16th COMPASS Analysis Phase international mini-workshop (COMAP-XVI)

21 May 2026, 13:25 → 25 May 2026, 20:15 Europe/Zurich

892/1-B09 (CERN)

Bakur Parsamyan (AANL, Turin section of INFN and CERN), Alessia Bongallino (Universidad del PaÍs Vasco (UPV/EHU)), Andrea Simonelli (INFN Roma 1), Lorenzo Rossi (University of Milan & INFN)

16th COMPASS "Analysis Phase" international mini-workshop will take place on May 21-22, 2026 at CERN in Geneva, Switzerland.
The workshop will be held in a hybrid mode.

The scientific program of the series of COMPASS "Analysis Phase" international mini-workshops is focused on the topics traditionally addressed by the COMPASS collaboration:

• Spin and 3D Structure of the Nucleon
• DIS, SIDIS, DVCS, DVMP, Drell-Yan measurements
• TMDs, GPDs and GTMDs
• Fragmentation Functions
• Meson Structure and Spectroscopy
• Search for Exotics
• Monte-Carlo simulation tools and techniques
• Technical aspects and analysis techniques

 

The 16th edition will focus on Drell–Yan cross-sections and phenomenology, as well as flavour-dependent TMD PDFs, helicity TMD PDFs, and comparisons of global analyses and models.

The workshop aims to foster discussion between experimentalists and theorists, with a focus on methodological developments, consistency of different approaches, and perspectives for future measurements and analyses. It will also contribute to the activities of the COST Action SHARP, promoting collaboration and exchange within the community working on hadron structure and related topics.

 

Thursday 21 May

13:40 → 18:40 Session 1: Nucleon and pion structure: TMD PDFs

Conveners: Mr Lorenzo Rossi (University of Milan & INFN), Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN)

14:30 Opening

Speaker: Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN)

14:40 TMD PDFs from Drell-Yan measurements

We present an extraction for the unpolarised transverse momentum dependent (TMD) parton distribution functions (PDFs) for the quarks in the pion from measurements of the Drell-Yan process.
The results are obtained from a fit of the non-perturbative part of the TMDPDFs in the framework of TMD factorisation
and rely on the non-perturbative input for the collinear PDFs.

As the data is limited, affected by a large normalisation uncertainty
and covers only a region of relatively large momentum fractions ( > 0.18 ),
the phenomenological extraction is at a disadvantage.
We illustrate the precise impacts of these drawbacks to make clear up to which extent such an extraction can be reliable in the first place.
Additionally such an extraction relies on several choices, which we explain and motivate those we make.

We obtain a good agreement with data in a qualitative and quantitative way
and as a result of our analysis strategy we do not observe a large normalisation issue between data and theory.
We highlight the developments from lattice (LaMET) which can eventually help to compliment the information we have from the experiments.
We also stress what the measurements from COMPASS can pragmatically improve in such kind of extractions.

Speaker: Valentin Moos (NYCU)

COMPAP16_Moos.pdf

15:10 Pion-induced Drell–Yan: a study of the full transverse-momentum spectrum

In this talk, we extend the study of low-invariant-mass Drell–Yan production over the full transverse-momentum spectrum, combining transverse-momentum resummation at small $q_T$ with fixed-order perturbative calculations at large $q_T$, together with a phenomenological description of non-perturbative effects.
We first consider proton–proton scattering as a benchmark to validate the approach. We then apply the same framework to pion-induced Drell–Yan processes, employing a pion non-perturbative input motivated by recent phenomenological models. This allows us to provide predictions for the transverse-momentum spectrum of low-mass Drell–Yan pair production in pion-induced reactions.

Speaker: Mr Lorenzo Rossi (University of Milan & INFN)

COMAP16_Rossi.pdf

15:40 Double quarkonium production in hadronic collisions at CERN fixed-target experiments

In this talk, we present new results for double quarkonium production in (un)polarized hadronic collisions at fixed-target experiments. Our approach incorporates the transverse momentum dependent factorization in combination with the Color-Singlet Model. We present new analytical expressions for the angular structure of the cross section for the $q \bar q$-induced channel, highlighting its similiarities with the Drell-Yan process, and provide predictions for the unpolarized cross section and transverse single-spin asymmetries for present and future fixed-target experiments at CERN and the LHC.

Speaker: Carlo Flore (Università di Cagliari & INFN)

Flore-di-onium-talk-COMAP.pdf

16:10 Coffee break

16:30 Meson parton distributions from global QCD analysis of experimental and lattice data

The pion and kaon are the pseudo-Goldstone bosons associated with chiral symmetry breaking as well as the lightest hadrons made up of quarks and gluons. Studying their structures allows us to understand more deeply the dynamics of QCD and emergent phenomena. Experimentally, the meson-induced Drell-Yan process provides sensitivity for the valence distributions at large momentum fraction, while the leading neutron electroproduction constrains the sea quark and gluon distributions in the pion at small momentum fraction. To complement the experimental measurements, we also use lattice QCD data of reduced pseudo-Ioffe time distributions and moment calculations. Future measurements at COMPASS, JLab, and the EIC will provide valuable information to flesh out the flavor structure of pions and kaons.

Speaker: Patrick Barry (Argonne National Lab)

Barry_COMAP26.pdf

17:00 The pion PDFs from the Fantômas framework

In light of recent progress on both the experimental and lattice-QCD sides, our understanding of the pion’s structure has evolved. Phenomenological, or global, QCD analyses play a crucial role in establishing a bridge between experimental data, theoretical predictions, and lattice-QCD studies. I will present recent efforts in the determination of the distribution of partons in the pion, with an emphasis on uncertainty quantification. The Fantômas project aims to explore the role of parametric forms for the distribution of quarks and gluons in the final uncertainty that results from the global analysis. To achieve this objective, the Fantômas project employs Bézier curves for generating diverse functional forms for the PDFs. The selection process for the final set of solutions hence involves sampling over the parametrization space, allowing for a comprehensive exploration of uncertainties. I will discuss the phenomenological interpretation of the FantoPDF uncertainties.

Speaker: Aurore Courtoy

COMAP_2026_AC.pdf

Friday 22 May

09:00 → 12:55 Session 2: Theory aspects

Conveners: Andrea Simonelli (INFN Roma 1), Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN)

10:00 Tensions in TMD PDF Extractions and the Phenomenological Description of TMD Observables

Transverse Momentum Dependent (TMD) parton distribution functions are rigorously defined QCD operators; however, their extraction remains challenging due to the dominance of non-perturbative effects at low energy scales. As a result, the role of phenomenological modeling is difficult to systematically control and may lead to potential misinterpretations of the information encoded in experimental data.
In this talk, commonly adopted extraction strategies are reviewed and critically assessed, with the aim of embedding the phenomenological description of TMDs within the broader framework of low-energy QCD.

Speaker: Andrea Simonelli (INFN Roma 1)

COMAP_meeting_2026.pdf

10:30 Impact of phenomenological prescriptions in TMD extractions

In this talk, I will present a phenomenological study aimed at assessing the impact of phenomenological choices in fits of Transverse Momentum Distributions (TMDs). In particular, I will focus on the so-called $b_*$ prescription, which is introduced in the standard Collins-Soper-Sterman (CSS) formalism to avoid the Landau pole. Using low invariant-mass Drell-Yan data, we observe that different choices of $b_*$ provide similarly good descriptions of experimental measurements and compatible behaviors of the extracted TMDs at small and large intrinsic transverse momentum, while significant differences emerge in the intermediate region. I will also discuss the crucial role of high-energy measurements and perturbative benchmarks for extracting reliable TMDs, and briefly comment on advantages and limitations of the $b_{\rm min}$ prescription.

Speaker: Matteo Cerutti (CEA Paris-Saclay)

Cerutti_COMAP.pdf

11:00 Coffee break

11:20 Pixel-Based TMD Extraction with Generative AI

We present a new nonparametric framework for the Bayesian inference of TMD parton distributions. By discretizing the impact parameter space into "pixels", we eliminate the biases of traditional functional forms. To sample the complex high-dimensional posterior, we employ a hybrid Normalizing Flow-driven Metropolis-Hastings algorithm. Using Singular Value Decomposition (SVD), we formally identify the resolution limits of the integral transform, introducing the concept of unobservable "null TMDs". Through multi-scale closure tests within the CSS formalism, we demonstrate how scale evolution combined with generative AI can break these degeneracies, enabling rigorous and unbiased 3D imaging of the nucleon.

Speaker: Marco Zaccheddu (Jefferson Lab)

Comap_Cern_slides_mz.pdf

11:50 Impact of COMPASS data on partonic polarized TMDs

We discuss the impact of different datasets on the extraction of the partonic Sivers function, with a particular focus on COMPASS SIDIS and DY measurements.

Speaker: Filippo Delcarro (Università di Pavia (IT))

Delcarro - COMAPXVI.pdf

12:20 Towards Precision TMD Phenomenology at Large-x

I will discuss the large-x behaviour of transverse momentum dependent distributions (TMDs) through the resummation of singular contributions in this regime to their collinear matching coefficients. The resummation is performed directly at the level of TMDs, preserving the process-independent structure of the formalism and allowing a unified treatment of different distributions relevant for phenomenology. The framework applies to both TMDPDFs and TMDFFs, including distributions that match onto twist-three collinear functions. I will present the general structure of the resummed results and discuss their phenomenological implications for TMD-sensitive observables, in particular the improvement of perturbative convergence and the reduction of theoretical uncertainties in the large-$x$ region. These results provide a useful step towards more precise and consistent phenomenological analyses of SIDIS and Drell–Yan measurements in kinematic regions relevant for current and future experiments.

Speaker: Óscar del Río García (Complutense University of Madrid)

COMAP26_Oscar.pdf

14:00 → 17:10 Session 3: Helicity PDF: measurements and extractions

Conveners: ALESSIA BONGALLINO (Universidad del PaÍs Vasco (UPV/EHU)), Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN)

14:00 Mapping the helicity transverse momentum distribution: status and perspectives

The helicity parton distribution function plays a crucial role in better understanding the quark and gluon spin structure of the proton. We study the transverse momentum dependence of the quark helicity distribution by analysing experimental data on semi-inclusive deep inelastic scattering off longitudinally polarised targets provided by the HERMES Collaboration. The fit is performed at next-to-leading order in perturbative QCD. Furthermore, we discuss the impact of double spin asymmetry data from the COMPASS and CLAS12 Collaborations.

Speaker: Alessia Bongallino (University of the Basque Country UPV/EHU)

COMAP_AB.pdf

14:30 Next-to-Next-to-Leading Order Global Analysis of Polarized Parton Distribution Functions

The study of the internal spin structure of the proton in terms of the contributions from quarks, anti-quarks and gluons, is a key focus of modern particle physics. These contributions can be described in terms of the longitudinally polarized parton distribution functions (pPDFs) or helicity PDFs, which are probed in high-energy scattering processes involving polarized nucleons.
In this talk I will present the results of our recent next-to-next-to-leading order (NNLO) fully-global QCD analysis of the proton's helicity PDFs. To obtain the distributions, we use data for longitudinal spin asymmetries in inclusive and semi-inclusive lepton-nucleon scattering as well as in weak-boson and hadron or jet production in proton-proton scattering. We analyze the data using QCD perturbation theory at NNLO accuracy, employing approximations provided by the threshold resummation formalism in cases where full NNLO results for partonic hard-scattering functions are not readily available. Our numerical results suggest a remarkable perturbative stability of the extracted helicity parton distribution functions.

Speaker: Ignacio Borsa (Tübingen University)

15:00 Coffee break

15:20 Longitudinal Spin Structure of the Proton in SIDIS at COMPASS

The longitudinal spin structure of the proton is studied with the COMPASS experiment using SIDIS data collected with a longitudinally polarized NH$3$ target and positive muon beams of 160 GeV/$c$ in 2007 and 200 GeV/$c$ in 2011. The analysis focuses on the double-spin asymmetry $A{LL}$, providing access to the spin-dependent structure function $g_1$, together with azimuthal asymmetries sensitive to longitudinal spin-dependent effects in SIDIS.

Speaker: Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN)

15:50 Longitudinal Target-Spin Effects in SIDIS with CLAS12

We present recent CLAS12 studies of target longitudinal-spin effects in semi-inclusive deep-inelastic scattering. The results provide new constraints on spin-dependent TMDs and the longitudinal spin structure of the nucleon.

Speaker: Harut Avakian (Jefferson Lab)

16:20 Longitudinal spin asymmetries at HERMES

In this contribution, we will recall the analysis details leading to the publication of the HERMES longitudinal-spin asymmetries, with a focus on A_LL in semi-inclusive DIS.

Speaker: Gunar Schnell

Monday 25 May

14:00 → 15:00 COST Action SHARP: WG1 meeting

Conveners: Dr Bakur Parsamyan (AANL, Turin section of INFN and CERN), Maria Zurita (Universidade de Santiago de Compostela)