  We study azimuthal asymmetries in heavy quark pair production in unpolarized
electron-proton and proton-proton collisions, where the asymmetries originate
from the linear polarization of gluons inside unpolarized hadrons. We provide
cross section expressions and study the maximal asymmetries allowed by
positivity, for both charm and bottom quark pair production. The upper bounds
on the asymmetries are shown to be very large depending on the transverse
momentum of the heavy quarks, which is promising especially for their
measurements at a possible future Electron-Ion Collider or a Large Hadron
electron Collider. We also study the analogous processes and asymmetries in
muon pair production as a means to probe linearly polarized photons inside
unpolarized protons. For increasing invariant mass of the muon pair the
asymmetries become very similar to the heavy quark pair ones. Finally, we
discuss the process dependence of the results that arises due to differences in
color flow and address the problem with factorization in case of proton-proton
collisions.
