Generalized parton distributions (GPDs) encoding multidimensional information of
hadron partonic structure appear as the building blocks in a factorized description of
hard exclusive reactions. The nucleon GPDs have been accessed by deeply virtual
Compton scattering and deeply virtual meson production with lepton beam. A
complementary probe with hadron beam is
shown to be
the exclusive pion-induced Drell-Yan process, $\pi^- p \to \mu^+ \mu^- n$,
as demonstrated by recent theoretical advances on describing this process in terms of QCD factorization as the
partonic subprocess convoluted with the nucleon GPDs and the pion distribution
and by the feasibility study for its measurement
via a spectrometer at the
High Momentum Beamline being constructed at J-PARC in Japan.
We also discuss
the possible soft partonic mechanisms beyond the QCD factorization framework,
and present an estimate of the soft mechanisms at J-PARC kinematics, making use of dispersion
relations and quark-hadron duality.
Realization of the measurement of the exclusive pion-induced Drell-Yan process at J-PARC will provide a new test of QCD descriptions of a novel class of hard exclusive reactions, and also offer the possibility of experimentally accessing nucleon GPDs at large timelike virtuality.