Abstract
The investigation of the partonic degrees of freedom beyond collinear approximation (toward a 3D description of the hadron structure) has been gaining increasing interest in the last decade. SIDIS reactions offer a rich phenomenology convoluting TMD parton distribution and fragmentation functions. In the recent years, several first measurements have been made that provide new insights on peculiar aspects of the parton dynamics, i.e. orbital motion and spin-orbits effects. Beyond spin asymmetries, unpolarized measurements, with their high-statistics data samples and natural connections among different fields of investigation, challenge our comprehension of TMD phenomena and are crucial for the TMD formalism assessment.
Highlights
The investigation of the partonic degrees of freedom beyond collinear approximation has been gaining increasing interest in the last decade
Promoted by the BNL and Jefferson National Laboratory (JLab) laboratories, the project for a polarized Electron-Ion Collider (EIC) in the States [21], has a comprehensive physics program ranging from the multi-dimensional spin physics investigation to the low-x physics saturation searches and the potentiality to address all the questions mentioned in this work
Hadron multiplicities in semi-inclusive deep-inelastic-scattering (SIDIS) reactions have since long time been used to study quark fragmentation, complementing the measurements done at higher energies at the e+e− collider machines
Summary
Few questions have challenged the interpretation of hadron structure and phenomena in Hadronic Physics in terms of perturbative QCD. The questions above relate to one of the fundamental degree of freedom of the elementary particles, the spin, and its correlation with the motion (i.e. transverse momentum) of the partons. As part of the most general mechanism of confinement, these correlations might manifest in unpolarized reactions where particle polarization is not directly observed or controlled. Based on the large amount of precise data provided by these experiments we have reached a good knowledge of the parton longitudinalmomentum and longitudinal-spin distributions of quarks in the nucleon, where "longitudinal" refers to the direction parallel to that of the exchanged virtual boson (the hard probe).
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