Learning the 3D structure of the proton – 3DLeaP

Protons make up a large part of the visible Universe and yet our understanding of their structure is surprisingly limited. Although Quantum Chromodynamics (QCD) has the potential to explain their emergence in terms of quarks and gluons, it presents us with formidable challenges when it comes to extracting quantitative information.

After decades of intense study, our knowledge of the 1D longitudinal structure of the proton, embodied in the collinear distributions, has reached a very high precision. 3DLeaP aims to obtain a comparably accurate 3D picture of the proton in terms of momentum and position distributions of quarks and gluons.

The specific objectives of 3DLeaP are:
1. the extraction of transverse-momentum distributions (TMDs) and generalised parton distributions (GPDs) that encode momentum and position distributions of quarks and gluons within the proton, respectively;
2. the determination of the joint momentum-position distributions as encoded in generalised TMDs (GTMDs).

TMDs and GPDs will be determined separately employing state-of-the-art theoretical calculations and a large experimental data set. GTMDs will then be extracted using the results of the previous step by means of a theoretically sound procedure.

Recent theoretical and experimental advances in the field of 3D proton structure make 3DLeaP particularly timely. Its realisation will deliver a detailed picture of the proton with important consequences ranging from stringent tests of QCD - shedding light on topics such as the proton spin decomposition - to the search of new phenomena at high-energy colliders such as the Large Hadron Collider and the future Electron-Ion Collider.