Abstract
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 normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D$^0$, D$^+$ and D$^{*+}$ mesons are measured in five $p_{\rm T}$ intervals from 1 to 20 GeV/$c$ and for $|y|<0.5$ via their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 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 D$^0$-meson relative yield is also measured as a function of the relative multiplicity at forward pseudorapidity. 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 $p_{\rm T}>1.3$ GeV/$c$ and $|y|<0.9$, and extrapolated to $p_{\rm T}>0$. 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.
Highlights
FB is the fraction of non-prompt J/ψ in each multiplicity interval, fB is the fraction in the multiplicity integrated sample [10], and dNJ/ψ/dy is the inclusive J/ψ relative yield measured for pT > 0 in each multiplicity interval normalized to its value in inelastic pp collisions [40]
In the first charged-particle multiplicity class Ntracklets ∈ [4, 8], which is used for the non-prompt J/ψ analysis presented here, the relative yield of inclusive J/ψ normalized to the inelastic cross section is dNJ/ψ/dy = 0.41±0.07±0.01
Charged-particle multiplicity at central rapidity was evaluated for events with at least a charged particle in the interval |η| < 1.0
Summary
The ALICE apparatus [51] consists of a central barrel detector covering the pseudo-rapidity interval |η| < 0.9, a forward muon spectrometer covering the pseudo-rapidity interval −4.0 < η < −2.5, and a set of detectors at forward and backward rapidities used for triggering and event characterization. The threshold was configured to select the ≈ 0.7% of the events with highest number of hits in the outer SPD layer This HM-trigger sample (Lint 14 nb−1) provides an increase of statistics by a factor of about 2.8 relative to the MB trigger for events with more than 50 tracklets, corresponding to about six times the average multiplicity. In the considered data samples, the instantaneous luminosity was limited to 0.6–1.2 × 1029 cm−2s−1 by displacing the beams in the transverse plane by 3.8 times the RMS of their transverse profile In this way, the interaction probability per bunch crossing was kept in the range 0.04–0.08, with a probability of collision pile-up below 4% per triggered event. The fraction of events with biased number of tracklets increases with increasing multiplicity and it was estimated to be below 2% at the highest multiplicities considered in this analysis, while the resulting bias on the measured number of tracklets was found to be negligible in all the multiplicity classes
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