arXiv subjects

Marco Zaccheddu

Publications and source records attributed to Marco Zaccheddu.

13 recordsLinked to original sources

arXiv2026

A Bayesian Pixel Based Approach for Model Independent TMD Reconstruction

We introduce a nonparametric pixel-based framework for the Bayesian inference and imaging of transverse momentum dependent (TMD) parton distributions. The methodology integrates TMD evolution within the Collins-Soper-Sterman formalism in a differentiable framework, and leverages generative AI through a hybrid normalizing flow-driven Metropolis-Hastings algorithm for efficient posterior sampling. The framework is validated through multi-scale closure tests of increasing complexity. Using singular value decomposition, we characterize the existence of null TMDs, functional components that remain unconstrained by observables, and demonstrate how multi-scale data break these degeneracies, enabling 3D partonic imaging.

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arXiv2026

Collins effect in pion-in-jet production in polarized $pp$ and $ep$ collisions

We study Collins azimuthal asymmetries for pion-in-jet production in polarized proton-proton and lepton-proton collisions. We adopt a hybrid transverse momentum dependent approach, with a collinear configuration for the initial state, and employ the transversity and Collins fragmentation functions extracted from semi-inclusive deep inelastic scattering and $e^+e^-$ annihilation data. After recalling the good description of the STAR data in $pp$ collisions, which supports the universality of the Collins function, we present predictions for Electron-Ion Collider kinematics, both at leading order and by including the quasireal photon exchange in the Weizs\"acker-Williams approximation. This contribution is sizable but does not spoil the dominance of quark-initiated channels. This implies that $\ell p$ processes allow for a clearer access to the transversity distribution, including its sea-quark component.

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arXiv2026

TMDs in the Lens of Generative AI: A Pixel-Based Approach to Partonic Imaging

This work introduces a novel, nonparametric pixel-based framework for the Bayesian inference and imaging of transverse momentum dependent (TMD) parton distributions. The methodology is built upon a fully differentiable framework that integrates TMD evolution with the Collins-Soper-Sterman formalism, enabling the simultaneous extraction of partonic distributions and the nonperturbative evolution kernel. To achieve efficient and exact sampling of the high-dimensional posterior, we leverage generative AI through a hybrid normalizing flow-driven Metropolis-Hastings approach. The framework is validated through multi-scale closure tests of increasing complexity, ranging from basic functional models to convoluted structure functions. Using singular value decomposition (SVD), we rigorously characterize the uncertainty of the reconstructed distributions and reveal the existence of null TMDs, which are functional components in the null space of the kernel that remain unconstrained by observables. The new framework provides the first integration of pixel-based discretization, generative AI, and SVD within a Bayesian context to solve the TMD inverse problem. This synergy between machine learning and multi-scale data removes inherent degeneracies and enables unbiased 3D partonic imaging.

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arXiv2026

Collins asymmetries for pion-in-jet production in polarized $\ell p$ collisions at the EIC

We study Collins azimuthal asymmetries for pion-in-jet production in polarized lepton-proton collisions, extending previous analyses of polarized $pp$ scattering to a complementary and theoretically simpler process. We keep adopting a simplified transverse momentum dependent (TMD) approach, with a collinear configuration for the initial state, and employ the transversity and Collins fragmentation functions as extracted from semi-inclusive deep inelastic scattering and $e^+ e^-$ annihilation processes. We then compute azimuthal asymmetries for the Electron-Ion Collider (EIC) kinematics, both within a leading order (LO) approach and by including quasireal photon exchange in the Weizs\"acker-Williams approximation. Although this contribution is relevant in the whole kinematical range explored, it does not spoil the dominance of quark-initiated channels, leaving only a marginal role to their gluon counterparts. In this respect, Collins asymmetries in lepton-proton processes allow for a much clearer access to the transversity distribution, including its sea-quark component. As we will argue, a comparison with future EIC data could represent a further step in testing the hypothesis of the universality of the Collins function as well as of the TMD factorization for this class of processes.

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arXiv2025

Collins function for pion-in-jet production in polarized $pp$ collisions: a test of universality and factorization

We present an updated study of the Collins azimuthal asymmetries for pion-in-jet production in polarized $pp$ collisions. To this aim, we employ a recent extraction of the transversity and Collins fragmentation functions from semi-inclusive deep inelastic scattering and $e^+ e^-$ annihilation into hadron pairs processes, obtained within a simplified transverse momentum dependent (TMD) approach at leading order in the strong coupling constant $\alpha_s$. In the present case we adopt a collinear configuration for the initial state, keeping transverse momentum effects only in the fragmentation mechanism. Our theoretical estimates, when compared against 200~GeV and 510~GeV data from the STAR Collaboration, show a generally good agreement for the distributions in the transverse momentum of the jet, the pion longitudinal momentum fraction and its transverse momentum with respect to the jet direction. While not being a proof, due the assumptions and limitations behind the entire approach, these findings corroborate the hypothesis of TMD factorization for such processes as well as of the universality of the Collins function and, once again, of a reduced impact of the proper TMD evolution on azimuthal asymmetries. We will also present predictions based on an extraction of the Collins and transversity distributions where information from data on single spin asymmetry for inclusive pion production in $p^\uparrow p$ collisions is included through a Bayesian reweighting procedure.

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arXiv2024

Transverse $Λ$ polarization in unpolarized $pp\rightarrow \, {\rm jet}\, Λ^\uparrow\, X $

In this Letter, we investigate the spontaneous transverse polarization of $Λ$ hyperons produced in unpolarized $pp$ collisions inside a jet, by adopting a TMD approach where transverse momentum effects are included only in the fragmentation process. We will present predictions based on the parametrizations of the $Λ$ polarizing fragmentation function as extracted from fits to Belle $e^+e^-$ data. These estimates will be compared against preliminary STAR data. We will then be able to explore the universality properties of the quark polarizing fragmentation function and, for the first time, the role of its gluon counterpart.

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arXiv2023

Transverse $Λ$ polarization in $e^+e^-$ annihilations and in SIDIS processes at the EIC within TMD factorization

We present a phenomenological study on the role of charm contribution and $SU(2)$ isospin symmetry in the extraction of the $Λ$ polarizing fragmentation functions from $e^+e^- \to Λ^\uparrow (\barΛ^\uparrow) \,h + X$ annihilation processes. We adopt the well-established transverse-momentum-dependent factorization formalism, within the Collins-Soper-Sterman evolution scheme at next-to-leading logarithm accuracy, carefully exploiting the role of the nonperturbative component of the polarizing fragmentation function. We then discuss the impact of these results on the predictions for transverse $Λ$, $\barΛ$ polarization in semi-inclusive deep inelastic scattering processes at typical energies of the future Electron-Ion Collider.

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arXiv2022

Transverse $Λ$ polarization in $e^+e^-$ processes within a TMD factorization approach and the polarizing fragmentation function

We perform a re-analysis of Belle data for the transverse $Λ$ and $\barΛ$ polarization in $e^+ e^-$ annihilation processes within a transverse momentum dependent (TMD) factorization approach. We consider two data sets, one referring to the associated production of $Λ$'s with a light unpolarized hadron in an almost back-to-back configuration, and one for the inclusive $Λ$ production, with the reconstruction of the thrust axis. We adopt the Collins-Soper-Sterman framework and employ the recent formulation on the factorization of single-inclusive hadron production. This extends a previous phenomenological study carried out in a more simplified TMD approach, leading to a new extraction of the polarizing fragmentation function (FF). While confirming several features of the previous analysis, here we include the proper QCD scale dependence of this TMD FF and carefully exploit the role of its nonperturbative component. The compatibility, within a unique TMD factorization scheme, of double and single-inclusive hadron production is discussed in detail. Moreover, we consider another data set for inclusive $Λ$ production, at much larger energies, from the OPAL Collaboration, in order to test the consistency of the entire approach. Finally, we elaborate on some fundamental issues like the role of $SU(2)$ isospin symmetry and the heavy quark contributions.

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arXiv2022

Extracting the $Λ$ Polarizing Fragmentation Function from Belle $e^+e^-$ Data Within the TMD Formalism

Data from the Belle Collaboration for associated production (with a light unpolarized hadron) and single-inclusive production of transversely polarized $Λ$-hyperons in $e^+e^-$ annihilation processes allowed to extract within a TMD approach, for the first time, the $Λ$ polarizing fragmentation function. Recent theoretical developments on the computation of cross sections for single-inclusive hadron production in $e^+e^-$ annihilation processes, within a Soft Collinear Effective Theory approach, combined with the CSS formalism for the double-hadron production case, can be used to perform a renewed analysis, adopting a full TMD factorization scheme. Preliminary results and a comparison between the two analyses are presented.

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arXiv2022

General Helicity Formalism for Two-hadron Production in $e^+e^-$ Collisions and the $Λ$ Polarizing Fragmentation Function

We present the complete structure of the azimuthal dependences and polarization observables for two-hadron production in $e^+e^-$ annihilation processes within a transverse momentum dependent (TMD) approach adopting the helicity formalism. The leading-twist TMD fragmentation functions (TMD-FFs) for spin-1/2 hadrons are fully accounted for. The role of the polarizing FF, together with its extraction from Belle data for the transverse polarization of $Λ$'s, are discussed as well. Finally, predictions for SIDIS processes at EIC are presented.

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arXiv2021

General helicity formalism for two-hadron production in $e^+e^-$ annihilation within a TMD approach

We present the complete leading-order results for the azimuthal dependences and polarization observables in $e^+e^-\to h_1 h_2 + X$ processes, where the two hadrons are produced almost back-to-back, within a transverse momentum dependent (TMD) factorization scheme. We consider spinless (or unpolarized) and spin-1/2 hadron production and give the full set of the corresponding quark and gluon TMD fragmentation functions (TMD-FFs). By adopting the helicity formalism, which allows for a more direct probabilistic interpretation, single- and double-polarization cases are discussed in detail. Simplified expressions, useful for phenomenological analyses, are obtained by assuming a factorized Gaussian-like dependence on intrinsic transverse momenta for the TMD-FFs.

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arXiv2021

$Λ$ polarizing fragmentation function from Belle $e^+e^-$ data

Experimental data from Belle Collaboration for the transverse polarization of $Λ$'s measured in $e^+ e^-$ annihilation processes are used to extract the polarizing fragmentation function (FF) within a TMD approach. We consider both associated and inclusive $Λ$ production, showing a quite consistent scenario. Good separation in flavor is obtained, leading to four independent FFs. Predictions for SIDIS processes at the EIC, crucial for understanding their universality and evolution properties, are also presented.

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arXiv2020

First extraction of the $Λ$ polarising fragmentation function from Belle $e^+e^-$ data

We present a thorough phenomenological analysis of the experimental data from Belle Collaboration for the transverse $Λ$ and $\barΛ$ polarisation, measured in $e^+e^-$ annihilation processes, for the case of inclusive (plus a jet) and associated production with a light charged hadron. This allows for the first ever extraction of the quark polarising fragmentation function for a $Λ$ hyperon, a transverse momentum dependent distribution giving the probability that an unpolarised quark fragments into a transversely polarised spin-1/2 hadron.

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