  We show that linearly polarized gluons inside unpolarized hadrons can be
directly probed in jet or heavy quark pair production in electron-hadron
collisions. We discuss the simplest cos (2 phi) asymmetries and estimate their
maximal value, concluding that measurements of the unknown linearly polarized
gluon distribution in the proton should be feasible in future EIC or LHeC
experiments. Analogous asymmetries in hadron-hadron collisions suffer from
factorization breaking contributions and would allow to quantify the importance
of initial and final state interactions.
