  We define a new strategy to scan jet substructure in heavy-ion collisions.
The scope is multifold: (i) test the dominance of vacuum jet dynamics at early
times, (ii) capture the transition from coherent to incoherent jet energy loss,
and (iii) study elastic scatterings in the medium, which are either hard and
perturbative or soft and responsible for jet thermalisation. To achieve that,
we analyse the angular distribution of the hardest splitting, $\theta_{\rm
hard}$, above a transverse momentum scale, $k_t^{\rm min}$, in high-$p_t$ jets.
Sufficiently high values of $k_t^{\rm min}$ target the regime in which the
observable is uniquely determined by vacuum-like splittings and energy loss,
leaving the jet substructure unmodified compared to proton-proton collisions.
Decreasing $k_t^{\rm min}$ enhances the sensitivity to the relation between
energy loss and the intra-jet structure and, in particular, to observe
signatures of colour decoherence at small angles. At wider angles it also
becomes sensitive to hard elastic scatterings with the medium and, therefore,
the perturbative regime of medium response. Choosing $k_t^{\rm min}\approx 0$
leads to order one effects of non-perturbative origin such as hadronisation
and, potentially, soft scatterings responsible for jet thermalisation. We
perform a comprehensive analysis of this observable with three state-of-the-art
jet-quenching Monte Carlo event generators. Our study paves the way for
defining jet observables in heavy-ion collisions dominated by perturbative QCD
and thus calculable from first principles.
