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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - 3DSPIN (3-Dimensional Maps of the Spinning Nucleon)

Teaser

Summary

How does the inside of the proton look like? What generates its spin?

3DSPIN will deliver essential information to answer these questions at the frontier of subnuclear physics.

Nucleons (protons and neutrons) are made of quarks and gluons, collectively called partons, confined inside nucleons by the forces described by Quantum Chromodynamics (QCD). Mapping the inner structure of the nucleon in higher and higher details is an essential step to better understand QCD and confinement.

3DSPIN will lead the way into a new stage of nucleon mapping, explore the distribution of quarks in full 3D momentum space and obtain unprecedented information on orbital angular momentum.

The goals of 3DSPIN are
1. extract from experimental data the 3D distribution of quarks (in momentum space), as described by Transverse-Momentum Distributions (TMDs);
2. obtain from TMDs information on quark Orbital Angular Momentum (OAM).

So far, we have detailed maps of the distribution of quarks and gluons in the nucleon in 1D (as a function of their momentum in a single direction). They are encoded in the well-known Parton Distribution Functions (PDFs). We also know that quark spins account for only about 1/3 of the spin of the nucleon.

Transverse Momentum Distributions (TMDs) are extensions of standard PDFs: they map the partonic distribution in 3D momentum space. The first goal of the research project is to reconstruct these three-dimensional maps from experimental data, a challenge at the frontiers of hadronic physics.

The situation may be compared to protein studies: one-dimensional mapping corresponds to knowing the sequence of amino acids. It is an extremely important piece of information, but insufficient to understand proteins. The study of their 3D structure literally revolutionized our understanding of protein chemistry. Similarly, 3D partonic imaging can bring exceptional developments in our understanding of the nucleon.

Out of the many questions that we can address using detailed 3D maps of the nucleon, one of the most important is the origin of the spin of the nucleon. Nowadays we know the spins of partons account for only about one-third of the total spin of the nucleon. Parton Orbital Angular Momentum (OAM) is still poorly known and should account for the remaining part. The 3DSPIN project will follow an innovative approach to quantify quark angular momentum, through its connections to the parton distribution in the full three-dimensional parton momentum space. This is the second goal of the project.

Work performed

3DSPIN is leading the way into a new phase in the study of Transverse Momentum Distributions (TMDs) and Orbital Angular Momentum (OAM).

In the last 18 months, 3DSPIN has brought the analysis of unpolarized TMDs to a new level of sophistication, in particular by including TMD evolution in the most effective way, critically considering the assumptions made in fitting procedures, simultaneously including measurements from different processes. No other existing analysis takes into consideration all these issues. We are performing the first global fit of unpolarized TMDs (distribution and fragmentation functions) and we are reaching a level comparable to the early global fits of standard PDFs. Results are almost ready for publication.

Final results

Mapping the 3D structure of chemical compounds has revolutionized chemistry. Similarly, mapping the 3D structure of the nucleon will have a deep impact on our understanding of the fundamental constituents of matter. 3DSPIN can open new perspectives on the dynamics of quarks and gluons and sharpen our view of high-energy processes involving nucleons.

Accurate three-dimensional maps of the inner structure of the nucleon can serve two purposes: on one hand, they can be used as tools to study any high-energy phenomena involving nucleons, make predictions, and possibly lead to the discovery of signs of physics beyond the Standard Model; on the other hand, they allow us to understand the dynamics of partons inside the nucleons and shed light on confinement, one of the deepest questions of subnuclear physics.

3DSPIN aims at estimating quark angular momenta through the study of TMDs. It will be part of a wider endeavor, including the study of collinear parton distributions (PDFs), generalized parton distributions (GPDs), gluon TMDs, and lattice QCD calculations. It is conceivable that, putting together all information obtained in the next twenty years, we will be able to give an unambiguous answer to the question of what generates the spin of the nucleon.

The results of our research can influence the plans for future experimental activities. In the near future: COMPASS II at CERN, and Hall A, B, C at Jefferson Lab. In the longer term: PANDA and PAX at GSI (Germany), LHeC (Large Hadron-electron Collider at CERN), AFTER (fixed-target experiment at LHC at CERN), EIC (electron-ion collider in the US). Also facilities that are not focused on hadronic physics will benefit from the outcomes of our project. For instance, the LHC relies on a detailed knowledge of hadron structure. Although TMDs do not appear in as many observable as PDFs, they are essential to understand several measurements. For instance, they can have an impact also on the determination of the W-boson mass, one of the essential parameters of the Standard Model. B factories (e.g., Belle at KEK, Japan) focus on flavor physics, but can be used also to study the physics of fragmentation functions.

On top of strictly technical results, the research project will have side effects on educational activities. Graduate students directly involved in the project will have the opportunity to learn research by working on a gratifying project, in a young group with an excellent worldwide reputation. Finally, our project will consolidate the world-class reputation of the European community working on hadronic physics and make Europe attractive for foreign researchers in this field.