Experimental importance of the QGP medium resolution length in jet modification

Establish experimentally the role of the quark–gluon plasma (QGP) medium resolution length—defined as the minimum angular separation at which the medium resolves two partons or subjets as independent color charges—in driving medium-induced modifications of jets in lead–lead collisions, and determine whether its effect is dominant relative to other jet–medium interaction mechanisms.

Background

The medium resolution length in a quark–gluon plasma sets the smallest angular separation at which two color charges can be resolved independently by the medium. It is directly connected to the transport coefficient \hat{q}, which characterizes transverse momentum broadening per unit path length in the medium. A denser medium (larger \hat{q}) implies stronger quenching and a smaller resolution length, with jets having more resolved constituents expected to be more strongly quenched.

Although multiple models predict a significant or even dominant impact of the medium resolution length on jet quenching and substructure, a direct experimental establishment of its importance has remained elusive. This paper measures groomed jet radius and jet girth for photon-tagged jets in PbPb and pp collisions to reduce selection biases and to probe medium-induced modifications, providing data that can be used to assess the resolution length’s role.