# Jet sub-structure and parton shower evolution in p+p and Au+Au collisions at STAR

## TL;DR

This paper presents detailed jet sub-structure measurements in p+p and Au+Au collisions at STAR, providing insights into parton shower evolution and jet quenching effects at different angular scales.

## Contribution

It offers the first measurements of jet sub-structure observables and their evolution in p+p and Au+Au collisions at STAR, advancing understanding of jet quenching and medium response.

## Key findings

- Jet sub-structure observables in p+p at 200 GeV are measured and compared to simulations.
- First measurement of jet angular structure ($ heta_{SJ}$) in Au+Au collisions at STAR.
- No significant difference in jet quenching observed across different angular scales.

## Abstract

Recent measurements of jet structure modifications at RHIC and LHC highlight the importance of differential measurements to study the nature of jet quenching. Since these jet structure observables are intimately dependent on parton evolution in both the angular and energy scales, measurements are needed to disentangle these two scales in order to probe the medium at different length scales to study its characteristic properties such as the coherence length. To that effect, the STAR collaboration presents fully unfolded results of jet sub-structure observables designed to extract fundamental quantities related to the parton shower via the SoftDrop shared momentum fraction, the groomed jet radius, and the jet Mass in p+p collisions at 200 GeV as a function of jet transverse momenta. We also showcase the first measurement of iterative softdrop groomed observables for first, second and third splits with an initiator prong transverse momenta ranging from 20-25 GeV. In comparing the un-corrected data to our simulation, we are able to look at snapshots of the jet clustering history leading towards an understand of the time evolution of the parton shower. Having established the p+p baseline, we present the first measurement of the jet's inherent angular structure in Au+Au collisions at 200 GeV via an experimentally robust observable the opening angle between the two leading sub-jets ($\theta_{SJ}$). In Au+Au collisions at STAR, we utilize a specific di-jet selection as introduced in our previous momentum imbalance ($A_{J}$) measurement and measure both the $A_{J}$ and the recoil jet spectra differentially as a function of the angular classes based on the $\theta_{SJ}$ observable. With such measurements, we probe the medium response to jets at a particular resolution scale and find no significant differences in quenching for jets of different angular scales as given by $\theta_{SJ}$.