A look inside the proton: polarized transverse momentum dependent parton distributions for modern collider experiments

One of the big open challenges in contemporary particle physics is the quest for the three-dimensional structure of the proton, a principal building block of all visible matter.

According to the theory of Quantum Chromodynamics, which is well-established since the mid-seventies, the proton is a composite particle built from elementary point-like particles known as quarks and gluons, or collectively 'partons'.

A simple picture in which these quarks and gluons are quasi-free and move alongside their parent proton, is often very successful in describing experiments at particle colliders.

However, this so-called 'collinear' framework is challenged in many experiments that are sensitive to the internal motion of the partons or to the spin degrees of freedom.

Moreover, being strictly one-dimensional, it cannot account for some of the proton's main properties such as its spin, a long-standing and still unresolved problem known as the proton spin puzzle.

Hence, in order to formulate an answer to such fundamental questions, it is necessary to explore the full three-dimensional structure of the proton.

This structure needs to be extracted from experimental data, and can be encoded into so-called transverse momentum dependent parton distribution functions (TMDs).

In this research project, I will study polarized TMDs, which are nowadays still very poorly known but are key to solve the aforementioned puzzles.

For my study, I will capitalize on a new approach to TMD phenomenology, the Parton Branching method, as well as recent data from COMPASS at CERN, HERMES at DESY, and Hall A at JLab.

This project is extremely timely, since the study of TMDs is a driving force behind several proposed new experiments in the US and in Europe, such as the recently approved Electron-Ion Collider.