'Differential measurements of jet substructure and partonic energy loss in Au plus Au collisions at root S-NN=200 GeV

The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in p p and Au + Au collisions ats root S-NN = 200 GeV. The substructure observables include SoftDrop groomed momentum fraction (z(g)), groomed jet radius (R-g), and subjet momentum fraction (Z(SJ)) and opening angle (theta(SJ)). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for p p collisions and are compared to leading-order Monte Carlo models. The subjet theta(SJ) distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in Au + Au collisions. We compare data from Au + Au collisions to those from p p which are embedded in minimum-bias Au + Au events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than z(g) and R-g. We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuumlike. We also report measurements of the differential dijet momentum imbalance (A(J)) for jets of varying theta(SJ). We find no qualitative differences in energy loss signatures for varying angular scales in the range 0.1 < theta(SJ) < 0.3, leading to the possible interpretation that energy loss in this population of high-momentum dijet pairs, is due to soft medium-induced gluon radiation from a single color charge as it traverses the medium.