Nuclear suppression of coherent J/ψ photoproduction in heavy-ion UPCs and leading twist nuclear shadowing

We determine the nuclear suppression factor SPb(x), where x=MJ/ψ²/Wγ p² with MJ/ψ the J/ψ mass and Wγ p the photon-nucleon energy, for the cross section of coherent J/ψ photoproduction in heavy-ion ultraperipheral collisions (UPCs) at the Large Hadron Collider (LHC) and Relativistic Heavy Ion Collider (RHIC) by performing the χ² fit to all available data on the cross section dσAA → J/ψ AA/dy as a function of the J/ψ rapidity y and the photoproduction cross section σγ A → J/ψ A(Wγ p) as a function of Wγ p. We find that while the dσAA → J/ψ AA/dy data alone constrain SPb(x) for x ≥ 10⁻³, the combined dσAA → J/ψ AA/dy and σγ A → J/ψ A(Wγ p) data allow us to determine SPb(x) in the wide interval 10⁻⁵ < x < 0.05. In particular, the data favor SPb(x), which decreases with a decrease of x in the 10⁻⁴ < x < 0.01 interval, and can be both decreasing or constant for x< 10⁻⁴. Identifying SPb(x) with the ratio of the gluon distributions in Pb and the proton Rg(x,Q₀²)=gA(x,Q₀²)/[A gₚ(x,Q₀²)], we demonstrate that the leading twist approximation (LTA) for nuclear shadowing provides a good description of all the data on dσAA → J/ψ AA/dy and σγ A → J/ψ A(Wγ p) as well as on the experimental values for SPb(x) derived from σγ A → J/ψ A(Wγ p). We also show that modern nuclear PDFs reasonably reproduce SPb(x) as well.