Evidence for nuclear gluon shadowing from the ALICE measurements of PbPb ultraperipheral exclusive J/{\psi} production

TL;DR

The ALICE experiment provides the first direct evidence of strong nuclear gluon shadowing at very small x in lead nuclei through ultraperipheral PbPb collisions, confirming theoretical predictions.

Contribution

This study offers the first experimental confirmation of nuclear gluon shadowing effects at x~10^{-3} using ultraperipheral collision data from ALICE.

Findings

Nuclear suppression factor S(x≈0.001)=0.61^{+0.05}_{-0.04}

Consistent with global nuclear parton distribution fits

Supports the leading twist theory of nuclear shadowing

Abstract

We show that the recent ALICE measurements of exclusive J/{\psi} 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/{\psi} 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.