[Paper Review] Charm-quark fragmentation fractions and production cross section at midrapidity in pp collisions at the LHC

Afd Subatomic Physics (SAP), S. Acharya+3 more|arXiv (Cornell University)|Jan 1, 2022

High-Energy Particle Collisions ResearchPhysics and Astronomy70 references77 citations

TL;DR

This paper presents the first measurement of charm-quark fragmentation fractions and the charm production cross section per unit of rapidity at midrapidity (|y| < 0.5) in pp collisions at √s = 5.02 TeV using ALICE data. It reports a 5σ significance for a 3.3-fold increase in the Λ⁺c fragmentation fraction compared to e⁺e⁻ collisions, challenging the universality of fragmentation functions across collision systems.

ABSTRACT

Recent pT-integrated cross-section measurements of the ground-state charm mesons and baryons, D0, D+, Ds+, Λc+, and Ξc0, are used to evaluate the charm fragmentation fractions and production cross section per unit of rapidity at midrapidity (|y|&lt;0.5), in pp collisions at √s=5.02 TeV at the LHC. The latter is dσcc¯/dy||y|&lt;0.5=1165±44(stat)-101+134(syst) μb. These measurements were obtained for the first time in hadronic collisions at the LHC, including the charm baryon states, recently measured by ALICE at midrapidity. The charm fragmentation fractions differ significantly from the values measured in e+e- and ep collisions, providing evidence of the dependence of the parton-to-hadron fragmentation fractions on the collision system, indicating that the assumption of their universality is not supported by the measured cross sections. An increase of a factor of about 3.3 for the fragmentation fraction for the Λc+ with a significance of 5σ between the values obtained in pp collisions and those obtained in e+e- (ep) collisions is reported. The fragmentation fraction for the Ξc0 was obtained for the first time in any collision system. The measured fragmentation fractions were used to update the cc¯ cross sections per unit of rapidity at |y|&lt;0.5 at √s=2.76 and 7 TeV, which are about 40% higher than the previously published results. The data were compared with perturbative-QCD calculations and lie at the upper edge of the theoretical bands.

Motivation & Objective

• To measure charm-quark fragmentation fractions and the production cross section per unit of rapidity at midrapidity in pp collisions at √s = 5.02 TeV.
• To test the universality of charm-quark fragmentation functions across different collision systems (pp vs. e⁺e⁻ and ep).
• To provide updated cross sections for cc production at √s = 2.76 and 7 TeV using new data on charm baryons.
• To constrain perturbative QCD calculations and improve modeling of charm hadroproduction in heavy-ion collisions.
• To support the study of charmonium regeneration in the quark–gluon plasma via accurate baseline cross sections.

Proposed method

• Measured pT-integrated cross sections for D⁰, D⁺, Dₛ⁺, Λ⁺c, and Ξ₀c baryons using reconstructed decays in ALICE detector data from pp collisions at √s = 5.02 TeV.
• Used a minimum-bias trigger with integrated luminosity of 19.3 ± 0.4 nb⁻¹ collected during the 2017 LHC run.
• Applied corrections for tracking efficiency, particle identification, reconstruction efficiency, and pT-extrapolation to reconstruct full cross sections.
• Propagated systematic uncertainties including luminosity, branching ratios, and feed-down contributions from beauty hadrons.
• Computed fragmentation fractions as ratios of hadron cross sections to the D⁰ cross section, with correlated uncertainties.
• Compared results with pQCD calculations (e.g., FONLL, PYTHIA 8) and statistical models (SHM) to assess theoretical consistency.

Experimental results

Research questions

• RQ1 Are charm-quark fragmentation fractions universal across different collision systems such as pp, e⁺e⁻, and ep?
• RQ2 What is the production cross section per unit of rapidity for charm quarks at midrapidity (|y| < 0.5) in pp collisions at √s = 5.02 TeV?
• RQ3 How do the fragmentation fractions for charm baryons (Λ⁺c and Ξ₀c) compare to those measured in e⁺e⁻ collisions?
• RQ4 To what extent do current pQCD calculations describe the measured cross sections, especially with the inclusion of charm baryons?
• RQ5 How do the new measurements affect the previously published cc cross sections at √s = 2.76 and 7 TeV?

Key findings

• The charm production cross section per unit of rapidity at midrapidity (|y| < 0.5) in pp collisions at √s = 5.02 TeV is measured as dσcc/dy||y|<0.5 = 1165 ± 44 (stat) +134⁄−101 (syst) µb.
• The fragmentation fraction for Λ⁺c in pp collisions is found to be approximately 3.3 times higher than in e⁺e⁻ or ep collisions, with a significance of 5σ.
• This is the first measurement of the Ξ₀c fragmentation fraction in any collision system, with a value of 0.022 ± 0.005 (stat) ± 0.004 (syst).
• The measured cross sections for charm baryons lead to a 40% increase in the updated cc cross sections at √s = 2.76 and 7 TeV compared to previous results.
• The data lie at the upper edge of theoretical pQCD bands, indicating that current calculations may underestimate the cross sections.
• The results challenge the assumption of universal fragmentation functions, indicating a strong dependence on the collision system.