Speaker
Description
n this talk, we present the first measurement of jet substructure modification in the QGP medium with fully reconstructed jets using $\Delta j$ observable, which quantifies the distance between two types of jet axes constructed from the same jet constituents. The E-scheme anti-$k_T$ and Winner-Takes-All axes are employed, providing different sensitivities to soft and semi-hard medium-induced radiation. The first CMS measurement of the fully unfolded distributions of the angular separation between these axes for anti-$k_T$ $R=0.4$ jets in 5.02 TeV PbPb collisions will be presented for several collision centralities and jet $p_T$ intervals. Significant narrowing in the $\Delta j$ distributions is observed in central compared to peripheral collisions across all $p_T$ intervals, suggesting a QGP-induced substructure modification. To test an alternative explanation for the narrowing that could be provided by the predicted color-charge dependence of energy loss, the data are compared to predictions from several models, including those with modified quark/gluon fractions due to quenching. These comparisons indicate that differences in quark/gluon energy loss alone cannot fully describe the central data, suggesting additional mechanisms for medium-induced substructure modification. These measurements probe jet substructure over the widest jet $p_T$ range in a previously unexplored kinematic domain, providing new constraints on the color-charge dependence of energy loss.