Numerical package for solving the JIMWLK evolution equation in C++

Abstract

Precise and detailed knowledge of the internal structure of hadrons is one of the most actual problems in elementary particle physics. In view of the planned high energy physics facilities, in particular the Electron–Ion Collider constructed in Brookhaven National Laboratory (National Academies of Sciences, Engineering and Medicine, 2018, [1]), the Chinese Electron–Ion Collider of China (Chen, 2018 [2]), or upgraded versions of CERN’s LHC experiments, it is important to prepare adequate theoretical tools to compare and correctly interpret experimental results. One of the model frameworks allowing to estimate hadron structure functions is the combination of the McLerran–Venugopalan initial condition model together with the JIMWLK equation which describes the evolution in rapidity of the initial distribution. In this package we present a parallel C++ implementation of both these ingredients. In order to allow a thorough assessment of systematic effects several discretizations of the JIMWLK kernel are implemented both in position and momentum spaces. The effects of the running coupling in three different definitions are provided. The main code is supplemented with test and check programs for all main functionalities. The clear structure of the code allows easy implementation of further improvements such as the collinear constraint (Hatta and Iancu, 2016).

Keywords

• Langevin equation
• JIMWLK equation
• Stochastic integration
• Gluon dipole distribution