Unintegrated gluon distribution from forward polarized $$\rho $$ ρ -electroproduction

We present here some arguments to support our suggestion that data on the helicity structure for the hard exclusive electroproduction of $$\rho $$ mesons at HERA (and in possible future high-energy electron-proton colliders) provide useful information to constrain the $$\kappa $$ -shape of the unintegrated gluon distribution in the proton