Dipartimento di Fisica

Descrizione del progetto

The formalism of transverse-momentum-dependent (TMD) factorization in Quantum Chromodynamics (QCD) is the cornerstone of all phenomenological studies of the hadron structure and of the hadronization mechanism in three-dimensional momentum space. TMD factorization allows us to connect specific measurements of cross sections or asymmetries with TMD parton distribution functions (PDFs) and fragmentation functions (FF), which characterize the “internal structure” of the QCD bound states (pions, nucleons, etc.) in terms of the elementary degrees of freedom of the theory, quarks and gluons [1]. Given the nonperturbative nature of QCD at the scales comparable to hadron masses, these TMD functions need to be studied by complementing perturbation theory with nonperturbative inputs, such as models, lattice QCD computations, or determinations from experimental data.

Nonperturbative effects associated with TMD functions can be observed in processes like Semi-Inclusive Deep Inelastic Scattering (SIDIS), where a lepton scatters off a hadron target and one other hadron is detected in the final state. In principle, comparison of experimental results with models, within a QCD approach, allows us to determine TMD PDFs and FFs. The intrinsic properties of hadrons are more readily accessible at low energies, that is when the virtuality Q of the photon that probes the hadronic target is of the order of a few GeV. In practice, this fact challenges TMD factorization because, for these values of Q, the (power) corrections to the approximations behind factorization theorems render large errors in cross section computations. Thus, in order to attain a reliable picture of hadronic structure, it becomes a necessity to improve those approximations made in TMD factorization and to reduce model dependence as much as possible.
It is also essential to demarcate the relevant kinematic regions where such improvements are more critical. Some theoretical progress has recently been made in these two respects. First, in a recent work by Prof. M. Boglione (Turin) and Prof. A. Prokudin (Pennsylvania State University) [2], where the kinematic regions of SIDIS were studied in detail and a dedicated computational tool was developed. Second, in the recent work by Prof. Ted C. Rogers (ODU), in collaboration with Dr. Gonzalez-Hernandez (Turin), where the low energy limit of TMD factorization was addressed [3]. However, these recent developments have yet to be implemented in phenomenological analyses to determine TMDs. In fact, the most recent determinations of TMDs still show some tension between theoretical cross section calculations and experimental data at moderate scales.
This project aims at relieving this tension between theory and measurements, by improving specific aspects of the formalism at low energies and quantifying the relevance of the improvements at the phenomenological level.
In particular, the factorization formalism that allows us to explore 1D and 3D hadron structure in momentum space can be connected up to power corrections of 1/Q, which need to be understood and kept under control. At the experimental level, multiplicities (cross section ratios) are particularly sensitive to these corrections [4]. By joining our expertise with researchers from Pennsylvania State University and from Jefferson Lab, we aim at classifying these corrections and testing their impact on the determinations of TMD distributions from SIDIS multiplicities.

This research line involves all the staff members of the Theoretical hadron physics group in Turin (Prof. Boglione, Dr. Gonzalez-Hernandez, Dr. Nocera, Dr. Signori), Prof. Alexey Prokudin from Pennsylvania State University and Prof. Ted Rogers from Old Dominion University and Jefferson Lab. To nurture a long-term collaboration among the involved Institutions, we also plan to involve a younger workforce, in particular Tommaso Rainaldi (PhD candidate - Old Dominion University and Jefferson Lab ), Patrick Barry (postdoc - Jefferson Lab), and Andrea Simonelli (postdoc - Old Dominion University and Jefferson Lab).