PARtonic Tomography Of Nucleon Software – PARTONS

The project undertaking requires:
- A software suite allowing simultaneous theoretical analysis of measurements related to several complementary experimental channels.
- A strategy to propagate statistical and systematic uncertainties related to experimental data, and an evaluation of modeling systematic uncertainties.
- An interactive visualizing software to compare experiments and theory and allow the release to the international community of the project scientific output.
- An interface to design new experiments on existing or future facilities.
- A database storing theoretical or experimental data of various kinds.
This requires a substantial improvement of the tools and technics usually used by the nuclear and particle physics communities. This project will be achieved by the synthesis of existing methods dedicated to the modeling and representation of complex systems, uncertainty handling and optimization of numerical methods.

This project began only six months ago but already has a scientific output thanks to the coordinated efforts of the four partners: Irfu (CEA), IPN and LPT (CNRS and Université d’Orsay) and CPhT (CNRS and Ecole Polytechnique). Let us mention :
- A study about the extension of the perimeter of applicability of a model which recently yield a satisfactory analysis of a large number of experimental data.
- The potential possibility to constrain the glue structure of the proton from a larger number of high precision experiments than previously thought.
- A preliminary study of the propagation of experimental uncertainties to obtain three-dimensional views of the proton structure.

The physics program related to the project requires very accurate measurements of a large number of observables. This experimental work will be going on for at least the next ten years in international laboratories, including the Thomas Jefferson National Laboratory (Jefferson Lab, or JLab) and The European Organization for Nuclear Research (CERN). Forthcoming years will see the release of measurements with an unmarked precision. The proton structure is also a key component of the physics case of a possible future Electron Ion Collider at the horizon 2025. The analysis of forthcoming data and the design of future experiments require a tool such as the one developed in the PARTONS project. This is a guaranty of the longevity of the project and a guideline for future developments.

Three articles have been published :
- H. Moutarde, B. Pire, F. Sabatie, L. Szymanowski and J. Wagner, Phys. Rev. D87 (2013) 054029 [arXiv:1301.3819 [hep-ph]].
- M. Guidal, H. Moutarde and M. Vanderhaeghen, Rept. Prog. Phys. 76 (2013) 066202 [arXiv:1303.6600 [hep-ph]].
- C. Mezrag, H. Moutarde and F. Sabatié, Phys. Rev. D88 (2013) 014001 [arXiv:1304.7645 [hep-ph]].

The aim of this project is the realization of a software framework dedicated to the study of the structure of hadrons in terms of their elementary constituents, quarks and gluons. This tool is critical for the achievement of a long-range program at the intersection of particle and nuclear physics involving hundreds of scientists worldwide.

During the 1970’s, physicists worked out a successful formulation of the strong interaction, although this formulation is still mysterious in many respects. It is named Quantum Chromo Dynamics (QCD). According to this fundamental theory, strongly interacting particles (hadrons) are made up of quarks and gluons, collectively referred to as partons. Quarks and gluons are the degrees of freedom through which QCD is defined, but this theory describes all strongly interacting particles: hadrons and nuclei. A major question is thus to understand the emergence of the properties of hadrons (mass, spin, etc.) from the collective organization of partons.

During the second half of the 1990’s, theorists exhibited the promising new theoretical concept of a Generalized Parton Distribution (GPD). For the first time in seventy years of study of the proton structure occurred the possibility of a three-dimensional representation of its internal structure as well as a possible path to the resolution of long-standing issues such as the origin of the proton spin. Theorists also proposed several different ways to access GPDs experimentally: GPDs indeed parameterize some observables of specific processes in a theoretically robust but very involved way. These findings demonstrated the feasibility of experimental tomography of hadrons.

The first convincing experimental evidences (obtained in electromagnetic scattering on hydrogen targets) were collected in the early 2000’s and results of the first dedicated experiments were published in 2006 and 2007. However the completion of the GPD physics program requires very accurate measurements of a large number of different observables to allow a complete experimental determination of GPDs. This experimental work is expected to continue at least during the next 10 years at several international facilities, including (among others) the Thomas Jefferson National Laboratory (Jefferson Lab, or JLab) and the European Organization for Nuclear Research (CERN). These forthcoming years will be the time of unprecedented high precision measurements. GPD physics is also one key component of the physics case of a possible future Electron Ion Collider (EIC) at horizon 2025.

Our project has been designed to fulfill the needs of the worldwide hadronic physics community. Its architecture consist of the following tools:
• A comprehensive database of experimental results;
• A comprehensive database of theoretical predictions;
• A fast and efficient software to extract GPDs from measurements of different observables of several specific processes;
• A robust strategy to propagate systematic and statistic uncertainties to the extracted GPDs, and to evaluate systematic uncertainties on GPD parametrizations;
• A visualisation software to compare experimental results and model expectations;
• An interface to connect the previous items to different experimental set-up descriptions to design new experiments;
• An interactive web site providing a free access to model and experimental values of GPDs first to the whole hadronic physics community, and second to a broader audience (science popularization and illustrative examples of current research trends in high school and undergraduate teaching).

First high precision measurements are expected by 2014 at CERN. At that time the first phase of the physics program of Jefferson Lab will be completed, and the second phase about to start. The release of the software components described here will be the suitable facility to take the next step to resume the physics program. It is expected to have a major phenomenological impact.