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There are three types of fragmentation functions (FFs) which are used to describe the twist-3 cross sections of the hard semi-inclusive processes under QCD collinear factorization, and they are called intrinsic, kinematical, and dynamical…
Transition to the reflective scattering mode which has emerged at the highest LHC energy of $\sqrt{s}=13$ TeV results in a relative shrinkage with the energy of the impact parameter region responsible for the inelastic hadron collisions.…
The solution of QCD equations for generating functions of {\it parton} multiplicity distributions reveals new peculiar features of cumulant moments oscillating as functions of their rank. It happens that experimental data on {\it hadron}…
The momentum and helicity density distributions of the strange quark sea in the nucleon are obtained in leading order from charged-kaon production in deep-inelastic scattering on the deuteron. The distributions are extracted from…
Sum rules in effective field theories, predicated upon causality, place restrictions on scattering amplitudes mediated by effective contact interactions. Through unitarity of the $S$-matrix, these imply that the size of higher dimensional…
Spin observables may reveal much deeper properties of non perturbative hadronic physics than unpolarized quantities. We discuss the polarization of hadrons produced in $e^+e^-$ annihilation at LEP. We show how final state $q \bar q$…
I discuss inclusive and semi-inclusive lepton-hadron scattering emphasizing the importance of polarization in order to study various single or double spin asymmetries and the importance of particle identification and angular resolution in…
Parton distribution functions (DFs) have long been recognised as key measures of hadron structure. Today, theoretical prediction of such DFs is becoming possible using diverse methods for continuum and lattice analyses of strong interaction…
Effective mass Klein-Gordon equation for the asymmetric Hulth{\'e}n potential is solved in terms of hypergeometric functions. Results are obtained for the scattering and bound states with the position dependent mass and constant mass, as a…
In a high-energy scattering process with hadrons in the initial state, color is involved. Transverse momentum dependent distribution functions (TMDs) describe the quark and gluon distributions in these hadrons in momentum space with the…
We discuss the recent interpretation of quark distribution functions in the plane transverse to the light-cone direction. Such a mapping is model independent and allows one to build multidimensional pictures of the hadron and to develop a…
We generalize a positivity constraint derived initially for parity-conserving processes to the parity-violating ones, and use it to derive non-trivial bounds on several Sivers functions, entering in the theoretical description of single…
The hyperfine interactions of the constituent quark model provide a natural explanation for many nucleon properties, including the Delta-N splitting, the charge radius of the neutron, and the observation that the proton's quark distribution…
We summarize the most significant aspects in the study of transverse spin phenomena over the last few decades, focusing on Semi-Inclusive Deep Inelastic Scattering processes and hadronic production in $e^+e^-$ annihilations. The…
Fragmentation functions represent a key ingredient to address the proton spin structure in semi-inclusive deep-inelastic scattering and proton-proton collisions. They can not be determined from perturbative Quantum Chromodynamics and have…
We discuss the production of hadrons in polarized lepton nucleon interactions and in the current jet fragmentation region; using the QCD hard scattering formalism we compute the helicity density matrix of the hadron and show how its…
The general formalism to describe single spin asymmetries in hadron-hadron high energy and large $p_T$ inclusive production within the QCD-improved parton model and assuming the factorization theorem to hold for higher twist contributions…
A reanalysis of collinear factorization for inclusive Deep Inelastic Scattering shows that a novel, non perturbative spin-flip term associated with the invariant mass of the produced hadrons couples, at large enough Bjorken $x_B$, to the…
We study the single-transverse spin asymmetry in semi-inclusive hadron production in deep inelastic scattering. We derive the leading contribution to the asymmetry at moderate transverse momentum $P_{h\perp}$ of the produced hadron in terms…
We calculate the spin density matrix of a back-to-back quark-antiquark pair inclusively produced in electron-nucleus scattering, taking into account the gluon saturation effect and the linearly polarized gluon distribution. We then…