QCD at the Extremes

1 Sept 2025, 13:45 → 11 Sept 2025, 16:00 Europe/Zurich

Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

 

International collaboration and free exchange of knowledge have been for many decades key assets in the progress of particle physics. In particular, international workshops have provided open forums for the sharing of new scientific results and fostering discussion. However, in the current political context the role and format of global scientific cooperation have become challenging. 

The online-only format of this workshop, marking the first in what we hope to be a series, reflects our commitment to universal open attendance and removes discriminating constraints such as those arising from politics, travel, cost for participation, and minimize environmental impacts.

The workshop will run for only 2h per day, to minimize the inconvenience across time zones, and to allow enough time for personal work as well as meetings and chats among participants. 

This first workshop with open access on "QCD at the Extremes" will consist of the following working group sessions:

• Onium production: Toponium and other Onia. Conveners: S. Baranov, A Grohsjean, Ch. Schwanenberger
• Quantum entanglement. Conveners: S. Baranov, K. Kutak
• Parton Densities and TMDs at high scales, including Heavy Boson PDFs. Conveners: A.M. Cooper-Sarkar, A. Glazov
• Parton Densities and TMDs at low scales, including saturation and recombination. Conveners: N. Raicevic, K. Kutak
• Strong coupling at small scales. Conveners: H. Jung, A. Kotikov

 

Organizing Committee:
H. Jung, K.Kutak, N. Raicevic, S. Taheri-Monfared

International Advisory Board:
J. Ellis, H. Jung, K. Kutak, M. Mangano, P. van Mechelen

Monday 1 September

13:45 → 14:00 Opening

Convener: Hannes Jung

Recording-H.Jung.mp4

Welcome.pdf

14:00 → 16:30 Strong coupling at small scales

Conveners: Anatoly Kotikov, Hannes Jung

14:00 Precise Determination of the Strong Coupling Constant from Dijet Cross Sections up to the Multi-TeV Range

We determine the value of the strong coupling α_s and study its running over a wide range of scales as probed by the dijet production process at hadron colliders. The analysis is performed using the complete next-to-next-to-leading order (NNLO) predictions in perturbative QCD and is based on dijet data published by ATLAS and CMS at center-of-mass energies of 7, 8, and 13 TeV. From a large subset of these data we infer a value of α_s(m_Z)=0.1178±0.0022 for the strong coupling at the scale of the Z-boson mass m_Z.
Complementing the LHC data with dijet cross sections measured at HERA, we extend the range to test the predicted running of the strong coupling towards smaller scales. Our results exhibit excellent agreement with predictions based on the renormalization group equation of QCD. This study represents the a very comprehensive test of the asymptotic behaviour of QCD, spanning over three orders of magnitude in energy scale from 7GeV up to 7TeV. (work based on arXiv:2412.21165)

Speaker: Daniel Britzger (Max-Planck-Institut für Physik München)

250901-alphas-running-britzger-QCDatExtremes-.pdf

Recording-D.Britzger-1.mp4

14:20 Theoretical Uncertainties on the determination of the strong coupling from Z pT

I will analyse the theoretical uncertainties inherent in the determination of the strong coupling from the transverse momentum (pT) spectrum of the Z boson. Such analyses require fine control of percent-level theoretical effects in small pT region, not only in terms of their magnitude but also of their shape and that of the corresponding theoretical uncertainties. This is theoretically extremely challenging. In this talk I will analyse the associated theoretical uncertainties via the novel theoretical nuisance parameter approach. In particular, I will focus on the perturbative uncertainty from missing higher orders in resummation, and that associated with the parton distribution functions, as well as others if time allows.

Speaker: Thomas Cridge (DESY)

Recording-Th.Cridge-1.mp4

TCridge_alphas_ZpT_QCDatextremes_Sep2025.pdf

14:40 Strong coupling from the IR to the UV extremes: A nonperturbative analytic approach

Following an introduction to light-front holographic QCD, a brief overview of recent analytic work by the HLFHS collaboration is presented. This includes the extension of the holographic strong coupling from the deep infrared (IR) region to the near-perturbative transition domain, where perturbative effects become significant. The analysis is then extended to the ultraviolet (UV) domain by imposing rigorous renormalization-group constraints at asymptotic infinity, revealing a nontrivial connection between the number of colors and the number of flavors, consistent with observations. The nonperturbative analytic model discussed here provides an accurate description of the strong coupling across all virtuality scales, up to the highest currently available data. As a further application of the holographic ideas to QCD, a relation between the transverse collision area at small longitudinal momentum fraction $x$ and deep inelastic scattering (DIS) entropy is discussed within the framework of the Holographic Principle.

HLFHS Collaboration\
https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.133.181901\
https://arxiv.org/abs/2505.19545

Speaker: Guy F de Teramond

gdtp QCD Extremes.pdf

Recording-G.F.deTeramond.mp4

15:00 Strong coupling from the IR to the UV extremes: Determination of $\alpha_s$ and prospects from EIC and JLab at 22 GeV

I will discuss how the Bjorken sum rule allows access to the QCD running coupling $\alpha_s$ at any scale, including in the deep infrared IR domain. The Bjorken sum data from Jefferson Lab, together with the world data on $\alpha_s$ reported by the Particle Data Group, allow us to determine the running of $\alpha_s(Q)$ over five orders of magnitude in four-momentum $Q$. I will then present two possible future measurements of the running of $\alpha_s(Q)$ using the Bjorken sum rule: the first at the EIC, covering the range $1.5 < Q < 8.7$ GeV, and the second at Jefferson Lab at 22 GeV, covering the range $1.0 < Q < 4.7$ GeV.

Speaker: Alexandre Deur (Jefferson Lab)

alpha_s_Deur.pdf

Recording-A.Deur.mp4

15:20 Break

15:30 Analytic QCD: latest results

Here an overview of the recent results, obtained in the framework of the fractional analytic QCD, is presented. The Higgs boson decay into a bottom-antibottom pair and the polarized Bjorken sum rule are considered as applications of the obtained results.

Speaker: Anatoly Kotikov (JINR)

KotikovQCDE.pdf

Recording-A.Kotikov.mp4

15:50 Equivalence principle and effective QCD coupling

The extraction of QCD coupling from Bjorken sum rule behavior at low Q2 is discussed. The small value of the slope justifying the approximate conformality is related, due to Gerasimov-Drell0-Hearn sum rules, to the closeness of the anomalous magnetic moments squared of proton and neutron. This, in turn is related to the equivalence principle for nucleon spin and its extension.

Speaker: Oleg Teryaev (JINR Dubna)

Recording-O.Teryaev.mp4

teryaev0109.pdf

16:10 Optimization of the factorization scale choice in QED Drell-Yan-like processes

The process of electron-positron annihilation into a virtual photon or Z boson is described within the QED structure function approach. The dependence of the electron PDF on the factorization scale is discussed. The results for several QCD-like prescriptions of the factorization scale choice are compared with the help of known exact two-loop results for this process. Similarities and differences between the description of Drell-Yan processes in QED and QCD are discussed.

The talk is based on our work:
[1] A.B. Arbuzov and U.E. Voznaya, Higher-order NLO initial state QED radiative corrections to e+e- annihilation revisited,'' Phys. Rev. D 109 (2024) no.11, 113002 [2] A.B. Arbuzov and U.E. Voznaya,Unpolarized QED parton distribution functions in NLO,'' J. Phys. G 50 (2023) no.12, 125004
and on some our recent yet unpublished results.

Speaker: Prof. Andrey Arbuzov (Joint Institute for Nuclear Research (RU))

arbuzov_QCD_extremes_2025.pdf

Recording-A.Arbuzov.mp4

Tuesday 2 September

14:00 → 17:05 Parton Densities and TMDs at high scales, including Heavy Boson PDFs

Conveners: Alexander Glazov (Deutsches Elektronen-Synchrotron (DE)), Amanda Sarkar (University of Oxford (GB))

14:00 Approximate N3LO PDFs: MSHT Updates

We will discuss several key updates to the MSHT approximate N3LO PDFs; including the combination of these aN3LO QCD PDFs with QED effects, the first determination of the strong coupling constant at aN3LO, and further updates based on new theoretical information available at N3LO including more precise splitting functions and newly determined transition matrix elements. All of this work has important consequences for phenomenology at the LHC.

Speaker: Thomas Cridge (DESY)

Recording-Th.Cridge.mp4

TCridge_PDFs_aN3LO_MSHT_QCDatExtremes_Sep2025.pdf

14:20 Intrinsic charm and the $D^+-D^-$ asymmetry produced in proton-proton collisions

We investigate the contribution
of the charm-anticharm ($c{\bar c}$) asymmetry of the proton eigenstate
obtained from QCD lattice gauge
to the asymmetry of $D^+,D ^-$ and $D^0,{\bar D}^0$ mesons produced
in $pp$ collisions at large Feynman variables $x$.
It is shown that an important tool for establishing the intrinsic charm
({\it IC}) content
of the proton is the charm hadron-antihadron asymmetry formed in $pp$
collisions.
Predictions for the asymmetry as function of $x$
for different {\it IC} probabilities are presented.
We show that the interference of the intrinsic $|uud c{\bar c}>$ Fock state
with the standard
contribution from the PQCD evolution leads to a large $D^+D^-$ asymmetry at
large Feynman $x$.

Speaker: Gennady Lykasov (Joint Institute for Nuclear Research (RU))

LYKASOV_02.09_2025.pdf

Recording-G.Lykasov.mp4

14:40 A parton shower consistent with parton densities at LO and NLO: PDF2ISR

We present a method for obtaining an initial-state parton shower model where the (backward) evolution fully consistent with the (forward) evolution of the collinear parton density used.
As a proof-of-concept we use parton densities obtained with the Parton Branching (PB) approach, and modify the default initial-state shower in PYTHIA8 with this method to be consistent with them. PB is ideally suited for checking the validity of our method since, in addition to producing collinear parton densities, it also produces the corre- sponding transverse-dependent (TMD) ones, and these can then be directly compared to the transverse momentum distribution obtained from the parton shower.
We show that TMD distributions which we in this way obtain from our modified PYTHIA8 shower using leading order (LO) parton densities and splitting functions are fully consistent with the corresponding leading order TMD densities. At next-to-leading order (NLO) it is not possible to achieve the same consistency using the built-in LO split- ting functions in the shower, but we show that by introducing NLO splitting functions using a reweighting procedure, we can achieve consistency also at NLO.
The method presented here, which we have named PDF2ISR, can be easily extended to any collinear parton densities, as long as the exact conditions for the evolution are known. With the PDF2ISR method we obtain an initial-state parton shower which in principle has no free parameters, and is fully consistent with collinear parton densities at LO and NLO.

Speaker: Mikel Mendizabal Morentin (Deutsches Elektronen-Synchrotron (DE))

QCDatExtremes-MikelMendizabal.pdf

Recording-M.Mendizabal.mp4

15:00 Drell-Yan Production Using a New Approach PDF2ISR in the PYTHIA Event Generator

A new approach, PDF2ISR, constructing an initial state parton shower which is fully consistent with the parton densities, is applied to Drell-Yan production. PDF2ISR is implemented inside the PYTHIA8 event generator and allows a full comparison with measurements.

This new approach is systematically cross-checked with predictions using the same parton densities, including Transverse Momentum Dependent (TMD) parton density functions, inside the CASCADE event generator. After completing the systematic analysis using the toy model, the PDF2ISR approach will be applied to the simulation of real Drell-Yan pairs and compared with the most recent experimental results of Drell-Yan production cross sections as a function of the pair's transverse momentum.

Speaker: Dusan Subotic (University of Montenegro (ME))

Recording-D.Subotic.mp4

Subotic_QCD.pdf

15:20 Break

15:30 Intrinsic $k_T$ in shower-based event generators: focus on PYTHIA

This talk explores the role of intrinsic transverse momentum - intrinsic $k_T$ and its energy dependence in parton shower models used in Monte Carlo event generators. Understanding intrinsic $k_T$ has long been a challenge: it is typically modeled with a Gaussian distribution, yet in practice, Monte Carlo generators require an intrinsic $k_T$ width that increases with collision energy, reaching unphysical values at high energies.
We investigate this behavior in \textsc{Pythia8} and demonstrate that it arises from the interplay between two non-perturbative effects: the intrinsic transverse motion of partons and soft-gluon emissions. These effects become particularly significant in Drell–Yan production at low transverse momentum, which serves as one of the most sensitive probes of initial-state parton evolution.
By analyzing the impact of the Sudakov form factor on the required intrinsic $k_T$ width, we clarify the origin of this unexpected energy dependence and provide new insights into parton shower modeling.

Speaker: Itana Bubanja (University of Montenegro (ME))

QCDatExtremes_Itana.pdf

Recording-I.Bubanja.mp4

15:50 Developments in collinear and TMD heavy boson probability density functions

Heavy boson densities are determined by solving extended DGLAP-type evolution equations that include all known strongly and weakly interacting partons and bosons. We use the Parton-Branching (PB) approach, which also allows for the direct extraction of Transverse Momentum Dependent (TMD) densities.

The initial distributions are fitted to high-precision Deep Inelastic Scattering data from HERA, ensuring accurate descriptions of the measurements. Photon and heavy boson densities are further validated using HERA cross-section data.

Speaker: Keila Moral Figueroa (CMS)

Keila_Moral_QCDextremes25.pdf

Recording-K.Moral.mp4

16:10 Spin-flip gluon generalized transverse-momentum-dependent parton distribution F1,2 at small-x.

The spin-flip processes in the deep inelastic scatterings are thought to be suppressed in the high energy. We found a positive intercept for the spin-flip generalized transverse momentum- dependent parton distribution (GTMDs) Re(F1,2). This is done by analytically solving the integro-differential evolution equation for Re(F1,2), in the dilute regime. Interestingly, the surviving solution corresponds to conformal spin n = 2 and carries an explicit cos 3φk∆ + cos φk∆ azimuthal dependence. As the imaginary part of F1,2, is related to the spin-dependent odderon or gluon Siver function and scales as Im(F1,2) ∼ x0, the positive intercept for Re(F1,2), implies that it is expected to dominate over the gluon Siver function in the small-x limit and may directly impact the modeling of unpolarised GTMDs and associated spin-flip processes.

Speaker: Nahid Vasim (Indian Institute Technology Delhi, New Delhi)

QCDatExtreme_Nahid.pdf

Recording-N.Vasim.mp4

16:30 NLL BFKL dijets with large rapidity separation for search of new physics at colliders

Dijets with large rapidity separation, described within the next-to-leading-logarithmic (NLL) BFKL evolution, have been observed recently at the LHC. Such dijets can be useful for search of new physics at colliders as the standard model background at large rapidities.
Dijets production from Kaluza-Klein graviton with extra-dimensions in trans-Planckian eikonal regime is discussed.

Speaker: Victor Kim (Petersburg Nuclear Physics Institute - PNPI NRC KI, Gatchina)

Kim_QCD@E2025.pdf

Recording-V.Kim.mp4

16:50 From Multiperipheral Models to BFKL Dynamics in Mueller–Navelet Jets

High-energy scattering often demands resumming large energy logarithms, beyond the reach of fixed-order QCD. Inspired by the old multiperipheral models, we study proton–proton collisions with a forward jet, a backward jet, and central mini-jets, using the BFKL framework. Jet rapidity distributions are used to search for BFKL effects, comparing PYTHIA8 predictions with those from the BFKLex Monte Carlo.

Speaker: Mr Dario Vaccaro (Laboratório de Instrumentação e Física Experimental de Partículas (LIP))

QCDatExtremes.pdf

Recording-D.Vaccaro.mp4

Wednesday 3 September

10:00 → 12:10 Parton Densities and TMDs at low scales, including saturation and recombination.

Convener: Natasa Raicevic (University of Montenegro (ME))

10:00 A phenomenological study of the proton charm and beauty content in the $k_t$-factorization formalism using KMR and MRW unintegrated parton distribution functions

In this paper, we study the phenomenology of the charm and beauty content of a proton by the $k_t$-factorization formalism using the integral version of the Kimber-Martin-Riskin ($KMR$) and Martin-Riskin-Watt ($MRW$) unintegrated parton distribution functions ($UPDF$) and the angular ordering constraint ($AOC$). For this purpose, the reduced beauty cross section ($\sigma_{red}^{b\overline{b}}(x‎, ‎Q^2)$) and the charm and beauty structure function ($F_2^{b\overline{b}}(x‎, ‎Q^2),q=c, b$) are calculated by the $k_t$-factorization formalism using $KMR$ and $MRW$ approaches and the $MMHT2014$-$PDF$ set as input. Recently, Guiot and van Hameren ($GvH$) demonstrated that the upper limit $k_{max}$ of the transverse-momentum integration in the $k_t$-factorization formalism should be approximately equal to $Q$, the hard scale, to avoid overestimating the proton structure function. In the present work, we show that this condition ($k_{max}=Q$) is not valid in the low and moderate energy regions. Furthermore, by considering both gluon and quark contributions at the same perturbative order and employing a physical gauge for the gluon, i.e., ‎$A^{\mu}q^{\prime}_{\mu}=0 $ in the calculation of $F_2^{q\overline{q}}$ within the $k_t$-factorization formalism, we do not encounter any overestimation of the theoretical predictions due to different choices of $k_{max}>Q$. Finally, the computed values of the reduced beauty cross section, $\sigma_{red}^{b\overline{b}}$, and the charm and beauty structure function, $F_2^{q\overline{q}}$, are compared with the experimental data of the $ZEUS$ and $H1$ collaborations in $HERA$ and theoretical predictions. In general, the results obtained from both $KMR$ and $MRW$ methods are in perfect agreement with experimental data and theoretical predictions, especially at high energies, but the $KMR$ framework is better consistent with experimental data and theoretical predictions than $MRW$ framework at low and moderate energies.

Speaker: Naeimeh Olanj (Department of Physics, Faculty of Science, Bu-Ali Sina University, 65178, Hamedan, Iran)

Olanj2025.pdf

Recording-N.Olanj.mp4

10:20 Photoproduction of charmonium-photon pairs in the dipole picture

In this talk, we discuss our results on $\eta_c\gamma$ and $\chi_c\gamma$ photoproduction, with a focus on ultraperipheral collisions at the LHC and the upcoming Electron-Ion Collider (EIC). At low values of $x$, where gluon saturation effects become significant, the interaction between heavy quarks and the nuclear gluon field is effectively described by the Color Glass Condensate (CGC) model. Within this approach, the cross sections of the investigated processes are determined by the dipole scattering amplitude, making these channels valuable tools for probing different phenomenological models of this fundamental quantity.

Our results include numerical predictions for the cross-sections and the expected counting rates, based on the technical specifications of current and future experimental facilities such as the LHC and EIC. We demonstrate that the predicted rates are sufficiently high for experimental analyses. Moreover, our results are consistent with recent theoretical predictions obtained within collinear factorization.

In addition, we evaluate the relevance of these processes as potential backgrounds to $\eta_c$ and $\eta_c$ photoproduction, which are among the proposed channels for studying $t$-channel odderon. We found that the contribution from meson-photon production (with an undetected photon) is comparable to the expected odderon contribution in the small momentum transfer kinematics.

This talk is partially based on materials of our recent publication Phys. Rev. D 111 (2025) 5, 056024

Speaker: IVAN ZEMLIAKOV

QCDExtr.pdf

Recording-I.Zemliakov.mp4

10:40 K-mesons production and isospin-symmetry in $pp$ and $AA$ collisions

Authors:
Alexey Guskov, Gennady Lykasov, Aleksandr Malakhov, Andrei Zaitsev

            JINR, Dubna, Moscow region

The production of K-mesons in $pp$ and $AA$ collisions at high energies is analyzed. We show that the ratio $R_K=(\sigma_{K^+}+\sigma_{K^-})/(2\sigma_{K^0_S})$
calculated within the similarity approach, including the gluon TMD
at low scales [1], and using the Lund model in the form of MC generator PYTHIA [2] is above 1 even at the conservation of the isospin-symmetry. It is about 1.18 at $\sqrt{s}=$ 11.9 GeV (the NA61/SHINE data [3]) and it decreases up to 1, when the energy increases up to a few TeV (ALICE data). The reason of this is related to the dynamics of the kaon production. This energy dependence of $R_K$ practically does not depend on the sort of beam or target.

Our calculations do not contradict to the world data at $\sqrt{s}=$
6-40 GeV and $\sqrt{s}=$ 3 TeV. In the energy region
40 GeV $<\sqrt{s}<$ 200 GeV there is an excess of the STAR data, having too large error bars, over our calculations. Therefore, it is desirable to check the isospin-symmetry violation by improving the measurement accuracy.

References
[1] G.I. Lykasov, A.I. Malakhov, A.A. Zaitsev, Eur.Phys.J.A 60, 239 (2024).
[2] T. Sjoestrand, Comp. Phys. Commun, 82, 4 (1994).
[3] The NA61/SHINE Collaboration, F. Giacosa, M. Gorenstein,R. Poberezhniuk,S. Samanta, Nature Communications, 16, 2849 (2025). arXiv:2312.06572v6 [nucl-ex].

Speaker: Gennady Lykasov (Joint Institute for Nuclear Research (RU))

GLMZ_31.0.8_2025_corr.pdf

Recording-G.Lykasov.mp4

11:00 Break

11:15 Double Unintegrated Parton Distribution Functions

The study of particle structure is one of the most fundamental and challenging scientific topics that has always maintained its attractiveness among researchers. They have pursued this goal with the
development of different tools and methods. One of these methods is by finding particle cross sections, which is accompanied by many complexities. Fortunately, the factorization theorem has made this path
easier by providing a solution. In this theory, using collinear and kt-factorization frameworks we can find the cross section of particle collisions. The kt-factorization theorem is actually an extension of the
collinear factorization theorem, which considers the transverse momentum of a parton in addition to its collinear component. Recently, another approach by extending the kt-factorization framework is proposed, which is called the (z, kt)-factorization. In this framework the kinematics of the last step emitted parton in contrast to the -factorization theorem is completely considered, and this parton plays an important role in cross section calculations. In this talk, we investigate the (z,)-factorization
framework by considering various hadronic differential cross sections, and also double unintegrated parton distribution functions of different approaches.

Speaker: Dr Somayeh Rezaie (School of Particles and Accelerators, Institute for Research in Fundamental Sciences (IPM), University of Tehran)

Recording-S.Rezaie.mp4

11:35 Unintegrated gluon density in nuclei with shadowing

We present a new unintegrated (transverse momentum dependent, TMD) gluon density in nuclei valid at small $x$ and incorporating the shadowing effect. We start from recently presented TMD gluon distribution in proton (LLM-2024), which shows saturation behavior, and extend it to nuclei using the geometric scaling. The saturation scale is changed in a way to describe available data on nuclear structure functions at low $x$ and $Q^2$ using color dipole approach. This method allows to take into account the shadowing effect at low $x$ in the TMD gluon distribution.

Speaker: Maxim Malyshev

malyshev.pdf

Recording-M.Malyshev.mp4

14:00 → 16:50 Parton Densities and TMDs at low scales, including saturation and recombination.

Convener: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk)

14:00 An investigation of different unintegrated parton distribution functions

We investigate unintegrated parton distribution functions (UPDFs) using the Kimber–Martin–Ryskin (KMR) and leading-order Martin–Ryskin–Watt (LOMRW) methods and compare them with transverse momentum dependent PDFs from the parton branching (PB) approach. The UPDFs are computed for various partons over a range of x, μ2, and kt , revealing that all methods yield similar results at small kt, while at higher kt , the LOMRW UPDFs remain larger and the PB distributions decrease more rapidly. We then compute the reduced cross section (RCS) in deep inelastic scattering at Q2=4.5 and 12 GeV2, finding that KMR and LOMRW predictions are consistent with PB results and experimental data at mid-x, though they overestimate the data at small x due to neglecting the longitudinal structure function. Finally, we calculate the Drell–Yan differential cross section for qˉq→e^- e^+, qg→qe^- e^+ and qˉq→e^- e^+ g subprocesses within the kt-factorization framework using the KATIE event generator and LOMRW UPDFs. Our results, particularly at higher lepton-pair transverse momentum, show good agreement with ATLAS experimental data, with the qˉq→e^- e^+ g contribution found to be negligible. These findings highlight the practical advantages of KMR and LOMRW methods for generating UPDFs, while underscoring the strong predictive power of the PB approach in describing experimental measurements.

Speaker: Zahra Badieian

Recording-A.Badieian.mp4

Z.Badieina-presentation.pdf

14:20 On the PB Sudakov: NNLL coefficient, CS kernel and intrinsic-kt

The TMD Parton Branching (PB) method was developed to include elements of the TMD physics in MC generators. Its relation to stringent CSS formalism remained initially unclear. In this talk, we shed light on the relation of the PB Sudakov form factor to that of CSS. We discuss both perturbative and non-perturbative components. We present recent developments to include NNLL coefficient in the PB Sudakov. We discuss the CS kernel extractions for different evolution scenarios. We discuss the recent studies on intrinsic-kt vs center-of-mass (in)dependence in different approaches and their relation to the non-perturbative Sudakov.

Speaker: Aleksandra Lelek (University of Antwerp (BE))

QCDatTheExtremes.pdf

Recording-O.Lelek.mp4

14:40 On the interplay between the BFKL resummation and high-energy factorization in Mueller-Navelet dijet production

We study the interplay between the high-energy resummation based on the BFKL equation and initial state radiation effects, taken into account within the framework of the high-energy factorization, in the production of Mueller-Navelet dijets at hadron colliders. We use the solution of the NLO BFKL equation built out of eigenfunctions perturbatively constructed up to NLO to avoid the need for a special renormalization scale setting. We demonstrate that the various data sets from the FNAL Tevatron and CERN LHC can be described in this way and both the initial state radiation effects of the high-energy factorization and the NLL BFKL resummation are crucial for the uniform description of the data across all values of rapidity difference between the jets.

Speaker: Alexey Chernyshev (BLTP JINR)

QCDatTheExtremes.pdf

Recording-A.Chernyshev.mp4

15:00 On the real NLO corrections in the Parton Reggeization Approach

We discuss real NLO corrections in the Parton Reggeization Approach in processes of heavy quark and isolated photon production at high energy hadron collisions. In part, the talk is based on published results:
1) M.Nefedov and V.Saleev, High-Energy Factorization for Drell-Yan process in $pp$ and $p{\bar p}$ collisions with new Unintegrated PDFs, Phys. Rev. D102 (2020), 114018
2) A.Karpishkov, M.Nefedov and V.Saleev, $B{\bar B}$ angular correlations at the LHC in parton Reggeization approach merged with higher-order matrix elements, Phys. Rev. D96 (2017) no.9, 096019
3) M.Nefedov and V.Saleev, Diphoton production at the Tevatron and the LHC in the NLO approximation of the parton Reggeization approach, Phys. Rev. D92 (2015) no.9, 094033
4) A.Chernyshev and V.Saleev, Single isolated photon production in the NLO* approximation of the parton Reggeization approach, Phys. Rev. D 110 (2024) no.11, 11

Speaker: Prof. Vladimir Saleev (BLTP JINR)

Recording-V.Saleev.mp4

report_qcd.pdf

15:20 Revisiting the gluon density from the Balitsky-Kovchegov equation with kinematical constraints and large x terms

We perform analysis of the small x nonlinear evolution equation formulated in momentum space supplemented by higher order terms. The equation is defined in the wide range of transverse momentum and longitudinal momentum fractions extending previous studies performed in Kutak and Kwiecinski [Screening effects in the ultrahigh-energy neutrino interactions, Eur. Phys. J. C 29, 521 (2003).] and in Kutak and Stasto [Unintegrated gluon distribution from modified BK equation, Eur. Phys. J. C 41, 343 (2005).]. The linear part of the equation is motivated by the renormalization group improved small x approach, which accounts for resummation of higher orders, and includes collinear splitting functions and kinematical constraints. The solution to the equation is then used to perform the fit to deep inelastic scattering reduced cross section data.

Speaker: Wanchen Li

Recording-W.Li.mp4

Resummed_BK_in_QCD_at_Extreme_2025.pdf

15:40 Break

15:55 Heavy-hadron production based on $k_t$-factorization with scale-dependent fragmentation functions

We present a comprehensive study of heavy-hadron production, including $D$ and $B$ mesons, heavy baryons, and the $B_c$ meson. Our calculations are based on the $k_t$-factorization and scale-dependent fragmentation functions, completing the program of implementing this formalism within a variable-flavor-number scheme, as initiated in Phys. Rev.D 104 (2021) 9 094038. Special emphasis is placed on the gluon-to-heavy-hadron contribution, which improves the description of data at small transverse momenta.

Speaker: guiot benjamin (Universidad Tecnica Federico Santa Maria)

QCD@theExtremes.pdf

Recording-B.Guiot.mp4

16:10 Neutron star as a signal for quark-gluon plasma formation

Studying neutron stars can help us understand the early formation of the universe. To understand and describe neutron stars, an appropriate equation of state that satisfies bulk nuclear matter properties is necessary. This study explores the quark matter EoS by applying the Density Dependent Quark Mass model which is an extension of the MIT Bag model. The EoS such as pressure, energy density, entropy, specific heat and speed of sound are calculated for a range of temperatures. The model results provide QGP EoS which are then compared with Lattice QCD data.

Speaker: Anisha Khatri (University of Delhi)

QCD_AT_EXTREMES_TALK (2).pdf

Recording-A.Khatri.mp4

Thursday 4 September

14:00 → 17:25 Quantum entanglement

Conveners: Krzysztof Kutak (Instytut Fizyki Jadrowej Polskiej Akademii Nauk), Sergey Baranov (P.N.Lebedev Physical Institute, Moscow)

14:00 Quantum detection of SM new physics in $t\bar t$ at the LHC

I illustrate how, thanks to the efforts made to observe entanglement in $t\bar t$ at threshold, evidence for a "toponium enhancement" has emerged.

Speaker: Prof. Fabio Maltoni (Universite Catholique de Louvain (UCL) (BE) and Università di Bologna)

maltoni-QCD-extreme-QI.pdf

Recording-F.Maltoni.mp4

14:35 Entanglememt entropy and its imprint in DIS hadron multiplicity distributions

Entanglement entropy has emerged as a novel tool for probing QCD phenomena. In this talk I present recent results in describing hadron production in Deep Inelastic Scattering (DIS) of electrons on protons.In particular I will discuss recent results on entanglement entropy in diffractive reactions, QCD evolution of entanglement entropy.

Speaker: Martin Hentschinski (Universidad de las Americas, Puebla)

QCDExtremes_Hentschinski_Sep0425.pdf

Recording-M.Hentschinski.mp4

14:55 Cascades of gluons at high energies and their QI measures

Speaker: Krzysztof Marek Kutak (Polish Academy of Sciences (PL))

Entanglement_online.pdf

Recording-K.Kutak.mp4

15:15 Spin Correlations

In recent works [1-3], we demonstrated that the spin correlation of back-to-back dihadron emerges even in unpolarized collisions thanks to the quantum entanglement of interacting partons, empowering unpolarized experiments to shed light on spin effects. We also extend our approach to relativistic heavy-ion collisions, opening up new avenues for investigating the quenching of polarized jets [4]. This talk presents our finding on both the longitudinal and transverse spin correlations of back-to-back dihadron.

References
[1] H.C. Zhang and S.Y. Wei, Probing the longitudinal spin transfer via dihadron polarization correlations in unpolarized ee and pp collisions, Phys. Lett. B 839 (2023), 137821.
[2] Z.X. Chen, H. Dong and S.Y. Wei, Dihadron helicity correlation in photon-nucleus collisions, Phys. Rev. D 110 (2024), 5.
[3] L. Yang, Y.K. Song and S.Y. Wei, Transverse spin correlation of back-to-back dihadron in unpolarized collisions, arXiv:2410.20917.
[4] X. Li, Z.X. Chen, S. Cao and S.Y. Wei, Correlations of dihadron polarization in central, peripheral, and ultraperipheral heavy-ion collisions, Phys. Rev. D 109 (2024), 014035.

Speaker: Prof. Shu-yi Wei (Shandong University)

Recording-S.Wei.mp4

15:35 Break

15:45 Investigating Medium Effects on Exotic $T_{cc}^{+}(3875)$ State with Thermal QCD Sum Rules

In this study, we examine the thermal behavior of the $T_{cc}^{+}(3875)$ meson, characterized by quantum numbers $J^{PC} = 1^{++}$, within the framework of thermal QCD sum rules. Employing a molecular interpolating current that matches the meson’s quantum numbers, we construct the two-point correlation function and incorporate nonperturbative contributions from QCD condensates up to dimension ten.The resulting thermal sum rules enable us to explore the medium-induced modifications of the mass and meson-current coupling constant of the $T$ resonance. Numerical analysis indicates that both parameters remain nearly stable up to a temperature of $T \simeq 120~\mathrm{MeV}$. Beyond this threshold, however, they begin to decrease noticeably with increasing temperature. At the deconfinement point, the mass is suppressed to approximately 30\% of its vacuum value.

Speaker: Sari Damen (university of kocaeli)

Recording-S.Damen.mp4

16:05 Machine Learning-Based Classification of Synthetic Quantum Entanglement–Like Events in High-Energy Collisions Using TMVA

Quantum entanglement is a fundamental phenomenon with potential implications in high-energy particle collisions, yet its direct identification in experimental data is challenging due to complex backgrounds and subtle correlation signatures. In this study, we develop a ROOT-based synthetic event generator to simulate collision events with two distinct classes: entangled-like events featuring correlated particle pairs with small azimuthal angle differences, and non-entangled events with uniformly distributed particle azimuthal angles. For each event, key observables such as particle multiplicity, mean azimuthal angle difference, and transverse spherocity—a measure of event shape—are computed to capture the underlying topology and correlation structures. Utilizing the Toolkit for Multivariate Data Analysis (TMVA), we train a Boosted Decision Tree (BDT) classifier on these features to discriminate between entangled-like and non-entangled events. The training is performed on a balanced dataset of 10,000 synthetic events, split into training and testing subsets. Results show that the BDT efficiently separates the two event classes, demonstrating the power of machine learning techniques to identify subtle quantum correlation patterns embedded in synthetic data. This approach lays the groundwork for applying advanced classification methods to real experimental data, potentially enhancing our ability to detect and analyze quantum entanglement phenomena in particle physics.

Speaker: Ms Thresia Michael (University Centre for Research and Development, Chandigarh University, Gharuan, Mohali-Punjab)

Recording-Th.Michael.mp4

16:25 Quantum Entanglement Dynamics in Electron-Positron Pair Production Under Time-Dependent Electromagnetic Fields

We investigate the quantum entanglement dynamics of electron-positron pairs produced from the vacuum under strong time-dependent electromagnetic fields, using a non-perturbative kinetic theory approach [1]. Within the framework of spinor quantum electrodynamics, we analyze the evolution of entanglement—quantified via logarithmic negativity—for multi-pulse laser field configurations, extending recent perturbative analyses [2]. The observed entanglement modulation stems from the external field’s non-adiabatic dynamics and the in-out state distinction, which together reveal non-equilibrium phase transition signatures as predicted in [3]. Our results demonstrate how structured ultra-intense fields (≳10²⁸ W/cm²) control entanglement generation in vacuum fluctuations, bridging predictions of non-perturbative QED with observable quantum information metrics.

References:
[1] S. A. Smolyansky et al., Russ. Phys. J. 59, 1731 (2017)
[2] Y. Li et al., Phys. Rev. D 95, 036006 (2017)
[3] C. Banerjee, M. P. Singh, Phys. Rev. D 100, 056016 (2019)

Speaker: DEEPAK SAH (Theory and Simulations Lab, Raja Ramanna Centre for Advanced Technology, Indore-452013, INDIA ; Homi Bhaba National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India)

Recording-D.Sah.mp4

16:45 Axial anomaly, entanglement and polarization

The pseudoscalar decays due to axial anomaly manifest the particular case of EPR effect. The Lorentz invariant description allows one to explore the seeming violation of causality in the most clear way. The related polarization effects in dilepton production are discussed.

Speaker: Oleg Teryaev (JINR Dubna)

Recording-O.Teryaev.mp4

17:05 Probing spin correlations, entanglement, and Bell nonlocality in bottom quark pairs at the LHC

While spin correlations and spin entanglement have been measured for top quarks at the LHC, they remain unexplored for other quark flavors. We propose analysis strategies for measuring spin correlations, entanglement, and Bell nonlocality in $b \bar b$ samples using the partial preservation of the spin information in $\Lambda_b$ baryons from bottom quark fragmentation. We find that certain measurements are feasible with existing datasets, while others will become possible at the HL-LHC. The proposed measurements will also provide new information on the currently poorly known polarized fragmentation functions.

Speaker: Yevgeny Kats (Ben-Gurion University)

Recording-Y.Kats.mp4

spin-correlations-QCD-extremes.pdf

Friday 5 September

13:40 → 17:25 Onium production: Toponium and other Onia

13:45 Intro

Speaker: Alexander Grohsjean (Hamburg University (DE))

Recording.A.Grohsjean.mp4

14:00 Quarkonium Parton Shower in Herwig 7

I will present the implementation of a fully automated quarkonium parton shower in Herwig 7, based on NRQCD factorisation with spin–colour projections. The framework systematically includes both colour-singlet and colour-octet production mechanisms, as well as gluon fragmentation and diquark production processes. Perturbative short-distance coefficients are combined with non-perturbative NRQCD long-distance matrix elements (LDMEs) to model the formation of heavy-quark bound states. I will also discuss the tuning of non-perturbative parameters and LDMEs. The implementation preserves spin correlations and polarisation effects, and accurately accounts for feed-down contributions.

Speaker: Aidin Masouminia (IPPP, Durham University)

QCD@Extremes-AM.pdf

Recording-A.Masouminia.mp4

14:25 On the threshold behaviour of heavy top production

Back in 1990 an article by Victor Fadin. Valery Khoze and the speaker studied the effects of Coulomb enhancements and pseudo-bound below-threshold contributions ("toponium").
The relevant formulae have now been recoded and made available as an add-on to Pythia8, both for singlet and octet states.
Cross sections can be explored under varying assumptions. Complete events can be generated, but t-tbar spin correlations are lacking.

Speaker: Torbjorn Sjostrand (Lund University (SE))

Recording-T.Sjostrand.mp4

topThreshold.pdf

14:50 Observation of a cross-section enhancement near the $t\bar{t}$ production threshold in $\sqrt{s}=$13 TeV $pp$ collisions with the ATLAS detector

I will present the recent ATLAS result on the observation of an excess near the $t\bar{t}$ threshold that is consistent with the formation of "toponium" quasi-bound states. Details of the experimental and statistical analysis techniques will be presented, with an emphasis on the modelling of both pQCD and pNRQCD $t\bar{t}$ production.

Speaker: Baptiste Ravina (CERN)

QCD at the Extremes ATLAS_CMS Toponium.pdf

Recording-B.Ravina.mp4

15:20 Toponium modelling at the LHC: from toy models to NRQCD predictions

We present recent developments in the description of toponium production at the LHC. Building upon a dedicated NRQCD framework, we compare predictions obtained with Green's function re-weighting to those originating from simplified pseudo-scalar models, including both narrow-width and broad-width setups. Using event generation with a consistent toolchain, we illustrate how these approaches differ at the level of key observables and discuss their implications for current and future searches.

Speaker: Benjamin Fuks

Recording-B.Fuks.mp4

Toponium_QCDExtreme.pdf

15:45 Updated predictions for toponium production at the LHC

We provide an update on QCD predictions for top-quark pair production close to threshold, including bound state effects at the Large Hadron Collider. We compute the top-quark pair invariant mass distribution, including Coulomb resummation for bound-state effects, as well as threshold resummation for emissions of soft and collinear gluons. We discuss uncertainty estimates and present a proposal for the use of these predictions in experimental analyses.

Speakers: Sven-Olaf Moch, Sven-Olaf Moch (Hamburg University (DE))

qcd-extreme-2025.pdf

Recording-S.Moch.mp4

16:10 How well does NRQCD describe quarkonium production data?

We perform a thorough investigation of the universality of the long distance matrix elements (LDMEs) of nonrelativistic QCD factorization based on a next-to-leading order (NLO) fit of $J/\psi$ color octet (CO) LDMEs to high transverse momentum $p_T$ $J/\psi$ and $\eta_c$ production data at the LHC. We thereby apply a novel fit-and-predict procedure to systematically take into account scale variations, and predict various observables never studied in this context before. In particular, the LDMEs can well describe $J/\psi$ hadroproduction up to the highest measured values of $p_T$, as well as $\Upsilon(nS)$ production via potential NRQCD based relations. Furthermore, $J/\psi$ production in $\gamma\gamma$ and $\gamma p$ collisions is surprisingly reproduced down to $p_T=1$ GeV, as long as the region of large inelasticity $z$ is excluded, which may be of significance in future quarkonium studies, in particular at the EIC and the high-luminosity LHC. In addition, our summary reveals an interesting pattern as to which observables still evade a consistent description.

Speaker: Mathias Butenschoen

Butenschoen_QCD_at_the_Extremes.pdf

Recording-M.Butenschoen.mp4

16:35 General Discussion

17:25 → 17:35 Closing of the Workshop

Recording-N.Raicevic.mp4

Monday 8 September

12:00 → 16:00 School: QCD at the Extremes

exercises-matplot.tgz

QCD_at_Colliders.pdf

12:00 Lecture: Monte Carlo technique

Lecture Notes1-1 (annotated)

Monte Carlo technique

12:01 Lecture: Structure of Matter

Lecture Notes1-2 (annotated)

Lecture: Structure of Matter (recording)

14:00 Exercises 1

Speaker: Hannes Jung

example-1.ipynb

example-1.py

example-2.ipynb

example-2.py

example-3.ipynb

example-3.py

example-4.ipynb

example-4.py

example-5.ipynb

example-5.py

exercise-1.pdf

Exercise1.pdf

Tuesday 9 September

12:00 → 16:00 School: QCD at the Extremes

exercises-matplot.tgz

QCD_at_Colliders.pdf

12:00 Lecture: Parton densities and evolution equations

Lecture Notes2-1 (annotated)

Lecture: Parton densities and evolution equations

12:01 Lecture: Parton densities and evolution equations (II)

Lecture Notes2-2 (annotated)

Lecture: Parton densities and evolution equations (II)

14:00 Exercises 2

Speaker: Hannes Jung

example-6.ipynb

example-6.py

example-7.ipynb

example-7.py

exercise-2.pdf

Exercise2.pdf

Wednesday 10 September

12:00 → 16:00 School: QCD at the Extremes

exercises-matplot.tgz

QCD_at_Colliders.pdf

12:00 Lecture: Soft gluon resummation and TMDs (I)

Lecture Notes3-1 (annotated)

Lecture: Soft gluon resummation and TMDs (I)

12:01 Lecture: Soft gluon resummation and TMDs (II)

Lecture Notes3-2 (annotated)

Lecture: Parton densities and evolution equations (II)

14:00 Exercises 3

Speaker: Hannes Jung

example-8.ipynb

example-8.py

example-9.ipynb

example-9.py

exercise-3.pdf

Thursday 11 September

12:00 → 16:00 School: QCD at the Extremes

exercises-matplot.tgz

QCD_at_Colliders.pdf

12:00 Lecture: TMDs and W/Z production

Lecture: TMDs and W/Z production

Lecture: TMDs and W/Z production

12:01 Lecture: W/Z parton densities and TMDs

Lecture: W/Z parton densities and TMDs

Lecture: W/Z parton densities and TMDs (annotated)

14:00 Exercises 4

Speaker: Hannes Jung