  We perform the first simultaneous extraction of parton collinear and
transverse degrees of freedom from low-energy fixed-target Drell-Yan data in
order to compare the transverse momentum dependent (TMD) parton distribution
functions (PDFs) of the pion and proton. We demonstrate that the transverse
separation of the quark field encoded in TMDs of the pion is more than $4
\sigma$ smaller than that of the proton. Additionally, we find the transverse
separation of the quark field decreases as its longitudinal momentum fraction
decreases. In studying the nuclear modification of TMDs, we find clear evidence
for a transverse EMC effect. We comment on possible explanations for these
intriguing behaviors, which call for a deeper examination of tomography in a
variety of strongly interacting quark-gluon systems.
