The energy dependence of exclusive heavy vector meson photoproduction cross-sections and NLO BFKL evolution

We study the energy dependence of the cross-section for exclusive photoproduction of vector mesons $J/\psi$ and $\Upsilon$, using a solution to the next-to-leading order (NLO) Balitsky-Fadin-Kuraev-Lipatov (BFKL) equation. Our goal is to use BFKL evolution as a benchmark to provide evidence for the presence of non-linear QCD dynamics and signs for the onset of gluon saturation at highest center of mass energies. Our approach determines initial conditions for the proton from the Bartels-Golec Biernat-Kowalski (BGK) dipole model, and evolves the resulting unintegrated gluon distribution using NLO BFKL evolution. For the nucleus, initial conditions are generated through both the impact parameter dependent saturation model (IP-Sat), using a Wood-Saxon distribution, and a $A^{1/3}$ scaling of the saturation scale of the original BGK model. We find that NLO BFKL evolution provides a very good description of the nuclear modification factor for $J/\psi$ production, if initial conditions are generated through a $A^{1/3}$ scaled BGK model, while the description fails, if initial conditions are created using the IP-Sat model.