  We develop the first systematic theoretical approach to dijet asymmetries in
hadron-hadron collisions based on the perturbative QCD (pQCD) expansion and the
Sudakov resummation formalism. We find that the pQCD calculation at
next-to-leading order is indispensable to describe the experimental data, while
the Sudakov resummation formalism is vital near the end points where the pQCD
expansion fails to converge due to the appearance of large Sudakov logarithms.
Utilizing our resummation improved pQCD approach, we obtain good agreement with
the most up-to-date fully corrected ATLAS data on dijet asymmetry in $pp$
collisions. Combining with the BDMPS jet energy loss formalism, we extract the
value of jet transport coefficient $\hat{q}_0 \sim
2$-$6~\textrm{GeV}^2/\textrm{fm}$ for the quark-gluon-plasma created in $PbPb$
collisions at 2.76A TeV. This work paves the way for a more complete and deeper
understanding of the properties of strongly-coupled QCD medium via the studies
of dijet asymmetries in relativistic heavy-ion collisions.
