Over the past year, the first results on the study of 3D structure of protons encoded in Transverse Momentum Dependent (TMD) and Generalized Parton Distributions (GPDs) functions became available from the JLab 12 GeV program. Additional studies provide new insights into topics such as hadronization dynamics and initial-final state correlations. The JLab program is therefore at the forefront of an international effort to elucidate the way the properties of the proton emerge dynamically from the interactions of the strong force. The precise data meanwhile challenges theory in several respects. On the one hand there are exciting new theoretical developments. For instance, there has been tremendous progress calculating TMDs and twist-3 functions on the lattice as well as in the description of nuclear effects on TMDs.
On the other hand the advent of precision data showed that theory struggles with a consistent description of transverse momentum shapes over a large range.
We are therefore holding a workshop in the established and successful series ``Correlations in Partonic and Hadronic Interactions (CPHI-2022)'' at Duke University to discuss recent JLab 12 results in the context of the results of other experiments, such as COMPASS and RHIC. This workshop will follow the successful CPHI-2020 workshop at CERN.
The goal of the workshop is to ensure that all experimental results are consistent, we understand the theory challenges and then to chart a way forward for theory and experiment.% as well as define experimental measurements that are needed for its validation and exploration.
This would also be an important basis for the development of a physics program for the proposed 24 GeV upgrade at JLab.
The study of the quark-gluon structure of nucleons and nuclei is one of the main goals of the nuclear physics program pursued at Jefferson Laboratory.
The fully correlated structure in the longitudinal momentum x carried by the struck quark, the impact parameter bT, the transverse momentum kT and the polarizations of quark and parent nucleon (sT,ST) is only described by so-called Wigner functions. However, these are very challenging to address experimentally. The current approach of the community is to concentrate on projections integrated over bT and kT respectively, leading to the definition of Transverse Momentum Dependent PDFs (TMDs) and Generalized PDFs (GPDs).
TMDs encode the correlation of partonic transverse momentum with x sT and ST whereas the GPDs do the same for the impact parameter bT
For recent reviews see e.g.and .
The precision measurement of TMDs and GPDs is at the center of the physics program of the 12 GeV upgrade of JLab. Recently first results were published  and other results are being finalized [4,5].
A sign that this is the start of a vibrant physics program are the newly proposed experiments, for example to study initial-final state correlations [6,7], and newly found connections of existing measurements to insights from other fields such as heavy ion physics or the proposal to use novel He3 target.
On the other hand, new measurements at JLab and other facilities around the world are stimulating progress in the theory community. Examples are new frameworks to handle QED radiation , progress calculating TMDs and twist-3 PDFs from first principles on the lattice, see e.g. Ref.  and references therein, and new insights into the modification of TMDs in the nuclear media .
However, theory is also challenged by the precision of the results and formulating a consistent framework for world data taken at different energies and in different processes like Semi-Inclusive Deep Inelastic Scattering and Drell-Yan.
A particular problem for a global description of data has been the multi-scale aspect of TMDs. In addition of the usual hard scale of the scattering Q2, the transverse momentum dependence of TMDs gives an additional scale. As our current theory is based on perturbative calculations, it is important to divide the phase space properly in regions which can be either treated in the perturbative framework or that are purely non-perturbative. However, the combination of these regions and the description of their overlap is currently problematic and does not describe data correctly [12-14]. This has been called by some the `
"qT" crisis, where qT is related to kT by inverse scaling with the energy fraction of the struck quark carried by the detected hadron.
The solution to this crisis is of great importance for polarized electroproduction experiments, and the JLab program, in particular, as most of the accessible phase space lies in the aforementioned overlap region. We believe that the cross-experimental aspect of these challenges to theory and experiment are also reflected in the makeup of the organizing committee.
We propose to organize a workshop in the series "Workshop on Correlations in Partonic and Hadronic Interactions" (CPHI). This workshop has been taking place every two years. The last iteration has been at CERN.
This workshop is dedicated to coordinate the efforts of the different experiments and theory to sort out the interpretation of the various multi-dimensional measurements which are key to our understanding of the delicate details of the 3D distributions of partons and their hadronization.
Science motivation and workshop outcomes
As outlined in the previous sections, the science motivation for the workshop is the evaluation of the state of the field of studying the 3D correlated structure of the proton in theory and experiment.
The workshop assembles a diverse list of speakers to discuss the pertinent issues.
The workshop will focus on understanding the current limitations and the steps needed for the further development of theory and phenomenology in the extraction and interpretation of TMDs and GPDs from existing data from e N, e+e-, and hadron-hadron facilities at BNL, CERN, DESY, FNAL, JLab, and KEK. It will also focus on identifying relevant systematic uncertainties in the measurements and informing possible future measurements.
In addition to the topics outlined in the previous sections, the following list represents a more complete list of current key questions of nuclear structure to be addressed by the proposed workshop.
Transverse Structure of nucleon and QCD issues associated with 3D structure
- Study of the QCD evolution properties of 3D PDFs.
- Unintegrated and Generalized Transverse Momentum Distributions.
- Evolution of TMDs and fits to physical cross sections
- MC generators for global analysis of 3D PDFs.
- Phenomenology of 3D parton distribution and fragmentation functions.
- 3D PDFs from Lattice QCD.
- Gluonic form factors
- Validation of extraction frameworks
- Extraction of PDFs from di-hadron production.
- Radiative corrections to hard scattering in exclusive and semi-inclusive
Partonic Structure beyond Densities
- Nuclear modifications of distribution functions.
- Nuclear modifications of fragmentation functions.
- Target fragmentation and conditional probabilities
- Higher twist asymmetries in SIDIS.
- New insights on 3D PDFs from non-perturbative models.\
Essential observables, which will direct the future experimental effort.
The global analysis of 3D PDFs require coordination of efforts from different experiments world wide including e N, e+e-, pp, pion-p anti-proton-proton, proton-Nucleon, and nucleon, nucleon facilities.
- Leptoproduction with fixed target facilities at CERN, JLab and EIC.
- Drell-Yan lepton pair production and Drell-Yan plus jets.
- Higgs boson production and Higgs boson plus jets
- Heavy flavor production
- Soft particle production and multi-parton interactions
QCD in the Nuclear Environment
- PDF nuclear modifications and short-range nucleon correlations
- Flavor dependence of nuclear modification effects
- Nuclear partonic distributions including x>1 region
- Tagged SIDIS processes off nuclear targets
- Hadronization processes in Nuclear SIDIS
- Hard nuclear QCD processes and Color transparency
The measure of the success of the workshop will be the development of a clear and specific understanding of the issues facing our understanding of the topics listed above. In particular, steps needed to be taken by theory and experiment to resolve the so-called "qT-crisis".
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