[Paper Review] Charge asymmetry in the photonic production of charmed mesons

A. Berezhnoy, A. K. Likhoded|arXiv (Cornell University)|Jun 13, 2005

Quantum Chromodynamics and Particle Interactions4 citations

TL;DR

This paper proposes that perturbative recombination in charm photoproduction explains the observed charge asymmetry in D* and Ds meson production at COMPASS, where the fragmentation of non-recombinated charm quarks and interference with valence quarks from the initial proton lead to significant asymmetries. The model predicts a large asymmetry of approximately 0.57–0.64 for Ds+ vs. Ds− production, which is robust against fragmentation model uncertainties.

ABSTRACT

Charge asymmetries for the charm meson production ($D^{*+}$--$D^{*-}$, $D^{*0}$--$\bar D^{*0}$ and $D^+_s$--$\bar D^-_s$) have been estimated for the COMPASS kinematic conditions in the framework of perturbative recombination model. Mass corrections have been taken into account in the calculations. The large asymmetry for $D^+_s$--$\bar D^-_s$ production has been predicted.

Motivation & Objective

• To explain the experimentally observed charge asymmetry in charmed meson production, particularly in the photon fragmentation region where pQCD factorization fails.
• To address the discrepancy between perturbative QCD predictions and experimental data showing non-zero asymmetries in D* and Ds meson yields.
• To investigate the role of non-recombinated charm quarks and light quark contributions in violating factorization theorems at low transverse momentum.
• To quantify the impact of valence quark interactions and interference effects on charge asymmetry in Ds meson production, despite the absence of strange quarks in the initial proton.

Proposed method

• Adopts the perturbative recombination model (PRM) to describe hadronization of charm quarks, including light quarks with mass ~300 MeV as an infrared cutoff.
• Introduces a fragmentation function f_{c→D(*)} proportional to αs²⟨O(3S1)⟩/(mq²mc) × I(zD, r), where r = mq/mc, to model non-perturbative effects via heavy quark propagator poles.
• Uses a modified factorization approach that includes interference terms between light quarks from the hadronic remnant and those from c-quark fragmentation.
• Calculates cross sections for γg, γq, and c-quark production processes, and combines them into asymmetry expressions via σ_{γq}^{D*+} and σ_{γq}^{c} terms.
• Evaluates asymmetry using two fragmentation models: one assuming full momentum transfer (case 1) and another using Kartvelishvili fragmentation (case 2).
• Derives asymmetry expressions (eqs. 12 and 13) that include contributions from non-recombinated c-quarks and gluon fusion, with normalization via experimental fragmentation probabilities.

Experimental results

Research questions

• RQ1 Why does the COMPASS experiment observe a significant charge asymmetry in D* and Ds meson production despite pQCD predictions of symmetric spectra?
• RQ2 To what extent can the perturbative recombination model explain the observed asymmetry without relying on factorization theorems?
• RQ3 How does the inclusion of light quark mass and interference effects alter the predicted asymmetry in Ds meson production?
• RQ4 Why is the Ds+ meson asymmetry large and robust, even though the initial proton contains no strange quarks?
• RQ5 How sensitive are the asymmetry predictions to assumptions about the fragmentation function of non-recombinated charm quarks?

Key findings

• The perturbative recombination model predicts a large charge asymmetry in Ds+ vs. Ds− production, with values of A^{D_{s}^{+}} ≈ 0.57 (case 1) and A^{D_{s}^{+}} ≈ 0.64 (case 2), depending on the fragmentation model.
• The asymmetry in D*+ vs. D*- production is small, with A^{D^{*+}} ≈ -0.03 (case 1) and A^{D^{*+}} ≈ -0.17 (case 2), indicating a weak dependence on fragmentation details.
• The Ds+ asymmetry is robust against uncertainties in the fragmentation model of non-recombinated charm quarks, making it a reliable prediction.
• The model explains the asymmetry through the production of an additional c-quark during recombination, which hadronizes into Ds+ mesons, even though the initial proton lacks strange quarks.
• The asymmetry arises due to backward scattering of valence quarks from the initial proton, leading to non-trivial interference and non-factorizable contributions at low PT.
• The model violates standard factorization at low transverse momentum due to power corrections ~mc²/pT⁶ and strong interference, especially at large x.