VA2-Virtual Access to 3DPartons (3DPartons)

3DPartons gives access to open-source code necessary for high precision phenomenology in the field of 3D hadron structure, with a specific emphasis on generalized parton distributions (GPDs) and transverse momentum dependent parton distributions (TMDs). This code covers the entire path between GPDs and TMDs up to observables (cross sections, asymmetries, etc.) including evolution equations and hard coefficient functions with various perturbative QCD approximations (twist or order in the strong running coupling). It can be used for experimental data fits, experiment design or phenomenological sensitivity study. It consists of C++ code organized within a fully modular and open architecture, which allows the possibility of permanent improvement by the addition of new models, channels or theoretical refinements. Partly driven by the need of high-precision phenomenology at the LHC, parton distribution functions (PDFs) have been accurately determined using tools and theoretical developments of increasing sophistication. Benefiting from the experience of decades of PDF studies, the GPD and TMD communities may find in 3DPartons a forum where they can mutualize knowledge and know-how of similar problems related to the complexity of the GPD and TMD computing chains. Services currently offered by the infrastructure: Existing tools are usually tied to one model or one experimental channel, and not as modular and generic as in the present proposal. The services offered would be completely unique worldwide. As it stands, 3DPartons will be based on parts of, or offer interfaces to, various existing codes:

• PDF (LHAPDF, APFEL, xFitter),
• GPD (PARTONS, KM),
• TMD (arTeMiDe, TMDlib, xFitter, CASCADE),

Most of these codes are either open-source or freely executable from a webpage. In their own domain, they have already been used for fits or event generations.

• PDF: the LHAPDF library provides standardized access to PDFs and is supported by many Monte Carlo (MC) generators and physics programs. xFitter is the first QCD fit framework to extract PDFs and assess the impact of some data sets. The resulting PDFs are provided in a format allowing immediate use with LHAPDF.
• GPD: KM and PARTONS have been used for global GPD fits and for prospective studies about a future electronion collider (EIC),
• TMD: arTeMiDe has been used to perform a global fit of Drell-Yan and Z-boson production data.

TMDlib provides a framework and an interface to a collection of different TMD parameterizations. It can make immediate use of the output of xFitter. CASCADE is a hadron level Monte Carlo event generator simulating the full TMD parton shower. TMDs and GPDs are topics of current experimental interest, at the core of key research programs e.g. at TJNAF or at a future EIC.