Speaker
Description
We study the polar and azimuthal decay angular distributions of $J/\psi$ mesons produced in semi-inclusive, deep-inelastic electron-proton scattering. For the description of the quarkonium formation mechanism, we adopt the framework of Non-Relativistic Quantum Chromodynamics, with the inclusion of the intermediate color octet-channels that are suppressed at most by a factor $v^4$ in the velocity parameter $v$, relative to the leading color-singlet channel. We put forward factorized expressions for the helicity structure functions in terms of transverse momentum dependent gluon distributions and shape functions, which are valid when the $J/\psi$ transverse momentum is small and correctly match with the collinear factorization results at high transverse momentum. It turns out that the $\cos 2\phi$ azimuthal decay asymmetry originates from the distribution of linearly polarized gluons inside an unpolarized proton.
We therefore suggest a novel experiment for the extraction of this so-far unknown parton density that could be performed, in principle, at the future Electron-Ion Collider.