  We discuss the physics interest and the experimental feasibility for
detecting high-mass dimuon pairs using the planned 50-GeV Proton Synchrotron
(PS) at the KEK/JHF and JAERI/NSP joint accelerator project. The Drell-Yan
measurement of $p+d$ versus $p+p$ at 50 GeV will provide unique information on
the flavor asymmetry of proton's up and down sea-quark distributions in the
large-$x$ region. A study of the nuclear dependences of Drell-Yan cross
sections can reveal the modification of antiquark distributions in nuclei.
Furthermore, the effect of energy loss for fast partons traversing nuclear
medium could also be sensitively measured. If polarized proton beam becomes
available at the 50-GeV PS, unique information on the sea-quark polarization
could be obtained. Study of heavy quarkonium production at the 50-GeV PS can
set important constraints on the mechanism of vector meson productions. Using a
prototype dimuon spectrometer, we have simulated the sensitivities for a
variety of measurements.
