  We present a first lattice QCD calculation of the unpolarized nucleon's
isovector transverse-momentum-dependent parton distribution functions
(TMDPDFs), which are essential to predict observables of multi-scale,
semi-inclusive processes in the standard model. We use a $N_f=2+1+1$ MILC
ensemble with valence clover fermions on a highly improved staggered quark
(HISQ) sea to compute the quark momentum distributions in a large-momentum
nucleon on the lattice. The state-of-the-art techniques in renormalization and
extrapolation in the correlation distance on the lattice are adopted. {The
perturbative kernel up to next-to-next-to-leading order is taken into account},
and the dependence on the pion mass and the hadron momentum is explored. Our
results are qualitatively comparable with phenomenological TMDPDFs, which
provide an opportunity to predict high energy scatterings from first
principles.
