  This letter presents the first measurement of the angle between different jet
axes (denoted as ${\Delta}R$) in Pb$-$Pb collisions. The measurement is carried
out in the 0$-$10% most-central events at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Jets
are assembled by clustering charged particles at midrapidity using the
anti-$k_{\rm T}$ algorithm with resolution parameters $R=0.2$ and $0.4$ and
transverse momenta in the intervals $40 < p_{\rm T}^{\rm ch jet} < 140$ GeV/$c$
and $80 < p_{\rm T}^{\rm ch jet} < 140$ GeV/$c$, respectively. Measurements at
these low transverse momenta enhance the sensitivity to quark$-$gluon plasma
(QGP) effects. A comparison to models implementing various mechanisms of jet
energy loss in the QGP shows that the observed narrowing of the Pb$-$Pb
distribution relative to pp can be explained if quark-initiated jets are more
likely to emerge from the medium than gluon-initiated jets. These new
measurements discard intra-jet $p_{\rm T}$ broadening as described in a model
calculation with the BDMPS formalism as the main mechanism of energy loss in
the QGP. The data are sensitive to the angular scale at which the QGP can
resolve two independent splittings, favoring mechanisms that incorporate
incoherent energy loss.
