Narrow Results

Sivers effect is describing the correlation between the transverse polarization of nucleon and the transverse momentum, $k_{\perp }$, of its unpolarized constituent partons. This effect is an outstanding subject and in this regard, a great deal in recent years has been considered from experimental and phenomenological points of view. It also plays an essential role to extend our understanding from nucleon structure. Semi-inclusive DIS (SIDIS) process provides us an opportunity to access to Sivers function which is dependent on transverse momentum of partons. In this paper, for the first time the covariant parton model is used to deliver us the $k_{\perp }$ and $x$ dependence part of Sivers function. Based on this model, this combinatory dependence is arising out from the HERAPDF parametrization group. In this paper the other required parametrized functions in Sivers function is also changed with respect to Ref. 1. The unknown parameters which exist in Sivers function can be extracted, doing a global fit over the recent available experimental data, including HERMES, COMPASS and JLAB collaborations for the single spin asymmetry (SSA) in ${\pi }^{-}$ and ${\pi }^{+}$ meson production as well as kaon production to constrain the evolved strange quark. This is done, considering advanced mathematical manipulations to overcome the difficulties which exist to compute the required multiple integrals and finally employing the CERN MINIUTE program to do a global fit. Our results for SSA are in good agreement with the available experimental data. For more confirmation a comparison between our results and the ones from Ref. 2 is also done.

The investigation of the partonic degrees of freedom beyond collinear approximation (3D description) has been gained increasing interest in the last decade. At the HERMES experiment, azimuthal asymmetries in hard exclusive reactions and in semi-inclusive deep-inelastic scattering of electrons and positrons off a (polarized) hydrogen and deuterium target have been measured. Such asymmetries provide new insights on crucial aspects of the parton dynamics. By measuring various hadron types in the initial and final states, flavor sensitivity is achieved. Non zero signals are reported for azimuthal asymmetries with respect the transverse target polarization in real-photon exclusive-electroproduction, which are related (still in a model dependent way) to the elusive quark orbital motion. Evidence is reported of the poorly known transversity function and of naive-T-odd transverse-momentum-dependent parton distribution functions related to spin-orbit effects. Evidence of spin-orbit effects in quark fragmentation is also observed, which are opposite in sign for favored and disfavored processes.

The exploration of the structure of the nucleon reaches a new level of sophistication when explicit dependences on transverse momenta of partons in both parton distributions and fragmentation functions are included. These dependences lead to a manifold of new correlations between the nucleon and quark spin as well as their momenta and thus a wealth of experimental observables. A particular example is the novel Sivers function that—according to firm QCD predictions—has to change sign when going from the Drell-Yan process to semi-inclusive deep-inelastic scattering. Here a review is given on which knowledge has been gathered about transverse-momentum-dependent (TMD) distribution and fragmentation functions from semi-inclusive deep-inelastic scattering experiments.

We present measurements of the ep → epπ⁰ cross section extracted at two values of four-momentum transfer Q² = 1.9 GeV² and Q² = 2.3 Gev² at Jefferson Lab Hall A. The kinematic range allows to study the evolution of the extracted hadronic tensor as a function of Q² and W. Results will be confronted with Regge inspired calculations and GPD predictions. An intepretation of our data within the framework of semi-inclusive deep inelastic scattering has also been attempted.

Single-spin asymmetries measured at the HERMES experiment for pions and charged kaons in semi-inclusive deep-inclastic scattering of electrons and positrons off a transversely polarized hydrogen target, are reported. Two of them reveal clear evidences of non-zero leading-twist parton distribution functions, such as the poorly known transversity and Sivers functions, describing crucial aspects of the dynamics of quarks in a transversely polarized nucleon. Evidences of a non-zero favored and disfavored Collins fragmentation function with opposite sign are also observed.