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Evidence for nuclear gluon shadowing from the ALICE measurements of PbPb ultraperipheral exclusive J/ψ production

V. Guzey, E. Kryshen, M. Strikman, M. Zhalov

TL;DR

This paper analyzes ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions to extract a nuclear suppression factor S(W_γp), which quantifies the impact of nuclear gluon shadowing at x ~ 10^-3. By comparing the measured coherent photoproduction cross sections with impulse-approximation predictions—using photon flux calculations, nuclear form factors, and forward γp→J/ψp data—the authors obtain S(19.6 GeV)=0.74^{+0.11}_{-0.12} and S(92.4 GeV)=0.61^{+0.05}_{-0.04}. These results are contrasted with multiple theoretical approaches, including VMD/Glauber, color-dipole models, StarLight, HIJING, global nuclear PDFs, and the leading-twist theory of nuclear shadowing; the data indicate substantial gluon shadowing at small x and are most compatible with leading-twist predictions (e.g., EPS09LO) within uncertainties. The work provides direct, relatively model-independent evidence for nuclear gluon shadowing and highlights the need for improved data to constrain nuclear PDFs at low x.

Abstract

We show that the recent ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon shadowing in lead at $x \sim 10^{-3}$. The evidence is based on the comparison of the nuclear suppression factor S(x\approx 0.001)=0.61^{+0.05}_{-0.04} found in the analysis of the coherent J/ψ photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing.

Evidence for nuclear gluon shadowing from the ALICE measurements of PbPb ultraperipheral exclusive J/ψ production

TL;DR

This paper analyzes ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions to extract a nuclear suppression factor S(W_γp), which quantifies the impact of nuclear gluon shadowing at x ~ 10^-3. By comparing the measured coherent photoproduction cross sections with impulse-approximation predictions—using photon flux calculations, nuclear form factors, and forward γp→J/ψp data—the authors obtain S(19.6 GeV)=0.74^{+0.11}_{-0.12} and S(92.4 GeV)=0.61^{+0.05}_{-0.04}. These results are contrasted with multiple theoretical approaches, including VMD/Glauber, color-dipole models, StarLight, HIJING, global nuclear PDFs, and the leading-twist theory of nuclear shadowing; the data indicate substantial gluon shadowing at small x and are most compatible with leading-twist predictions (e.g., EPS09LO) within uncertainties. The work provides direct, relatively model-independent evidence for nuclear gluon shadowing and highlights the need for improved data to constrain nuclear PDFs at low x.

Abstract

We show that the recent ALICE measurements of exclusive J/ψ production in ultraperipheral PbPb collisions at 2.76 TeV provide the first direct experimental evidence for the strong nuclear gluon shadowing in lead at . The evidence is based on the comparison of the nuclear suppression factor S(x\approx 0.001)=0.61^{+0.05}_{-0.04} found in the analysis of the coherent J/ψ photoproduction cross sections measured by ALICE with the nuclear gluon shadowing predicted by the global fits of nuclear parton distributions and by the leading twist theory of nuclear shadowing.

Paper Structure

This paper contains 8 sections, 25 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Calculation of the suppression factor
  3. Cross section of coherent $J/\psi$ photoproduction in ALICE measurements
  4. $J/\psi$ photoproduction on Pb in the impulse approximation
  5. Estimation of the suppression factor $S(W_{\gamma p})$
  6. Comparison of the suppression factor with theoretical predictions
  7. Conclusions
  8. Acknowledgements

Figures (3)

  • Figure 1: The fit to the forward $J/\psi$ photoproduction cross section data hera.
  • Figure 2: Comparison of the ALICE suppression factors with the estimates in the Glauber model with the color dipole cross section and in the Starlight approach.
  • Figure 3: Comparison of the ALICE suppression factors with predictions of the nuclear gluon shadowing in HIJING 2.0 (top), global QCD fits (middle), and in the leading twist approximation (bottom).