Parton Showers and Resummation 2025

14 Jul 2025, 08:30 → 18 Jul 2025, 19:00 Europe/Zurich

4/3-006 - TH Conference Room (CERN)

Alba Soto-Ontoso, Alexander Karlberg (CERN), Daniel Reichelt, Luca Buonocore (CERN), Pier Francesco Monni (CERN)

The 2025 annual workshop on Parton Showers and Resummation (PSR2025) will be held on July 14-18 at CERN. The workshop brings together researchers from the  QCD communities involved in analytic resummations and parton shower event generators, focusing on their connection. Recent advances in resummation techniques, parton shower algorithms and QCD phenomenology will be presented, and  the latest developments and applications will be discussed.

The conference will take place in association with a dedicated school the week prior, and a focused workshop in the two weeks following. 

Registration for all events are now open.

Dinner-Menu.pdf

Monday 14 July

13:15 → 13:25 Welcome

Speakers: Alba Soto Ontoso (Universidad de Granada (ES)), Dr Alexander Karlberg (CERN), Daniel Reichelt, Luca Buonocore (CERN), Pier Francesco Monni (CERN)

13:25 → 14:10 Current limitations of event generators in experimental analyses at the LHC

Speaker: Jennifer Roloff (Brown University (US))

14:20 → 14:50 Conceptual and technical challenges in the matching of parton showers to higher-order calculations

Speaker: Peter Skands (Monash University (AU))

15:00 → 15:20 LHC phenomenology with KrkNLO matching

The systematic combination of perturbative QCD with parton-shower resummation is achieved at NLO by a small, but growing, number of 'NLO matching' methods.

Among them, the KrkNLO method is unique in exploiting a modification of the PDF factorisation scheme, from the conventional $\overline{\mathrm{MS}}$ scheme, to a scheme (the 'Krk' scheme) in which the NLO corrections can be applied as a simple multiplicative reweight. This is positive by construction, since it does not use subtraction, and unlike other methods has no dependence on an unphysical choice of shower-scale or suppression factor.

We summarise the KrkNLO method, its recent implementation for general colour-singlet processes in Herwig 7, and present results for LHC processes involving the production of massive and massless vector bosons. We will summarise the results of a systematic comparison with MC@NLO, and also review the properties of the Krk scheme among other factorisation schemes proposed as alternatives to $\overline{\mathrm{MS}}$, including their positivity.

Speaker: James Christopher Whitehead

15:30 → 16:00 Coffee

16:00 → 16:30 Logarithmically-accurate and positive-definite NLO shower matching with ESME

Speaker: Dr Alexander Karlberg (CERN)

16:40 → 17:10 Current status of NNLOPS simulations at LHC

Speaker: Luca Rottoli (Universita & INFN, Milano-Bicocca (IT))

17:20 → 17:40 NNLO matching in VINCIA

Fixed-order matching of Monte Carlo showers remains a critical ingredient to push forward the precision frontier of collider phenomenology. Beyond the conceptually simple leading-order matching, the past two decades have seen multiple techniques to achieve next-to-leading order (NLO) matching which have further been fully automated and made available in public codes. This pushed the precision frontier to NNLO.

We outline a new technique to perform NNLO matching in which the shower Monte Carlo is used as the phase space generator. In the context of a simple process, Z decay to hadrons, we focus on the VINCIA sector shower which utilises the antenna formalism. In the sector approach, the phase space is partitioned into non-overlapping sectors which greatly simplifies the inclusion of 2->4 antenna functions. The matching formulae are derived by comparing the fully differential jet production cross section computed both in the shower and at fixed-order. We develop a technique to compute the complex sector integrals which control the NLO correction to the 3-jet production cross section. The latter is the only missing ingredient to achieve NNLO matching in the process Z decay to hadrons. Finally, we discuss the flowchart of the numerical implementation.

Speaker: Basem El-Menoufi

17:50 → 18:10 Final-state NLO matching in Alaric

In this talk, we present steps towards automated matching of the Alaric parton shower to NLO QCD matrix elements for lepton collisions in Sherpa. We validate our implementation against jet production in $e^+e^- \to$ jets for up to five jets. We show numerical tests of the subtraction in the infrared limit, benchmarking against the Catani-Seymour subtraction. We then present first hadron-level matched results compared against LEP data.

Speaker: Peter Meinzinger (Zürich University)

18:30 → 19:30 Reception

Tuesday 15 July

09:00 → 09:20 Status of NLL-Accurate Parton Showers in Herwig

Speaker: Siddharth Sule (The University of Manchester (GB))

09:30 → 10:00 PanScales efforts towards NNLL showers

Speaker: Gregory Soyez (IPhT, CEA Saclay)

10:10 → 10:40 The Alaric project

Speaker: Stefan Hoeche (Fermilab)

10:50 → 11:20 Coffee

11:20 → 11:50 Recent progress in subleading-colour effects and amplitude-level evolution

Speaker: Simon Platzer (University of Graz (AT))

12:00 → 12:30 Hadronization models with NLL-accurate showers in Herwig

Speaker: Andre Hoang (Austrian Academy of Sciences (AT))

12:40 → 14:00 Lunch

14:10 → 14:40 Non-perturbative corrections to the projected N-point energy correlators

Speaker: Dr Aditya Pathak (DESY)

14:50 → 15:20 Non-perturbative power corrections using the PanScales shower

Speaker: Silvia Zanoli (University of Oxford)

15:30 → 16:00 Coffee

16:00 → 18:20 Parton densities and soft resummation

16:00 Four-top production at the LHC: improving predictions with invariant-mass threshold resummation.

We present new results for the invariant-mass distribution and total cross section for four-top production, incorporating the resummation of soft-gluon corrections at improved next-to-leading logarithmic accuracy, including non-logarithmic $\mathcal{O}(\alpha_s)$ terms (NLL’ accuracy). After discussing the challenges of invariant-mass threshold resummation in processes with six coloured legs, we present our resummed predictions matched to fixed-order NLO calculations. Our results modify the shape of the fixed-order invariant-mass distribution and significantly reduce theoretical uncertainties.

Speaker: Tommaso Saracco (Nikhef)

16:30 Factorization and Resummation in Threshold PDFs

We study factorization at next-to-leading power (NLP) in deep inelastic scattering (DIS) in the endpoint region $x\to1$ within the framework of soft-collinear effective theory (SCET). The full QCD process is matched onto two SCET currents, whose matrix elements factorize into individual component functions. By employing endpoint reshuffling theorems that relate these component functions at endpoint kinematics, we remove all endpoint divergences. We then derive the relevant renormalization-group (RG) equations and solve them at leading order in RG-improved perturbation theory, thereby resumming large logarithms to all orders. Our main finding is that, in the endpoint limit, new structures emerge for the parton distribution functions (PDFs), necessitating a modification of the standard PDF factorization scheme .

Speaker: Marvin Schnubel (BNL)

17:00 Threshold resummation for VV pair production to NNLO+NNLL at the LHC (Zoom)

We perform the threshold resummation for massive vector boson pair production processes ($ZZ$ and $W^{+}W^{-}$) in hadron collisions to Next to Next Leading Log accuracy. The resummed cross-sections are then matched with NNLO fixed order results, which are obtained using the MATRIX code. We present our results for the invariant mass distribution to NNLO+NNLL accuracy in QCD for the current LHC energies. The NNLL contributions enhance the cross section by a few per cent in the high invariant mass regions. In these regions, the uncertainties due to unphysical scales ($\mu_{R}$ and $\mu_{F}$) in the fixed-order results, 4.6\% for ZZ and 4.2\% for WW (at $Q=1.3$ TeV) get reduced to
3.2\% and 3.0\%, after resummation.

Speaker: Vaibhav Pandey (Indian Institute of Technology Guwahati)

17:30 Large-x Resummation for Transverse Momentum Distributions

In this talk, I discuss the resummation of leading logarithmic contributions to the collinear matching coefficients of Transverse Momentum Dependent distributions (TMDs) in the large-$x$ regime. Resummation is performed directly at the level of TMDs, preserving their process-independence and, for the first time, covering distributions that match onto twist‑three collinear Parton Distribution Functions (PDFs). I will present general resummation formulas valid for all leading power functions, including TMDPDFs and TMDFFs, except the pretzelosity distribution, which is associated with a twist-four operator. Additionally, I demonstrate that the resummation accuracy can reach N$^3$LL, surpassing some of the current fixed-order results for several TMDs. The application of these formulas improves perturbative convergence, helps estimate unknown higher‑order terms, and constrains non‑perturbative model inputs, thereby providing a robust framework for phenomenology of the different TMD processes.

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

18:00 Small-x Resummed Gluon Densities from HELL and Applications with JETHAD (Zoom)

We review the state of the art in applying the small-x resummation to parton distribution functions in the proton, with particular emphasis on the gluon content. In the first part, we briefly discuss small-x resummed 1D collinear distributions, highlighting their connections with the 3D transverse-momentum dependent counterparts at both small and moderate x, including the effects of gluon-proton spin correlations. In the second part, we present a novel determination of unintegrated gluon densities, obtained from the small-x resummation formalism as implemented in the HELL approach. Finally, we discuss applications to HERA and LHC phenomenology, leveraging the JETHAD multimodular interface.

Speaker: Dr Francesco Giovanni Celiberto (UAH Madrid)

16:00 → 18:20 Recent developments in Monte Carlo generators

16:00 Decomposition of QCD splitting functions into scalar radiators and pure remainders

In this talk, I will present a new decomposition of QCD splitting functions, carried out systematically up to second order in the strong coupling. The core idea is to separate the splitting functions into two components: scalar dipole radiator functions and pure remainders. Unlike conventional approaches, our construction does not rely on any soft or collinear approximations. The splitting functions can be used to construct an overlap free subtraction, or for the construction of accurate Parton showers.

Speaker: Max Knobbe (University of Göttingen)

16:30 A novel algorithm for spin correlations in parton evolution

Spin correlations between QCD emissions have so far been neglected in most of the parton shower algorithms commonly used for LHC predictions. However, their inclusion is crucial to achieve NLL accuracy or to include NLO contributions in these algorithms.
In this talk, we present a new algorithm for incorporating spin correlations into parton shower algorithms. Instead of using spin density matrices as in the Collins-Knowles algorithm, the common choice for this task, our algorithm relies on soft currents to exchange spin information between emissions. This has the advantage of reducing the number of amplitude evaluations and, thus, improving the performance compared to the standard algorithm. During the talk I will discuss the idea of the algorithm and the results of its first implementation for final-state particles. Once fully developed, it will be the basis for the inclusion of spin correlations in the parton shower code ALARIC.

Speaker: Diana Mareen Hoppe (Technische Universitaet Dresden (DE))

17:00 Parton shower recoil with jets

Speaker: Jack Helliwell (Monash University (AU))

17:30 Towards two-loop EW corrections at NLL in OpenLoops

At energies above the Electroweak (EW) scale, higher-order EW corrections exhibit a logarithmic enhancement which is driven by the ratio of the typical scattering energy to the gauge-boson mass. At next-to-leading order (NLO) these corrections lead to factors amounting to several tens of percent in tails of kinematic distributions of crucial LHC processes, and still contribute a few percent at next-to-next-to-leading order (NNLO). As such, their inclusion is essential to reduce theoretical uncertainties arising from missing higher-order corrections.
In this talk, I will review the key features of the algorithm implemented in OpenLoops (OL) for calculating one-loop EW corrections in the logarithmic Sudakov approximation, namely at next-to-leading logarithmic (NLL) accuracy, and present the status towards its extension to the two-loop level. This approximation efficiently implements the Denner-Pozzorini algorithm using an effective vertex approach, enabling to reproduce the full one- and two-loop results with percent-level accuracy while preserving tree-level computational complexity.

Speaker: Lorenzo Mai (University of Genova)

Wednesday 16 July

09:00 → 09:30 Timelike Factorization in jet physics

Speaker: Ian Moult

09:40 → 10:10 Recent developments in jet substructure calculations

Speaker: Prof. Simone Marzani (Università di Genova and INFN Sezione di Genova)

10:20 → 10:40 Automation of NNLL calculations

Speaker: Andrea Banfi (University of Sussex)

10:50 → 11:20 Coffee

11:20 → 11:50 Recent theoretical advancements in the description of energy correlations

Speaker: Huaxing Zhu (ZJU - Zhejiang University (CN))

12:00 → 12:30 TBD

Speaker: Gherardo Vita

12:40 → 14:00 Lunch

14:00 → 15:00 TH colloquium: The Strong Interaction at 50 Years: less Puzzling, more Rich, and still Mysterious

https://indico.cern.ch/event/1513473/

Speaker: Prof. Iain Stewart

15:00 → 15:45 Coffee

15:45 → 16:15 Thrust distribution in electron-positron annihilation at full NNNLL+NNLO (and beyond) in QCD

Speaker: Giancarlo Ferrera (Università degli Studi e INFN Milano (IT))

16:25 → 16:55 On Determining $\alpha_s(m_Z)$ from Dijets in $e^+e^-$ Thrust

We update a previous N$^3$LL$^\prime$+${\cal O}(\alpha_s^3)$ determination of the strong coupling from a global fit to thrust data. Detailed discussions are provided concerning the stability of the results under variations of the fit range and the importance of summing up higher-order logarithmic terms for convergence and stability. We also demonstrate that a number of additional effects associated to power corrections have a small impact on this fit result, including modifications to the renormalon substraction scheme for dijet power corrections and the inclusion of three-jet power correction models.

Speaker: Miguel Benitez (University of Salamanca)

17:05 → 17:35 Non-perturbative power corrections to $e^+e^-$ event shapes in the three-jet region and strong coupling fits

Speaker: Paolo Nason

17:45 → 18:05 A Precise Determination of $\alpha_s$ from Heavy Jet Mass

A global fit for $\alpha_s(m_Z)$ is performed on available $e^+e^-$ data for the heavy jet mass distribution. The state-of-the-art theory prediction includes $\mathcal{O}(\alpha_s^3)$ fixed-order results, N$^3$LL$^\prime$ dijet resummation, N$^2$LL Sudakov shoulder resummation, and a first-principles treatment of power corrections in the dijet region. Theoretical correlations are incorporated through a flat random-scan covariance matrix. The global fit results in $0.1145^{+0.0021}_{-0.0019}$, compatible with similar determinations from thrust and $C$-parameter.
Dijet resummation is essential for a robust fit, as it engenders insensitivity to the fit-range lower cutoff; without resummation the fit-range sensitivity is overwhelming.
In addition, we find evidence for a negative power correction in the trijet region if and only if Sudakov shoulder resummation is included.

Speaker: Arindam Bhattacharya

19:30 → 21:30 Dinner at Bistrot du lion d'or

https://liondorcarouge.ch/en/

Thursday 17 July

09:00 → 09:30 High-energy limit of QCD

Speaker: Einan Gardi

09:40 → 10:10 Reggeization in Color

Speaker: Prof. Iain Stewart

10:20 → 10:40 Vector boson+dijets at large mjj

We present the first calculation of high-energy corrections to the
process of photon+dijet production. The high-energy corrections stabilise
the fixed-order perturbative behaviour and lead to a significant
improvement in the description of data.

Speaker: Jeppe Andersen (IPPP, University of Durham)

10:50 → 11:20 Coffee

11:20 → 11:50 TBD

Speaker: Luca Buonocore (CERN)

12:00 → 12:30 Super-leading logarithms from effective field theory

Factorization theorems for non-global observables at hadron colliders can be used to resum super-leading logarithms (SLLs) and we present a phenomenological analysis of their numerical impact in pp -> 2 jets.

SLLs are closely related to collinear factorization breaking and are driven by a double-logarithmic evolution equation in an effective field theory. The compatibility of this double-logarithmic evolution with single-logarithmic PDF evolution at low scales implies stringent consistency conditions on the low-energy matrix element of this effective theory. 

We will derive these conditions in our talk; the explicit computations necessary to verify consistency at the three-loop level will be covered in a separate presentation.

Speaker: Thomas Georg Becher (Universitaet Bern (CH))

12:40 → 13:00 Differential threshold resummation for hadronic and SIDIS processes

Using renormalization group arguments in dQCD, I derive an analytic resummation formula for the threshold resummation of differential distributions with fixed longitudinal partonic kinematics. By matching to existing fixed-order NNLO computations I obtain NLL resummation for the Drell-Yan rapidity distribution with fixed partonic rapidity and for semi-inclusive deep-inelastic scattering. For Drell-Yan I compare to previous results obtained using SCET.

Speaker: Stefano Forte (Università degli Studi e INFN Milano (IT))

13:10 → 14:30 Lunch

14:30 → 15:30 Factorisation and non-perturbative aspects

14:30 Towards a Quantum Information Theory of Hadronization

Speaker: Johannes Michel (Nikhef/University of Amsterdam)

15:00 Renomalon analysis of SCET-II observables in the large-$\beta_0$ approximation

We present a derivation of nonperturbative corrections to SCET-II-type observables in the large-$\beta_0$ approximation based on the bubble-sum approximation within the SCET framework. We demonstrate how to obtain the leading-power nonperturbative corrections using the collinear anomaly approach and further discuss their impact on the corresponding partonic cross sections.

Speaker: Kevin Brune

14:30 → 15:30 Recent developments in jet physics

14:30 NNLO predictions with nonlocal subtractions and fiducial power corrections

Speaker: Alessandro Broggio

15:00 The Lund b-jet plane

In this talk, I will present our research on the substructure of jets containing heavy flavour. Our main goal is to better understand these jets from a theoretical perspective, using resummed perturbative techniques that are specially designed for jets coming from heavy quarks. In particular, we provide analytical predictions for several key jet substructure observables, including jet angularities and the Lund plane density for $b$-jets. I will highlight the most significant differences between our results and those obtained in the massless quark approximation.

Speaker: Andrea Ghira (Università degli studi di Genova)

15:30 → 16:00 Coffee

16:15 → 18:25 Factorisation and non-perturbative aspects

16:15 Exponentiation of next-to-soft gluon and soft quark emissions from the replica trick

Speaker: Leonardo Vernazza (INFN, Torino)

16:45 NNLO soft function for 0-jettiness in tt production

Speaker: Sebastian Edelmann

17:15 Three-loop jet function for boosted heavy quarks

Previous studies have shown that certain observables at massless $e^+e^−$ colliders producing primary top quarks can be used to measure the top quark mass with an uncertainty smaller than $\Lambda_{\rm QCD}$. The maximal sensitivity to the top mass is attained in the peak of the distribution, where Effective Field Theories (EFTs) enable the factorization of the differential cross section across different physical scales. The jet function —previously known at two loops— appears as a common ingredient in many factorization theorems, motivating the computation of this matrix element at higher perturbative orders.

In this talk, we will review the basic EFT concepts and the factorization theorem necessary to introduce the jet function. We will outline the workflow of a fixed-order calculation and present our analytic result for the three-loop jet function for boosted heavy quarks. Our result can be used to improve the calibration of the top quark mass parameter in parton-shower Monte Carlo generators and enhances the perturbative accuracy of the jet invariant mass distribution for reconstructed top quarks.

Speaker: Alberto Martín Clavero

17:45 Soft anomalous dimensions with massive legs

Speaker: Zehao Zhu (University of Edinburg)

16:15 → 18:25 Recent developments in jet physics

16:15 Super-leading logarithms and PDF factorization

I analyze the low-energy dynamics of gap-between-jets cross sections at
hadron colliders, for which phase factors in the hard amplitudes spoil
collinear cancellations and lead to double (`super-leading') logarithmic
behavior. Based on a method-of-regions analysis, I identify three-loop
contributions from perturbative active-active Glauber-gluon exchanges with the correct structure to render the cross section consistent with PDF
factorization below the gap veto scale. From this, two important conclusions may be drawn: first, no double-logarithmic terms appear in the scale evolution below the veto scale, and second, the DGLAP evolution must be correctly matched as well.

Speaker: Dominik Schwienbacher

16:45 The perturbative structure of jet vetoes in azimuthal gaps

The distribution of transverse energy flow into an azimuthal strip presents novel theoretical features and accompanying challenges in its all-order description. Its resummation structure depends non-trivially on the axis definition, with the standard thrust axis breaking naive double-logarithmic exponentiation. Moreover it receives non-global logarithmic (NGL) enhancements that start with $\mathcal{O}(\alpha_s^2L^3)$ rather than the usual single-logarithmic NGL structure. We investigate the logarithmic structure of this observable at both lepton and hadron colliders and present resummed analytic results at the next-to-double-logarithmic order, as well as a numerical algorithm for its leading-logarithmic resummation at leading $N_C$. We find that standard widely used dipole showers fail to describe the observable beyond crude double-logarithmic estimates. This study shows that simple modifications to well-studied observables can lead to complex theoretical structures, underscoring the need for flexible numerical tools with high logarithmic precision.

Speaker: Alexander Fraley (University of Manchester)

17:15 SLLs in ttbar production

Speaker: UPALAPARNA BANERJEE (JGU Mainz)

17:45 Partial N3LL + NNLO Resummed Predictions for the Drell-Yan Process in Rapidity Dependent Jet Veto Observables

Jet vetoes are important tools that can be used to separate hard processes. A commonly used variable in which jets are identified and vetoed is the transverse momentum of a jet. It can also be useful to vary the tightness of such cuts depending on the rapidity of a jet depending on the focus on central or forward jets. This naturally leads to a class of such variables related to the leading jets transverse momentum weighed by some rapidity dependent function. Here we explore various types of rapidity dependent jet veto variables.

Applying tight cuts on such variables requires resummation of large logarithms, which can cause fixed order predictions to be unphysical. Here we present cutting edge partial N3LL + NNLO results for the Drell-Yan process in 2 rapidity dependent jet veto variables. These predictions act as a test for SCET and can be used to probe into the properties of QCD radiation.

Speaker: Thomas Clark

Friday 18 July

09:00 → 09:30 Overview of jet physics in heavy-ion collisions

Speaker: Joao Lourenco Henriques Barata

09:40 → 10:10 Theoretical understanding of QCD cascades in heavy-ion collisions

Speaker: Dr Guilherme Milhano (LIP-Lisbon & CERN TH)

10:20 → 10:40 KNO scaling in quark and gluon jets at the LHC

The Koba-Nielsen-Olesen (KNO) scaling of hadron multiplicity distributions, empirically confirmed to hold approximately in $e^+e^-$ collisions and Deep Inelastic Scattering, has been observed to be violated in hadron-hadron collisions. In this work, we show that the universality of KNO scaling can be extended to hadron-hadron collisions when restricted to QCD jets. We present a comprehensive study of KNO scaling in QCD jets produced in proton-proton collisions at the LHC. Using perturbative QCD calculations in the double logarithmic approximation and PYTHIA simulations, we find that KNO scaling approximately holds for both quark and gluon jets across a broad jet $p_T$ range, from $0.1$ TeV to $2.5$ TeV, at both the parton and hadron levels. Especially, we highlight characteristic differences between the KNO scaling functions of quark and gluon jets, with the quark-jet scaling function lying above that of gluon jets at both low and high multiplicities. This distinction is essential for interpreting inclusive jet data at the LHC. Furthermore, we propose direct experimental tests of KNO scaling in QCD jets at the LHC through quark-gluon discrimination using jet substructure techniques, as demonstrated by applying energy correlation functions to PYTHIA-generated data.

Speaker: Dr Lin CHEN (Instituto Galego de Fisica de Altas Enerxias - IGFAE)

10:50 → 11:20 Coffee

11:20 → 11:50 Recent developments in the calculation of higher-order matrix-element in heavy-ion collisions

Speaker: Peter Arnold (University of Virginia (US))

12:00 → 12:10 Farewell

Speakers: Alba Soto Ontoso (Universidad de Granada (ES)), Dr Alexander Karlberg (CERN), Daniel Reichelt, Luca Buonocore (CERN), Pier Francesco Monni (CERN)

12:40 → 13:55 Lunch