# Measurement of the Shared Momentum Fraction $z_g$ using Jet Reconstruction in p+p and Au+Au Collisions with STAR

## TL;DR

The STAR collaboration measures the shared momentum fraction z_g in p+p and Au+Au collisions at 200 GeV, finding no significant modification in jet splitting functions despite di-jet imbalance, providing insights into parton energy loss models.

## Contribution

First measurement of the jet splitting function z_g in both p+p and Au+Au collisions at RHIC energies using SoftDrop grooming.

## Key findings

- No significant modification of z_g observed in Au+Au compared to p+p.
- Di-jet pairs show imbalance in Au+Au that is restored when soft constituents are included.
- Results support the universality of the Altarelli-Parisi splitting functions in heavy-ion collisions.

## Abstract

One key difference in current energy loss models lies in the treatment of the Altarelli-Parisi, AP, splitting functions. It has been shown that the shared momentum fraction, henceforth called Jet Splitting Function $z_g$ as determined by the SoftDrop grooming process can be made a Sudakov-safe measurement of the symmetrized AP functions in p+p collisions. The STAR collaboration presents the first z_g measurements at $\sqrt{s_{NN}}=200$ GeV in p+p and Au+Au collisions, where in Au+Au we use the specific di-jet selection introduced in our previous momentum imbalance measurement. For a jet resolution parameter of $R=0.4$, these di-jet pairs were found to be significantly imbalanced with respect to p+p, yet regained balance when all soft constituents were included. We find that within uncertainties there are no signs of a modified Jet Splitting Function on trigger or recoil sides of this di-jet selection.