The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}} =200\mathrm{GeV}$. The substructure observables include SoftDrop groomed momentum fraction ($z_g$), groomed jet radius ($R_g$), and subjet momentum fraction ($z_{\mathrm{SJ}}$) and opening angle (${\theta }_{\mathrm{SJ}}$). 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 ${\theta }_{\mathrm{SJ}}$ distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in $\mathrm{Au}+\mathrm{Au}$ collisions. We compare data from $\mathrm{Au}+\mathrm{Au}$ collisions to those from $p+p$ which are embedded in minimum-bias $\mathrm{Au}+\mathrm{Au}$ 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 ${\theta }_{\mathrm{SJ}}$. We find no qualitative differences in energy loss signatures for varying angular scales in the range $0.1< {\theta }_{\mathrm{SJ}}<0.3$, 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.

• Received 20 September 2021
• Accepted 11 January 2022

DOI:https://doi.org/10.1103/PhysRevC.105.044906