Description
Jets are well-established tools for studying the properties of the QGP. In this talk, we present a new measurement of jet substructure modification using the observable $\Delta j$, which characterizes the distance between two types of jet axes constructed from the same jet constituents. We use E-scheme and WTA axes, which have different sensitivities to soft and semi-hard medium-induced radiation. The reported fully unfolded distributions represent the first CMS measurements of the angular separation between these axes for anti-$k_T$ $R=0.4$ jets in 5.02 TeV PbPb collisions across several collision centralities and jet $p_T$ intervals. Significant modifications in the $\Delta j$ distributions are observed in central compared to peripheral collisions across all $p_T$ intervals, suggesting a progressive narrowing of angular correlations, likely due to QGP-induced jet substructure modification. Alternatively, this narrowing could be explained by the predicted color-charge dependence of energy loss, leading to a larger migration of gluon-initiated jets toward lower final-state energies. We compared the data to predictions from several models, including two energy loss models with modified quark/gluon fractions due to in-medium energy loss. We found that differences in quark/gluon energy loss alone cannot fully describe the central data, suggesting the need to account for medium-induced substructure modification. These measurements probe jet substructure in a previously unexplored kinematic domain and provide new constraints on the color-charge dependence of energy loss.
