Heavy flavor production in p–p collisions as a function of multiplicity using different modes of Pythia 8

The production of mesons containing charm quarks in p–p collisions at s=7 TeV has been explored as a function of multiplicity using Pythia 8 event generator. The effects of various processes at the partonic level such as different modes of color reconnection and rope hadronization have been studied. The comparison of relative yields (Yield/⟨Yield⟩) of D-mesons and J/ψ as a function of charged-particle multiplicity for various transverse momentum (p T) ranges, measured by the ALICE experiment with the estimations of the Pythia 8 model is performed. The results are in reasonable agreement within the framework of microscopic processes. The relative yields of B-mesons have also been predicted for various pT intervals (1


Introduction
The occurrence of hard and semi-hard partonic scattering in a single collision due to multipartonic interactions (MPIs), is closely associated with the charged-particle multiplicity.Heavy flavors are predominantly produced via hard scattering processes.This allows to study the particle production as a function of event activity at the partonic level and can shed light on the interplay between hard and soft mechanisms.
Recent studies [1] reveal the successful description of strangeness enhancement in p−p collisions with the microscopic model of color reconnection along with the rope hadronization mechanism implemented in Pythia 8 [2][3].This does not incorporate the traditional assumption of a de-confined and thermalized plasma state.The aim of this study is, to investigate the effect of overlapping colored strings that form colored ropes on the heavy flavour production.These colored ropes have higher effective string tension and eventually hadronize into particles with higher mass in the high-multiplicity regime.The D-meson, B-meson, and J/ψ relative yields are studied as a function of charged-particle multiplicity with and without rope hadronization mechanism for three different modes of color reconnection mechanism [6], in p−p collisions at √ s = 7 TeV.The relative yield of D 0 as a function of relative charged-particle multiplicity is obtained in the p T ranges of 2 < p T < 4 GeV/c for different modes of color reconnection.The yields of D-mesons (D 0 , D + , D *+ ), B-mesons (B 0 , B + ) and their charge conjugates are obtained within |y| < 0.5 (mid-rapidity region) for five p T intervals, from 1 GeV/c to 20 GeV/c.To avoid auto-correlation, the charged-particle multiplicity class definition is obtained within ALICE V0 detector acceptance ( −3.7 < y < −1.7 and 2.8 < y < 5.1).Afterward, the charged-particle multiplicity was obtained for each multiplicity class in |η| < 0.5.

Results and discussion
The relative yield of D 0 as a function of relative charged-particle multiplicity in p T interval 2 < p T < 4 GeV/c for the various modes of color reconnection is compared to the measured yields by ALICE [5].This is shown in Fig. 1 (Left Panel).The formation of color ropes, which is more pronounced in high multiplicity pp collisions, together with the color reconnection mechanism describes the measured data qualitatively for all multiplicity classes.The D 0 relative yield increases non-linearly with increasing multiplicity.The non-linear rise at higher multiplicity could be due to an increased production of heavy flavor particles from hadronizing ropes.
The inclusive J/ψ relative yield is obtained as a function of relative charged-particle multiplicity for p−p collisions at √ s = 7 TeV in Pythia 8.The comparison of obtained results with the measured ALICE data is shown in Fig. 1 (Right Panel).The formation of color ropes along with the color reconnection mechanism in Pythia 8 qualitatively describes the increase in the relative yield of inclusive J/ψ measured in data.This could suggest that J/ψ mesons production is not affected significantly by underlying event mechanisms.
The relative yields of D-mesons (avg.D 0 , D ± , D *± ) and B-mesons (B 0 , B + ) are shown as a function of the relative charged-particle multiplicity in p−p collisions at √ s = 7 TeV for five p T intervals in Fig. 2 left and right, respectively.The different combinations of microscopic processes show a stronger than linear increasing trend with increasing relative charged-particle multiplicity.Implementation of rope hadronization exhibits a faster increase at high-multiplicity

Summary
The multiplicity-dependent study of the open charm (D-mesons), open beauty (B-mesons), and hidden charm (J/ψ) meson production in p−p collisions at √ s = 7 TeV has been investigated using the different microscopic mechanisms of Pythia 8 model like color reconnection mechanism and rope hadronization.A comparison of these estimations has been done with recent measurements of the ALICE experiment at the LHC.The overlap of many strings resulting in the formation of ropes in high multiplicity events together with the partonic color reconnection mechanism brings forth a reasonable quantitative and good qualitative description of the multiplicity dependence of the relative yields of D-mesons and J/ψ in p−p collisions at √ s = 7 TeV measured by the ALICE experiment.A non-linear increasing trend of relative yields of D-mesons and B-mesons with an increase in relative charged-particle multiplicity is observed for all p T intervals.This observation is influenced by the production processes of the cc and b b.This study can act as an interesting baseline for the upcoming measurements at the LHC in heavy flavor sector.

Figure 1 .
Figure 1.Left panel: Relative D 0 yield as a function of relative charged-particle multiplicity in p−p collisions at √ s = 7 TeV for the D 0 p T range 2 < p T < 4 GeV/c.The measured ALICE data, denoted as closed data points, are normalized to the inelastic cross-section.The open data points are not corrected for the trigger selection efficiency and are normalized to the visible cross-section.Right panel: The relative yield of inclusive J/ψ as a function of relative charged-particle multiplicity in p−p collisions at √ s = 7 TeV for p T > 0 GeV/c.

Figure 2 .
Figure 2. Left panel: Relative D-meson (avg. of D 0 , D ± , D *± ) yield as a function of relative charged-particle multiplicity in p−p collisions at √ s = 7 TeV for different p T ranges.Right panel: Relative B-meson (avg. of B 0 , B + and their charge conjugate) yield as a function of relative charged-particle multiplicity in p−p collisions at √ s = 7 TeV for different p T intervals.