Sar WorS 2025 - 4th Sardinian Workshop on Spin

11 Jun 2025, 09:00 → 13 Jun 2025, 16:55 Europe/Rome

A Workshop on the 3D structure of the nucleon: quark and gluon TMDs from SIDIS, pp collisions, e+e- annihilation processes and related quarkonium production.

Sar WorS 2025 is the fourth edition of the Sardinian Workshop on Spin and related issues, following the previous ones in 2019, and 2021 in Cagliari and the 2023 one in Nora.

The main objective of the workshop is to bring together, in a lively and informal environment, experts in the study of Transverse Momentum Dependent parton distributions and fragmentation functions (TMDs), to discuss the state of the art and provide new ideas and perspectives in the field, both on the theory and the experimental side. Young researchers will also have the unique opportunity to present their own work and projects in a highly stimulating context. The workshop will focus on open issues and new tools to investigate the 3D structure of the nucleon as well as the fragmentation mechanism within a TMD approach.
All presentations will serve as a starting point for a more in-depth common discussion on the main topics of the workshop and for a fruitful and educated brainstorming among participants.
The workshop will be held at the Hotel Baia di Nora in Pula (CA) and the meeting, from Wednesday morning at 9am to Friday afternoon at around 4 pm, is expected to be in-person. The number of participants is limited to about 30/35, and the number of presentations, upon invitation only, to around 20-25.
The workshop is supported by CEA Paris-Saclay, the Universities of Cagliari and Pavia and the INFN units of Cagliari and Pavia.

 

  

   

 

Conference picture

Sar-Wors-logo-2025.png

Wednesday, 11 June

Opening

Convener: Cristian Pisano (Istituto Nazionale di Fisica Nucleare)

TMD description of quarkonium production

Convener: Cristian Pisano (Istituto Nazionale di Fisica Nucleare)

1 cos 2 phi asymmetry in almost back-to-back production of J/\psi and jet at EIC and effect of TMD evolution

A back-to-back semi-inclusive J/ψ- jet production is a promising process to study gluon transverse momentum distribution (TMD) at the future electron-ion collider. A back-to-back configuration allows a higher transverse momentum for J/\psi We present an extension of a previous work where we studied cos 2φ azimuthal asymmetry within the TMD factorization framework for this process. We present and compare the effect of TMD evolution on the asymmetry, in two approaches that differ in the parametrization of the perturbative tails of the TMDs and the nonperturbative factors. We show that the asymmetry depends on the parametrizations of the nonperturbative Sudakov factors in the larger bT region and on the perturbative part of the evolution kernel. We use nonrelativistic QCD to estimate the J/ψ production and show the effect of using different long-distance matrix element sets. Overall, the asymmetry after incorporating TMD evolution is small, but increases with the transverse momentum imbalance of the J/ψ-jet pair.

Speaker: Asmita Mukherjee (Physics Department, IIT Bombay)

1-Asmita.pdf

2 TMD shape function and its effect in J/ψ photo- and electroproduction

In this talk, I present predictions for the unpolarized differential cross-section of J/ψ electroproduction at the Electron-Ion Collider (EIC) in the low transverse momentum region. I also compare with the experimental data for photoproduction at HERA. The calculation was performed at next-to-leading order (NLO) in the hard function and resummed at next-to-next-to-leading logarithmic accuracy in both the gluon transverse momentum dependent (TMD) parton distribution function and the TMD shape function with the shape function non-cusp anomalous dimension at NLO. The analysis emphasizes the impact of the TMD shape function on the theoretical predictions, with a focus on modeling choices and the associated theoretical uncertainties coming from different sources as scale variation and LDME sets. Finally, preliminary results for other relevant processes are also presented and briefly discussed.

Speaker: Samuel F. Romera (University of the Basque Country)

SarWorS25_SFR.pdf

3 Gluon TMDs from quarkonium production

Extraction of observables sensible to gluon TMDs is one of the main goals of many proposed high-energy experiments. In this talk, I will present results for the production of C-even quarkonium states in proton-proton collisions. The quarkonium formation mechanism is described within the framework of Non-Relativistic QCD (NRQCD). Our computation for the cross sections incorporates the polarization of both the incoming protons and we also show the results for the maximal values of the transverse single-spin asymmetries, demonstrating that these observables could, in principle, be measure at LHCSpin, a fixed target experiment planned at the LHC.

Speaker: Nanako Kato (Istituto Nazionale di Fisica Nucleare)

Kato_SarWorS.pdf

10:55 Coffee break

Fragmentation functions: Phenomenology

Convener: Francesco Murgia (Istituto Nazionale di Fisica Nucleare)

4 Modeling baryon production in polarized string fragmentation

We present an extension of the string+3P0 model of hadronization that includes the production and decays of polarized spin-1/2 baryons. The new model predicts: a Collins effect for baryon emission in transversely polarized quark fragmentations, the transverse spin transfer from the quark to the baryon, a baryon transverse polarization with respect to the production plane in unpolarized quark fragmentations. The latter effect emulates the polarizing TMD FF of quarks to baryons.

Speaker: Albi Kerbizi (Lund University and INFN Trieste)

Kerbizi_SarWors2025.pdf

5 A new extraction of di-hadron fragmentation functions, from BELLE $e^+ e^-$ data at $\sqrt{S} = 10.58$ GeV.

Di-hadron fragmentation functions represent an alternative method for extracting the transversity PDF, complementary to the Collins effect.
However, data on unpolarized inclusive cross sections for charged di-hadron production have only become available recently.
In this talk, I will present a new extraction of the unpolarized di-hadron fragmentation functions up to NNLO accuracy, carried out in collaboration with Paris-Saclay, using both a model-inspired and a neural network parameterization.

Speaker: Luca Polano (Istituto Nazionale di Fisica Nucleare)

SarWors_LPolano.pdf

6 First insight into transverse-momentum-dependent fragmentation physics at photon-photon colliders

Future planned lepton colliders, both in the circular and linear configurations, can effectively work as virtual and quasi-real photon-photon colliders and are expected to stimulate an intense physics program in the next few years. In this paper, we suggest to consider photon-photon scattering as a useful source of information on transverse momentum dependent fragmentation functions (TMD FFs), complementing semi-inclusive deep inelastic scattering and $e^+e^-$ annihilation processes, which provide most of the present phenomenological information on TMD FFs. As a first illustrative example, we study two-hadron azimuthal asymmetries around the jet thrust-axis in the process $\ell^+\ell^-\to\gamma^* \gamma\to q\bar q\to h_1 h_2 + X$, in which in a circular lepton collider one tagged, deeply-virtual photon scatters off an untagged quasi-real photon, both originating from the initial lepton beams, producing inclusively an almost back-to-back light-hadron pair with large transverse momentum, in the $\gamma^*\gamma$ center of mass frame. Similar processes, in a more complicated environment due to the presence of initial hadronic states, can also be studied in ultraperipheral collisions at the LHC and the planned future hadron colliders.

Speaker: Simone Anedda (Istituto Nazionale di Fisica Nucleare)

SarWorS2025.pdf

13:10 Lunch

TMD experiments at CERN

Convener: Isabella Garzia (Istituto Nazionale di Fisica Nucleare)

7 Spin, Structure, and Synergy: COMPASS and the Experiments of Tomorrow

The COMPASS experiment at CERN has played a pivotal role in advancing our understanding of the spin structure and dynamics of the nucleon, as well as the internal landscape of hadrons. With its versatile muon and hadron beams, COMPASS has delivered key insights into transverse momentum-dependent distributions (TMDs), generalized parton distributions (GPDs), and the nucleon’s spin puzzle. As we look ahead, COMPASS stands as a critical bridge to a new era of precision QCD studies, completing and complementing the physics programs of AMBER, the Electron-Ion Collider (EIC), the JLab 22 GeV upgrade, and emerging LHC spin initiatives. This talk will highlight recent COMPASS results, their theoretical impact, and how the experiment’s legacy and ongoing efforts synergize with the next generation of hadron structure investigations.

Speaker: Bakur Parsamyan (Istituto Nazionale di Fisica Nucleare)

Parsamyan_SarWorS-2025.pdf

8 LHCspin: a polarized fixed-target experiment at the LHC

A polarized gaseous target, operated in combination with the high-energy and high-intensity LHC beams, has the potential to open new physics frontiers and to deepen our understanding of the strong interaction in the non-perturbative regime of QCD. Specifically, the LHCspin project aims to perform spin-physics studies in high-energy polarized fixed-target collisions using the LHCb detector. Being designed and optimized for the detection of heavy hadrons, LHCb will allow to probe the nucleon structure through, e.g., the inclusive production of c- and b-hadrons, and ideal tool to access the essentially unexplored spin-dependent gluon TMDs. This configuration will allow to explore the nucleon internal dynamics at unique kinematic conditions, including the poorly explored high x-Bjorken and high x-Feynman regimes. With the installation of the proposed setup, LHCb will become the first experiment delivering simultaneously unpolarized beam-beam collisions at 14 TeV and both polarized and unpolarized beam-target collisions at center-of-mass energies of the order of 100 GeV. The status of the LHCspin project is presented along with a selection of physics opportunities.

Speaker: Prof. Luciano Libero Pappalardo (University of Ferrara)

Pappalardo_SarWors 2025.pdf

Discussion

Convener: Alessandro Bacchetta (Istituto Nazionale di Fisica Nucleare)

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Thursday, 12 June

Single spin asymmetries

Convener: Mariaelena Boglione (Istituto Nazionale di Fisica Nucleare)

9 NLO corrections and factorization for transverse single-spin asymmetries

We present next-to-leading order QCD corrections for the cross sections of
$\ell p^\uparrow\to (h\,\mathrm{or}\,jet)X$ with transversely polarized initial protons.
These cross sections are known to be power-suppressed in QCD
and probe twist-3 parton correlation functions in the proton. Our calculation exhibits the full complexity of a
perturbative QCD analysis beyond leading power, involving in particular various derivatives of the
parton correlation functions. We demonstrate that collinear factorization for these single-spin observables holds at one loop.
We also present exploratory phenomenological results for the NLO single-spin asymmetry in $ep^\uparrow\to hX$ and
compare to data from the HERMES experiment.

Speaker: Daniel Rein (Universität Tübingen)

rein_SarWorS_2025.pdf

10 Quark and gluon Sivers functions in dijets at the EIC

We explore the quark and the gluon Sivers functions in dijet production at the EIC. Our analysis includes a comprehensive investigation of the sin⁡($\phi_S - \phi_r$) modulated cross-sections for both quark and gluon channels. Furthermore, we state the possibility of re-defining the resummation method, altering the treatment of large logarithmic corrections. We also test different models for the gluon Sivers function, offering insights relevant to future experimental studies.

Speaker: Patricia Andrea Gutierrez Garcia (Universidad Complutense de Madrid)

patricia_gutierrez_sar_wors_2025.pdf

11 The Sivers effect in atomic Compton scattering

In this talk I present a QED-based analog of the Sivers effect in QCD. In particular, Compton scattering off an ionized Helium-4 target is used as a process to access the Sivers effect for the electron in the atomic target.
To account for the motion of the bound electron in the Helium target, hydrogen-like wave functions in momentum space are employed.
Both the unpolarized TMD distribution function and the Sivers function for the electron bound in the target are calculated. Predictions for the cross section and for the single spin asymmetry are discussed.

Speaker: Andrea Signori (Istituto Nazionale di Fisica Nucleare)

Signori_SarWors_2025.pdf

10:45 Coffee break

Collinear QCD

Convener: Dr Carlo Flore (Università di Cagliari e INFN, Sezione di Cagliari)

12 Heavy quark mass effects in the energy-energy correlation in the back-to-back region

We consider the Energy-Energy Correlation (EEC) in electron-positron
annihilation into hadrons at high energies, in the back-to-back (two-jet) region. We propose a simple formalism to consistently incorporate
heavy-quark mass effects in the QCD Sudakov form factor, which governs the resummation of large infrared logarithms. We also briefly discuss the extension of our results to the transverse momentum distribution in the Drell-Yan process.

Speaker: Giancarlo Ferrera (Milan University & INFN)

sarwors25-ferrera.pdf

13 Hadronic Cross Sections in Cosmic Antiproton Production & Diquark Capture of Seaquarks in Hadrons

Bringing collider-based QCD observables to the observed astrophysical excess of antiprotons in cosmic rays appears to be a useful cross-disciplinary venture. Astrophysical observations require a precise calculation of the cross sections for direct proton-proton to antiproton production vs. indirect production (in which the protons first produce an antineutron that decays down to an antiproton with branching fraction 1). Cross section calculations using PDFs and fragmentation functions for both 17.2 and 8000 GeV center-of-mass energies (i.e., NA49 and LHCb) tentatively find a significant discrepancy between the claimed ~30% difference in direct and indirect antiproton production and the predictions from the numerical studies. In addition to this study, and somewhat at right angles to it, the effects of the "diquark capture" model on higher Fock states of the proton will be discussed, particularly the mass and spin effects in the strange quark sector.

Speaker: Jennifer Rittenhouse West (INFN Turin and University of Turin)

JRW-SarWors2025_tiny_fixes.pdf

14 Parton distributions of light nuclei within the Poincaré covariant light-front approach

In this talk I will review and discuss the main results obtained from our collaboration regarding the study and calculations of various parton distributions for light nuclei including 2H, 3H, 3H and 4He. In particular, I will review the evaluation of the nuclear EMC effect 3H, 3H and 4He where the nuclear structure functions incorporated in the EMC ratio have been calculated within the relativistic Poincar´e covariant Light-Front approach, which permits the utilization of sophisticated nuclear wave-functions derived from realistic nuclear potentials [1, 2]. We show that while results for 3He align with experimental data, indicating that conventional nuclear physics adequately predicts the EMC ratio in this case, the agreement with data for 4He is less satisfactory. This outcome suggests that for 4He, effects beyond conventional nuclear physics must be incorporated to explain the EMC effect. This analysis therefore establishes a critical baseline for future calculations. This approach has been similarly applied to calculate spin-dependent 3He structure functions. Results demonstrate excellent agreement with experimental data, and we have proposed a procedure to extract the corresponding neutron structure functions. We also review findings from our studies of nuclear GPDs and their associated asymmetries. Specifically, we provide predictions 3He and for 4He we discuss the agreement with current experimental data [5, 6, 7, 8]. Finally, we present our recent investigation of double parton scattering off light nuclei, specifically to propose experiments at the future Electron Ion Collider. To this aim, we provide the first realistic and relativistic calculation of the 2H double parton distributions. In conclusion, these studies have significant relevance for forthcoming experimental programs at JLab and EIC.

References
[1] E. Pace, M. Rinaldi, G. Salmè and S. Scopetta, Phys. Lett. B 839, 137810 (2023)
[2] F. Fornetti, E. Pace, M. Rinaldi, G. Salmè, S. Scopetta and M. Viviani, Phys. Lett. B
851, 138587 (2024)
[3] M. Rinaldi and S. Scopetta, Phys. Rev. C 85, 062201 (2012)
[4] M. Rinaldi and S. Scopetta, Phys. Rev. C 87, no.3, 035208 (2013)
[5] S. Fucini, S. Scopetta and M. Viviani, Phys. Rev. D 101, no.7, 071501 (2020)
[6] S. Fucini, S. Scopetta and M. Viviani, Phys. Rev. C 102, 065205 (2020)
[7] S. Fucini, M. Rinaldi and S. Scopetta, Few Body Syst. 62, no.1, 3 (2021)
[8] S. Fucini, M. Hattawy, M. Rinaldi and S. Scopetta, Eur. Phys. J. A 57, no.9, 273 (2021)

Speaker: Matteo Rinaldi (Istituto Nazionale di Fisica Nucleare)

Rinaldi_Sar.pdf

13:00 Lunch

Drell-Yan processes

Convener: Asmita Mukherjee (Physics Department, IIT Bombay)

15 A new framework for TMD phenomenology beyond conventional approaches

The phenomenological extraction of Transverse Momentum Dependent (TMD) parton distributions is key to probing the three-dimensional structure of hadrons and gaining deeper insights into the strong force and Quantum Chromodynamics (QCD). While TMDs are rigorously defined through QCD operators, their extraction from experimental data is complicated by the dominance of non-perturbative effects at low energy scales.
Achieving agreement with data requires substantial modeling, extending beyond the simple fitting of unknown non-perturbative functions at low transverse momentum. In this talk, I will examine the impact of these choices through a phenomenological analysis of low-energy Drell–Yan data, comparing different prescriptions and evaluating their respective strengths and limitations. I will go beyond conventional treatments by revisitin $q_T$-resummation and presenting a novel procedure that enables consistent computation directly in momentum space, while completely avoiding phenomenological biases. This new approach isolates the true non-perturbative core of transverse-momentum physics, rooted in the deep infrared behavior of the strong coupling and the DGLAP splitting functions.

Speaker: Andrea Simonelli (Istituto Nazionale di Fisica Nucleare)

SarWors25.pdf

16 Description of the full $q_T$ spectrum of low $Q$ Drell-Yan production

In this talk, we present a description at the highest available perturbative accuracy of the complete transverse-momentum $q_T$ spectrum in low-invariant-mass Drell–Yan production, with special attention to the challenging high-$q_T$ region.

Speaker: Lorenzo Rossi (Istituto Nazionale di Fisica Nucleare)

SarWors.pdf

Discussion

Convener: Dr Valerio Bertone (CEA Paris-Sclay)

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Friday, 13 June

TMD factorization and resummation

Convener: Andrea Signori (Istituto Nazionale di Fisica Nucleare)

17 Affinity tool for kinematic region analysis

I will present a new, refined, version of the "affinity" tool, based on an Monte Carlo event-by-event mapping of the SIDIS kinematic regions. Affinity is a diagnostic tool that allows us to quantify our level of confidence in the application of the appropriate factorization theorem (TMD or collinear) in each kinematic bin (x,Q^2,z,P_T) in which the final hadron is produced in SIDIS processes.

Speaker: Prof. Mariaelena Boglione (Istituto Nazionale di Fisica Nucleare)

Boglione_SarWorS25.pdf

18 Joint thrust and TMD resummation in SIDIS

We present the framework for obtaining precise predictions for the transverse momentum of hadrons with respect to the thrust axis in ep collisions. This will enable a precise extraction of TMD parton distributions. To this end, we make use of the derived factorization theorems within Soft Collinear Effective Theory, and calculate and renormalize the present functions. That is, the thrust-soft, thrust-TMD soft, collinear-soft and hemisphere-TMD-thrust soft functions with the delta regulator at NLO. Finally, we present our predictions for the cross-section differential in transverse momentum and thrust for SIDIS at NNLO.

Speaker: Daniel Díaz Fernández (Universidad Complutense de Madrid)

Presentación_Sar_Wors.pdf

19 Resummations and approximations for TMDs

I will show recent progresses in the treatment of TMD in the large-x limit and in the leading-TMD approximations. These treatments of the TMD are particularly useful when perturbative information is still not precise enough and allow to make predictions on higher-twist TMD, TMD moments, and angularities.

Speaker: Ignazio Scimemi (Universidad Complutense Madrid)

SarsWors2025Scimemi.pdf

10:45 Coffee break

Fragmentation functions: Experiments & Phenomenology

Convener: Umberto D'Alesio (Istituto Nazionale di Fisica Nucleare)

20 Selected results on hadronization in e+e-, hadron-hadron, and lepton-hadron interactions

Fragmentation functions as well as parton distributions are fundamental non-perturbative objects that allow knowledge obtained from different processes to be used for the understanding of other processes through QCD factorization theorems. They also serve as a test bed of the theoretical framework. Recent results on hadronziation from several key processes: hadron-hadron interaction, e+e- annihilation, and deep-inelastic lepton-nucleon scattering, with a particular focus on strange and charmed hadron production, will be reviewed.

Speaker: Gunar Schnell (University of the Basque Country UPV/EHU)

21 Fragmentation function studies at BESIII

Fragmentation functions play a crucial role in describing the hadronization process.
We report the measurements of normalized differential cross sections of inclusive pi0, Ks, and eta production as a function of hadron momentum at six energy points with q^2 transfer from 5 to 13 GeV^2 at BESIII. In addition, new recent results of single inclusive charged pions and kaons at center of mass energies from 2.000 to 3.671 GeV are presented. The comparisons with several theoretical models are also discussed.

Speaker: Isabella Garzia (Istituto Nazionale di Fisica Nucleare)

Pula_Garzia.pdf

22 Tests of TMD FFs universality

In this talk, we present some recent updates on the TMD fragmentation functions (FFs), namely the unpolarized and the Collins FFs. The impact of $A_N$ data measured in inclusive processes is estimated through an extended version of the Bayesian reweighting method, and the universality properties of these functions are tested through estimates of the Collins asimmetry for $\pi$-jet production in $pp$ collisions.

Speaker: Dr Carlo Flore (Università di Cagliari e INFN, Sezione di Cagliari)

FF-universality-talk-SarWorS2025.pdf

13:00 Lunch

Discussion

Convener: Marco Radici (Istituto Nazionale di Fisica Nucleare)

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Closing