Di-hadron fragmentation and mapping of the nucleon structure

TL;DR

This paper reviews how di-hadron fragmentation functions serve as tools to probe nucleon structure, transversity, twist-3 distributions, and potential CP-violation effects, highlighting their theoretical formalism, experimental observables, and future research directions.

Contribution

It provides a comprehensive overview of di-hadron fragmentation functions, emphasizing their role in mapping nucleon structure and accessing fundamental QCD properties.

Findings

Extraction of transversity distribution via di-hadron asymmetries

Connection of di-hadron functions to twist-3 distribution e(x)

Potential to explore QCD vacuum CP-violation effects

Abstract

The fragmentation of a colored parton directly into a pair of colorless hadrons is a non-perturbative mechanism that offers important insights into the nucleon structure. Di-hadron fragmentation functions can be extracted from semi-inclusive electron-positron annihilation data. They also appear in observables describing the semi-inclusive production of two hadrons in deep-inelastic scattering of leptons off nucleons or in hadron-hadron collisions. When a target nucleon is transversely polarized, a specific chiral-odd di-hadron fragmentation function can be used as the analyzer of the net density of transversely polarized quarks in a transversely polarized nucleon, the so-called transversity distribution. The latter can be extracted through suitable single-spin asymmetries in the framework of collinear factorization, thus in a much simpler framework with respect to the traditional one in single-hadron fragmentation. At subleading twist, the same chiral-odd di-hadron fragmentation function provides the cleanest access to the poorly known twist-3 parton distribution $e(x)$, which is intimately related to the mechanism of dynamical chiral symmetry breaking in QCD. When sensitive to details of transverse momentum dynamics of partons, the di-hadron fragmentation functions for a longitudinally polarized quark can be connected to the longitudinal jet handedness to explore possible effects due to $CP-$violation of the QCD vacuum. In this review, we outline the formalism of di-hadron fragmentation functions, we discuss different observables where they appear and we present measurements and future worldwide plans.