How to test path-length dependence in energy loss mechanisms: analysis leading to a new observable

Understand

When traversing QCD medium, high $p_\perp$ partons lose energy, which is typically measured by suppression, and also predicted by various energy loss models.

• A crucial test of different energy loss mechanisms is how the energy loss depends on the length of traversed medium (so-called path-length dependence).
• The upcoming experimental results will allow to, at least in principle, for the first time, clearly observe how the energy loss changes with the size of the medium, in particular, by comparing already available $Pb+Pb$ measurements with now upcoming $Xe+Xe$ data at the LHC.
• However, in practice, to actually perform such test, it becomes crucial to chose an optimal observable.