Measurement of azimuthal asymmetries of positive pions in DIS off nuclei with the CLAS detector at Jefferson Lab*

Hadronization is the process whereby energetic quarks evolve back to color-neutral hadrons, thus satisfying confinement.

This process has been studied in electro-nuclear scattering with an unpolarized electron beam, by measuring the absorption or enhancement of positive pions produced in the interaction with different nuclear targets made of D, C, Fe, and Pb.

The cross section for this process is known to have three linearly independent terms in the form $A+B \, cos(\phi)+C \, cos(2\phi)$, where $A$, $B$, and $C$ depend on four kinematic variables and $\phi$, which is the azimuthal angle of the pion in the virtual-photon-nucleon reference frame.

Nuclear modifications of the cross section to date have been performed using only the first term, while the second term is only known to be smaller, and the third the smallest of them.

The orders of magnitude of these parameters were found to agree with this claim.

Preliminary, at a qualitative level, the first asymmetry (related to the $B$ term) presents a reduction with hints of a nuclear size dependency.

Although the dependence on certain quantities, such as the hadron energy given by the virtual photon ($Z_h$), do not show reduction at all.

The measurement of these quantities will illustrate the extent to which quantum effects are needed to describe the medium-modified scattering, which thus far have only been described using semi-classical approaches, and would help to determine the mechanisms that enforce QCD confinement.