplicity measured in the VELO detector (blue). The purple point indicates the value measured in $e^{+}{e}^{-}$ collisions at LEP [61].

Figure 4

Ratio of ${\mathrm{\Lambda }}_b^0$ to $B^0$ cross sections as a function of $p_{\mathrm{T}}$, in bins of (a) the total multiplicity measured in the VELO detector and (b) the backwards track multiplicity. The purple point shows the value measured in $e^{+}{e}^{-}\to Z^0\to b\overline{b}$ reactions at LEP [61].