Production and Hadronization of Heavy Quarks

TL;DR

The paper develops a comprehensive model for the production and hadronization of heavy quarks (charm and bottom) within the Lund string fragmentation framework, highlighting how hadronization depends on the production environment through color flow, beam remnants, and string dynamics. It blends perturbative production mechanisms (pair creation, flavour excitation, gluon splitting) with nonperturbative hadronization treatments (normal fragmentation, cluster decay, cluster collapse) and beam-remnant effects, implemented in a Pythia-based Monte Carlo. The authors demonstrate broad applicability by exploring fixed-target to LHC energies, predicting observable effects such as rapidity shifts (beam remnants drag) and potentially sizable asymmetries at low energies, while showing small asymmetries at central LHC energies. They also discuss high-p⊥ asymmetries, correlations between heavy-quark pairs, and photoproduction, emphasizing that a complete description requires integrating perturbative dynamics with the hadronization environment, with predictions that can be tested by current and future experiments.

Abstract

Heavy long-lived quarks, i.e. charm and bottom, are frequently studied both as tests of QCD and as probes for other physics aspects within and beyond the standard model. The long life-time implies that charm and bottom hadrons are formed and observed. This hadronization process cannot be studied in isolation, but depends on the production environment. Within the framework of the string model, a major effect is the drag from the other end of the string that the c/b quark belongs to. In extreme cases, a small-mass string can collapse to a single hadron, thereby giving a non-universal flavour composition to the produced hadrons. We here develop and present a detailed model for the charm/bottom hadronization process, involving the various aspects of string fragmentation and collapse, and put it in the context of several heavy-flavour production sources. Applications are presented from fixed-target to LHC energies.

Figures (25)

• Figure 1: Examples of heavy-flavour production diagrams. (a,b) Leading order. (c) Pair creation (with gluon emission). (d) Flavour excitation. (e) Gluon splitting. (f) Events classified as gluon splitting but of flavour-excitation character.
• Figure 2: Example of a string configuration in a $\mathrm{p}\overline{\mathrm{p}}$ collision. (a) Graph of the process, with brackets denoting the final colour singlet subsystems. (b) Corresponding momentum space picture, with dashed lines denoting the strings.
• Figure 3: Distribution of $\chi$ variable at 40 GeV for (a) a meson and (b) a baryon. Full curves show the default intermediate option, dashed the even one and dotted the uneven one, corresponding to $\langle \chi \rangle=$ 0.22, 0.50 and .12 for mesons and .13, .33 and .076 for baryons.
• Figure 4: The total (a) charm and (b) bottom cross sections for pp collisions as a function of $E_{\mathrm{CM}}=\sqrt{s}$. The contributions from pair creation, flavour excitation and gluon splitting are shown separately.
• Figure 5: Dependence of the charm cross section on model aspects, for pp collisions as a function of $E_{\mathrm{CM}}=\sqrt{s}$. Shown is the ratio of cross sections: pair creation for $m_{\mathrm{c}}=1.7$ GeV/$m_{\mathrm{c}}=1.3$ GeV, flavour excitation for GRV 94L/CTEQ 5L parton distributions, and gluon splitting for $Q^2_{\mathrm{max}} = M^2_{\mathrm{max}} = Q^2$/$Q^2_{\mathrm{max}} = M^2_{\mathrm{max}} = 4 Q^2$.