Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at $\mathbf{\sqrt{\textit s}}=7$ TeV

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7 TeV
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Prompt D meson and non-prompt $J/\psi$ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton--proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV. The results are reported as a ratio between yields in a given multiplicity interval normalized to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalized to the average value for inelastic collisions (relative charged-particle multiplicity). D0, D+ and D*+ mesons are measured in five pt intervals from 1 GeV/c to 20 GeV/c and for $|y|<0.5$ via their hadronic decays. The D meson relative yield increases with increasing charged-particle multiplicity. For events with multiplicity six times the average multiplicity of inelastic collisions, a yield enhancement of about a factor of 15 relative to the multiplicity integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D0-meson relative yield is also measured as a function of the relative multiplicity at forward pseudo-rapidity. The non-prompt $J/\psi$, i.e. the B hadron, contribution to the inclusive $J/\psi$ production is measured in the di-electron decay channel at central rapidity. It is evaluated for pt>1.3 GeV/c and $|y|<0.9$, and extrapolated to pt>0. The fraction of non-prompt $J/\psi$ in the inclusive $J/\psi$ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA~8, EPOS~3 and percolation calculations.