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Single-parton scattering versus double-parton scattering in the production of two $c \bar c$ pairs and charmed meson correlations at the LHC

Andreas van Hameren, Rafal Maciula, Antoni Szczurek

TL;DR

The paper addresses whether double charm production at LHC energies is driven by double-parton scattering (DPS) or single-parton scattering (SPS). It computes SPS with exact matrix elements for $gg\to c\bar c c\bar c$ and $q\bar q\to c\bar c c\bar c$ and models DPS via a $k_t$-factorization framework using a factorized double-parton distribution with $\sigma_{eff}=15\text{ mb}$, comparing to high-energy approximations and LHCb data. The main findings are that the $gg$-initiated SPS contribution is about a factor of $50$ larger than the $q\bar q$ contribution, but DPS overwhelms SPS in the total cross section at $\sqrt{s}=7$ TeV; differences between exact and approximate SPS appear mainly at small invariant masses, where gluon splitting is relevant. Hadron-level predictions for $D$ mesons qualitatively reproduce LHCb distributions in relative rapidities and invariant masses, with DPS providing the correct trend but some normalization deficiency that may require incorporating additional mechanisms such as $3\to4$ processes. Overall, the work confirms DPS as the dominant mechanism in double-charm production at LHC energies and provides a quantitative framework for comparing to experimental data.

Abstract

We compare results of exact calculations of single parton scattering (SPS) and double parton scattering (DPS) for production of $c \bar c c \bar c$ and for $D$ meson correlations. The SPS calculations are performed in collinear approximation with exact matrix element for $g g \to c \bar c c \bar c$ and $q \bar q \to c \bar c c \bar c$ subprocesses. It is shown that the contribution of gluon-gluon subprocess is about factor 50 larger than that for quark-antiquark annihilation. The new results are compared with results of previous calculation with the approximate matrix element for $g g \to c \bar c c \bar c$ in the high-energy approximation. The cross section for the present exact calculation is bigger only at small invariant masses and small rapidity difference between two $c$ quarks (or two $\bar c$ antiquarks). We compare correlations in rapidities of two $c$ (or two $\bar c$) for DPS and SPS contributions. Finally we compare our predictions for $D$ mesons with recent results of the LHCb collaboration for invariant mass, rapidity distance between mesons and dimeson invariant mass. The predicted shapes are similar to the measured ones, however, some strength seems to be lacking. Our new calulations clearly confirm the dominance of DPS in the production of events with double charm.

Single-parton scattering versus double-parton scattering in the production of two $c \bar c$ pairs and charmed meson correlations at the LHC

TL;DR

The paper addresses whether double charm production at LHC energies is driven by double-parton scattering (DPS) or single-parton scattering (SPS). It computes SPS with exact matrix elements for and and models DPS via a -factorization framework using a factorized double-parton distribution with , comparing to high-energy approximations and LHCb data. The main findings are that the -initiated SPS contribution is about a factor of larger than the contribution, but DPS overwhelms SPS in the total cross section at TeV; differences between exact and approximate SPS appear mainly at small invariant masses, where gluon splitting is relevant. Hadron-level predictions for mesons qualitatively reproduce LHCb distributions in relative rapidities and invariant masses, with DPS providing the correct trend but some normalization deficiency that may require incorporating additional mechanisms such as processes. Overall, the work confirms DPS as the dominant mechanism in double-charm production at LHC energies and provides a quantitative framework for comparing to experimental data.

Abstract

We compare results of exact calculations of single parton scattering (SPS) and double parton scattering (DPS) for production of and for meson correlations. The SPS calculations are performed in collinear approximation with exact matrix element for and subprocesses. It is shown that the contribution of gluon-gluon subprocess is about factor 50 larger than that for quark-antiquark annihilation. The new results are compared with results of previous calculation with the approximate matrix element for in the high-energy approximation. The cross section for the present exact calculation is bigger only at small invariant masses and small rapidity difference between two quarks (or two antiquarks). We compare correlations in rapidities of two (or two ) for DPS and SPS contributions. Finally we compare our predictions for mesons with recent results of the LHCb collaboration for invariant mass, rapidity distance between mesons and dimeson invariant mass. The predicted shapes are similar to the measured ones, however, some strength seems to be lacking. Our new calulations clearly confirm the dominance of DPS in the production of events with double charm.

Paper Structure

This paper contains 6 sections, 11 equations, 12 figures.

Table of Contents

  1. Introduction
  2. Some details of the calculation
  3. Numerical Results
  4. $c \bar{c} c \bar{c}$ production
  5. $D$ meson correlations
  6. Conclusions

Figures (12)

  • Figure 1: Rapidity (left panel) and transverse momentum (right panel) distributions of charm quarks for $c \bar{c} c \bar{c}$ production. We compare contributions of gluon fusion and quark-antiquark annihilation.
  • Figure 2: Distribution in rapidity distance between two $c$ quarks (or two $\bar{c}$ antiquarks) and in invariant mass of $c c$ (or $\bar{c} \bar{c}$) system.
  • Figure 3: Rapidity (left panel) and transverse momentum (right panel) distributions of charm quarks for $c \bar{c} c \bar{c}$ production.
  • Figure 4: Uncertainties of the SPS contribution due to the choice of renormalization and factorization scale.
  • Figure 5: Uncertainties of the SPS contribution due to the choice of charm quark mass.
  • ...and 7 more figures