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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>J</a:mi> <a:mo>/</a:mo> <a:mi>ψ</a:mi> </a:mrow> </a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi mathvariant="normal">ϒ</c:mi> </c:math> , 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 nonlinear QCD dynamics and signs for the onset of gluon saturation at the 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 <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:msup> <f:mi>A</f:mi> <f:mrow> <f:mn>1</f:mn> <f:mo>/</f:mo> <f:mn>3</f:mn> </f:mrow> </f:msup> </f:math> 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 <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mi>J</h:mi> <h:mo>/</h:mo> <h:mi>ψ</h:mi> </h:math> production if initial conditions are generated through an <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:msup> <j:mi>A</j:mi> <j:mrow> <j:mn>1</j:mn> <j:mo>/</j:mo> <j:mn>3</j:mn> </j:mrow> </j:msup> </j:math> scaled BGK model, while the description fails if initial conditions are created using the IP-Sat model.