On systematic effects in the numerical solutions of the JIMWLK equation

In the high energy limit of hadron collisions, the evolution of the gluon density in the longitudinal momentum fraction can be deduced from the Balitsky hierarchy of equations or, equivalently, from the nonlinear Jalilian–Marian–Iancu–McLerran–Weigert–Leonidov–Kovner (JIMWLK) equation. The solutions of the latter can be studied numerically by using its reformulation in terms of a Langevin equation. In this paper, we present a comprehensive study of systematic effects associated with the numerical framework, in particular the ones related to the inclusion of the running coupling. We consider three proposed ways in which the running of the coupling constant can be included: “square root” and “noise” prescriptions and the recent proposal by Hatta and Iancu. We implement them both in position and momentum spaces and we investigate and quantify the differences in the resulting evolved gluon distributions. We find that the systematic differences associated with the implementation technicalities can be of a similar magnitude as differences in running coupling prescriptions in some cases, or much smaller in other cases.

PHENIX heavy flavor highlights

Heavy flavor production is a sensitive probe of the initial gluon density in the nucleon and is modified by the entire evolution of the hot quark and gluon medium created in high-energy nucleus–nucleus collisions. Besides, it is a process that can be calculated by perturbative QCD because of their large mass. The PHENIX experiment at RHIC studied the heavy flavor productions for a broad momentum and rapidity ranges using single leptons from the semileptonic decay of charm and bottom hadrons, and dileptons from [Formula: see text] decays in [Formula: see text], [Formula: see text]A, and Au [Formula: see text] Au collisions at [Formula: see text][Formula: see text]200[Formula: see text]GeV. In these proceedings, the recent experimental results in [Formula: see text], Au [Formula: see text] Au, and the small collision systems are presented and the heavy flavor productions and their modifications are discussed.

EMC effect at small Bjorken x values

The Bessel-inspired behavior of parton densities at small Bjorken x values is used along with “frozen” and analytic modifications of the strong coupling constant [1] to study the so-called EMC effect. Among other results, this approach allowed predicting small x behavior of the gluon density in nuclei.

Determination of the TMD gluon density in a proton using recent LHC data

Unintegrated (or transverse momentum dependent, TMD) parton distributions in a proton are important in high-energy physics. Using the latest LHC data on the hadron production in pp collisions, we determine the parameters of the initial TMD gluon density, derived in the framework of quark-gluon string model at the low scale μ0 ~ 1 − 2 GeV and refine its large-x behavior using data on the tt̅ production at $\sqrt s = 13\,{\rm{TeV}}$. Then, by using the Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation, we extend the obtained TMD gluon density to the whole kinematic region. We tested the proposed TMD gluon density to the inclusive Higgs production in different decay modes, t-channel single top production at the LHC and to the proton structure functions $F_2^c(x,\,{Q^2})$ and $F_2^b(x,\,{Q^2})$ in a wide region of x and Q2. Good agreement with the latest LHC and HERA data is achieved.

Overview of ALICE results on ultra-peripheral collisions

Lead nuclei, accelerated at the LHC, are sources of strong electromagnetic fields which can be used to measure photon-induced interactions in a new kinematic regime. These interactions can be studied in ultra-peripheral p–Pb and Pb–Pb collisions where impact parameters are larger than the sum of nuclear radii and hadronic interactions are strongly suppressed. Heavy quarkonium photoproduction is of particular interest since it is sensitive to gluon distributions in target hadrons. An overview of ALICE results on vector meson photoproduction in ultra-peripheral Pb–Pb and p–Pb collisions will be presented. Implications to study gluon density distributions and nuclear gluon shadowing will be discussed.