A journey into the proton structure: Progresses and challenges

TL;DR

This paper reviews recent progress and ongoing challenges in understanding the three-dimensional structure of protons, focusing on transverse momentum dependent parton densities and their relation to high-energy gluon distributions.

Contribution

It provides an overview of advances in extracting TMD parton densities, especially for polarized gluons, and discusses their connection to unintegrated gluon distributions in high-energy physics.

Findings

Progress in extracting gluon TMDs

Identification of challenges in polarization measurements

Connections established between TMDs and UGDs

Abstract

Unraveling the inner dynamics of gluons and quarks inside nucleons is a primary target of studies at new-generation colliding machines. Finding an answer to fundamental problems of Quantum ChromoDynamics, such as the origin of nucleon mass and spin, strongly depends on our ability of reconstructing the 3D motion of partons inside the parent hadrons. We present progresses and challenges in the extraction of TMD parton densities, with particular attention to the ones describing polarization states of gluons, which still represent an largely unexplored field. Then, we highlight connections with corresponding parton densities in the high-energy limit, the so-called unintegrated gluon distributions or UGDs and, more in general, to recent developments in high-energy physics.