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The exploration of the inner structure of protons and neutrons (nucleons) in terms of the quark and gluon degrees of freedom of QCD has made impressive progresses in the last years, with the recognition of the importance of the parton intrinsic motion, of the transverse spin and the correlations of transverse momentum of partons and spin. For the first time a full picture of the nucleon structure based on parton spin, orbital angular momentum and internal motion is emerging, much improving the usual simple representation in terms of collinear partonic motion.

The new knowledge about the nucleon content is embedded in the so-called transverse momentum dependent parton distribution and fragmentation functions (TMDs), which generalize the standard parton distributions. These contain all the information on the intrinsic motion of quarks and gluons and on spin-orbit correlations, and will be the key to construct the ultimate picture of the internal structure of the nucleon going beyond the collinear approximation.

The TMDs can be experimentally revealed by looking at, often neglected, azimuthal and transverse Single Spin Asymmetries (SSA) in many physical processes. Such experiments are currently running at CERN (COMPASS Collaboration), DESY (HERMES Collaboration), JLab, KEK and RHIC. In addition, this type of physics is an essential part of the program of future facilities such as the FAIR-project at GSI, J-PARC and the proposed EIC at JLab or RHIC. It will be intriguing to explore the potential of studying these new developments in hadron physics at the LHC, and indeed suggestions and proposals start appearing in the literature.

The currently most accessible way to obtain information about TMDs is to measure spin and azimuthal asymmetries, and in particular single-spin asymmetries, in Semi-Inclusive Deep Inelastic Scattering (SIDIS). Such measurements involve longitudinal beam polarization and/or longitudinal or transverse target polarization.

The complexity of the task of mapping out TMDs requires the interplay of diverse and complementary measurements of various different observables over a wide kinematical range involving experiments with polarized beams and targets. In the near future more SIDIS data are expected to come from JLab experiments and from the COMPASS measurement on a transversely polarized proton target. The analysis of the BELLE data in electron-positron annihilation will also

produce new results on the spin dependent fragmentation functions, and especially for the fragmentation into two-hadrons. On a longer time scale, a wealth of single- and double-spin observables related to TMDs will be measured at the upgraded JLab 11 GeV polarized electron beam, at COMPASS, with the pion beam and the transversely polarized proton target, and at GSI, colliding transversely polarized antiprotons on transversely polarized protons.

The proposed network will bring together theorists and experimentalists working in this field in order to study in detail the key physics issues of TMDs measured in lepton-lepton, lepton-hadron and hadron-hadron scattering processes. The envisaged combination of experimental and theoretical efforts will provide the synergy for unraveling this intriguing aspect of hadron physics.

The global physics project involves measuring azimuthal and Single Spin Asymmetries, extracting TMDs from data, combining different information from different physical processes, studying the QCD properties of TMDs, modeling them, planning future experiments and so on. It is a project centered around a well defined goal - the nucleon structure - which requires complementary activities, collaborative work, coordinated planning and organization, frequent exchange of information and common moments of interpretation and elaboration of results.

In order to achieve the best work organization, distribution of the tasks and synergetic collaboration among the different groups, 6 teams (3 experimental and 3 theoretical ones) have been formed, according to Table 1.

Table 1 - Teams, participating groups, and related Activity Leaders

In addition, 15 different physical subprojects have been defined, as in Table 2, and assigned to the various teams, according to Table 3.

Table 2 - Projects of the TMD networking activity

Table 3 - Tasks distribution among participating teams

The expected outcomes of this networking activity are summarized in Table 4, with the expected month of delivery. As most activities will proceed with regular constant progress, the delivery months are indicated at the end of each year, or each second year, or the end of the project.

Table 4 - Foreseen deliverables, with expected months of delivery.
 In brackets are given the corresponding projects.

The complexity of the task of mapping out the transverse and the spin-orbit structure of the nucleon requires the interplay of diverse and complementary measurements. It calls for a precision study of a large variety of measurable asymmetries and, in particular, of their behavior over a wide kinematical range and their dependence on many different kinematical observables.

The primary goal of the project is twofold: to promote and stimulate co-operation between experimentalists who are active in data analysis within the different research infrastructures COMPASS, HERMES and JLab on the one hand, and between experimentalists and theory experts in the field, on the other. This latter collaboration is a very important aspect of the project as there is a considerable amount of genuine theoretical groundwork to be performed before data analysis. Over the course of several years of data analysis the closest of interactions between experimentalists and theorists will be required for the determination of new experimental quantities and their theoretical interpretation and modeling. For the experimental groups, this collaboration will be crucial to promote common protocols and inter-operability. It will allow the mutual coordination and rationalization of their data-analysis capabilities and the pooling of resources (such as Monte Carlo event production, study of systematic effects and tests of proposed analysis algorithms). This represents a very significant improvement in the capabilities of the three research infrastructures for the measurements of semi-inclusive deep-inelastic scattering of leptons off nucleon targets.

From a global perspective, the two major collaborations studying the spin structure of the nucleon are based in Europe: COMPASS at CERN and HERMES at DESY. Moreover, a group of European physicists will analyze in Europe data collected at JLab. These facilities can very fruitfully combine their efforts as they will operate in different kinematical regions, planning to measure azimuthal, single- and double-spin asymmetries in hadron lepton-production at beam energies of 100-160 GeV, 27 GeV and 6-11 GeV, respectively. This will allow for a more detailed study of the x- or z-dependence and of the Q²-evolution of TMDs.

On the theory side, most experts on TMD-related issues are working in European universities and institutes, a major part of them participating in this project.

The proposed network brings together scientists from 20 European Institutions, representing 8 European Countries. The envisaged combination of experimental and theoretical efforts guarantees considerable synergy, which will further strengthen the leading role of European scientists in this sector of hadron physics.

In addition, this type of physics is an essential part of the program of the largest future hadron facility in Europe, the FAIR-project at GSI.