  This work presents the first theoretical investigation of the medium
modification of jet broadening as an event-shape observable in multijet final
states due to jet quenching in high-energy nuclear collisions. The partonic
spectrum of $pp$ collisions with next-to-leading order (NLO) accuracy at
$\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV is provided by the POWHEG$+$PYTHIA8 event
generator, while the linear Boltzmann transport (LBT) model is utilized to
investigate the energy loss of fast partons as they traverse through the hot
and dense QCD medium. We present the jet broadening distributions in multijet
final states for both $pp$ and PbPb collisions at $\sqrt{s_{\mathrm{NN}}} =
5.02$ TeV, then observe an enhancement at the small jet broadening region and
suppression at the large jet broadening region in PbPb collisions relative to
that in $pp$. This suggests that medium modification with parton energy loss in
the QGP leads to a more concentrated energy flow in all observed multijet
events in PbPb reactions. We also demonstrate that the intertwining of two
effects, the jet number reduction and the restructured contribution, results in
the novel behavior of nuclear modification of the jet broadening observable in
PbPb collisions.
