Probing the hardest branching of jets in heavy ion collisions

TL;DR

This paper presents the first calculation of the distributions of momentum sharing and angular separation between leading subjets in heavy ion collisions, revealing significant early modifications in the parton shower compared to proton-proton collisions.

Contribution

It introduces a novel theoretical framework for analyzing the hardest jet branchings in heavy ion collisions using soft-collinear effective theory with Glauber gluons.

Findings

Qualitative and quantitative agreement with CMS data

Early in-medium modifications of jet shower observed

Proposes new measurement for angular distribution of hardest branching

Abstract

We present the first calculation of the momentum sharing and angular separation distributions between the leading subjets inside a reconstructed jet in heavy ion collisions. These observables are directly sensitive to the hardest branching in the process of jet formation and are, therefore, ideal for studying the early stage of the in-medium parton shower evolution. The modification of the momentum sharing and angular separation distributions in lead-lead relative to proton-proton collisions is evaluated using the leading-order medium-induced splitting functions obtained in the framework of soft-collinear effective theory with Glauber gluon interactions. Qualitative and in most cases quantitative agreement between theory and preliminary CMS measurements suggests that the parton shower in heavy ion collisions can be dramatically modified early in the branching history. We propose a new measurement which will illuminate the angular distribution of the hardest branching within jets in heavy ion collisions.