Parton Showers and Resummation 2026

Jul 8, 2026, 8:45 AM → Jul 10, 2026, 7:10 PM Europe/London

Jocelyn Bell Burnell Lecture Theatre, Schuster Building (University of Manchester)

Emmet Byrne, Thomas Cridge (DESY), Mrinal Dasgupta (The University of Manchester (GB)), Caterina Doglioni (The University of Manchester (GB)), Jeffrey Forshaw, Jonathan Richard Gaunt (University of Cyprus (CY)), Jack Holguin (University of Manchester), Saad Nabeebaccus (University of Manchester), Andrew Pilkington (University Of Manchester), Mike Seymour (The University of Manchester (GB))

The 2026 annual workshop on Parton Showers and Resummation (PSR2026) will be held on July 8-10 at the University of Manchester. 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.

Accommodation at select hotels is available at a discounted rate until June 10th, see Practical information for more details.

Note that PSR2026 will be held in the week just before BOOST 2026 in Krakow. For attendees of both workshops, Manchester and Krakow are well-connected by direct flights.

Most visitors to the UK now require an Electronic Travel Authorisation (ETA) before travelling. Applications are made through the official UK Government website at gov.uk/eta, and costs £20.

 

List of confirmed speakers:

 

Carlota Andres

Andrea Banfi

Thomas Becher

Luca Buonocore

Jon Butterworth

Leticia Cunqueiro Mendez

Silvia Ferrario Ravasio

Terry Generet

Andrea Ghira

Patrick Hager

Jack Holguin

Sebastian Jaskiewicz

Daniele Lombardi

Simone Marzani

Saad Nabeebaccus

Zoltan Nagy

Paolo Nason

Simon Plätzer

Daniel Reichelt

Jennifer Roloff

Joanne Roper

Gavin Salam

Nicolas Schalch

Dominik Schwienbacher

Mike Seymour

Gregory Soyez

Giovanni Stagnitto

Siddharth Sule

James Whitehead

 

This workshop is supported by the IPPP Durham.

 

   

PSR_photo.png

Wed, July 8

Introduction

PSR--intro.pdf

Talks: Wednesday 1

Convener: Caterina Doglioni (The University of Manchester (GB))

1 Getting the final state right matters: An experimentalist’s perspective

With the HL-LHC on its way, and FCC-ee on the horizon, we are entering an era of precision collider physics, motivated not only by probing the limits of the Standard Model, but by looking for clues as to a bigger theory that contains it. The improved calculational techniques under discussion at this meeting will be critical. However, issues such as reproducibility of analyses, reliability of uncertainty estimates, and reinterpretability of measurements will also be of increasing importance, and are in some ways challenged by the growing use of sophisticated machine-learning-enabled analysis techniques. I will discuss some ways in which these challenges can be and are being addressed.

Speaker: Jonathan Butterworth (UCL)

psrjmb.pptx

2 The parton shower in heavy-ion collisions

Speaker: Leticia Cunqueiro Mendez (Roma Sapienza University)

PartonShowerinHeavyIons.pdf

coffee break

Talks: Wednesday 2

Convener: Caterina Doglioni (The University of Manchester (GB))

3 Cell Reweighting and Unbinned Validation for Negative-Weight Mitigation using Optimal Transport

As the accuracy of experimental results increases in high energy physics, so too must the precision of Monte Carlo simulations. Currently, event generation at next-to-leading order (NLO) accuracy in QCD and beyond results in the production of negatively-weighted events. The presence of these weights increases strain on computational resources by degrading the statistical power of MC samples, and can be pathological in the context of machine learning. We have developed a post hoc "cell reweighting" scheme that applies an IRC-safe metric in the multidimensional space of events so that nearby events are reweighted together, with the metric implemented using Optimal Transport techniques borrowed from computer vision to address this longstanding problem in computational particle physics. We compare the performance of the algorithm under different choices of metric and explicitly demonstrate its behaviour on simulated events with a Z boson and two jets produced at NLO accuracy.

To validate that these full phase-space reweightings preserve the physical fidelity of the underlying model — a task for which comparisons to marginalized 1D kinematic histograms can mask subtle biases — we additionally introduce an unbinned figure of merit based on the "Cross-Section-Mover's Distance," an Optimal Transport-based quantity that measures the work required to transform one theoretical prediction into another. This complementary metric provides a principled way to benchmark reweightings performed with different metric choices (e.g., Euclidean vs. Energy-Mover's Distance) and can be applied more broadly wherever phase-space reweighting biases must be studied in an unbinned way.

Speaker: Jennifer Roloff (Brown University (US))

cellResampling.pdf

4 Monte Carlo Event Generators: status and progress

Monte Carlo Event Generators form a bridge between the Standard Model Lagrangian and collider measurements. To do so they combine complementary approximations and models across all scales: fixed-order perturbation theory for the hard scattering, matching to or merging with the parton shower, the semi-classical shower itself for the cascade, and semi-empirical models of hadronisation and MPI. In this talk we review the fundamentals of event generation alongside recent progress, and point to the challenges ahead.

Speaker: James Whitehead

Whitehead.pdf

lunch

Talks: Wednesday 3

Convener: Mike Seymour (The University of Manchester (GB))

5 Showers and hadronization on GPUs

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

s-sule-gpus-psr-2026.pdf

6 Updates on amplitude evolution

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

slides.pdf

7 Formulation of parton shower evolution beyond leading order

The parton shower beyond the leading order is the next step to improve our
understanding of hadronic final states in collider experiments. This is rather challenging since we have to deal with
more Feynman graphs and more complicated integrals. At first order level the parton shower algorithm is rather
intuitive but higher order level the quantum interference are more important and it cannot be understood based on
classical or semi-classical probabilities. We need a general definition of the parton showers at all order level. For
this we introduce the concept of infrared renormalization and the shower cross sections are defined as the solution of
the corresponding renormalization group equations. In this talk we also discuss the mathematical framework of the
parton shower evolution that is adequate to go beyond the leading order.

Speaker: Zoltan Nagy

znagy-psr26.pdf

8 Recent Results from e^{+}e^{-} Alliance

Speaker: Hannah Bossi (Massachusetts Inst. of Technology (US))

HBossi_PSR_2026.pdf

coffee break

Talks: Wednesday 4

Convener: Silvia Zanoli (University of Oxford)

9 Resonance aware QCD+EW MC@NLO matched parton showers

Parton showers are an essential feature of event generation that provide a numerical resummation of the large logarithms that enhance soft and collinear radiation. Matching them to higher order calculations requires careful treatment in order to avoid double-counting real radiation and preserve fixed order accuracy through matching algorithms such as MC@NLO. As we approach HL-LHC, there is a need for increasing precision of theoretical predictions so the inclusion of NLO EW alongside QCD corrections and matching to a QED shower becomes necessary. QED parton showers present addition complications, including the potential for negative splitting kernels and many more spectators to each splitting. In a dipole implementation, these spectators absorb the recoil required to satisfy momentum conservation and on-shell conditions in each splitting, however this 'reshuffling' of momenta can lead to distortion of the lineshape if it spans a resonance. Thus, the need for a resonance aware implementation is much greater for a QED shower than QCD due to the expanded set of spectators.

I will present the first implementation of automated QCD+EW MC@NLO matching in the event generator Sherpa, along with an approach to treat resonances by factorising the parton shower into production and decay for hard emissions, using Drell-Yan to illustrate the cases of both a neutral and charged resonance.

Speaker: Joanne Roper (Durham University)

PSR_July26.pdf

10 Resonance and Width-aware simulation of $e^+e^- \to t\bar{t}$ at the FCC-ee

In this talk I will discuss a new technique for the next-to-leading order accurate simulation of $e^+e^- \to t\bar{t}$ that respects the resonant nature of the process above and near the top-quark pair production threshold.
Standard parton shower simulations do not account for resonances appearing in the hard process when simulating the radiation of soft gluons from external legs, neither preserving their on-shellness in the distribution of recoil, nor taking into account width suppressed effects in the splitting kernels.
I discuss the implementation of a new method, solving both problems at NLO close to the threshold, in the Alaric parton shower within the Sherpa framework, as well as the application to $e^+e^- \to t\bar{t}$ at a future FCC-ee at $\sqrt{s} = 365$ GeV.

Speaker: Daniel Reichelt (CERN)

PSR_2026_DReichelt.pdf

11 Resonance-Aware NLO+PS Matching for Off-Shell Top–Antitop Production at Lepton Colliders

The matching of NLO predictions to parton showers has long been considered a solved problem. However, it has recently attracted renewed attention with the development of a new generation of parton showers capable of achieving next-to-leading logarithmic (NLL) or even higher logarithmic accuracy. This progress poses new challenges for existing matching schemes and makes it worthwhile to revisit established approaches and explore new solutions.

In this work, we present a resonance-aware strategy for matching full off-shell NLO QCD predictions at lepton colliders to the PYTHIA 8 Simple Shower. As a case study, we focus on processes involving top–antitop pair production. Fixed-order predictions are obtained using the MC@NLO method combined with Catani-Seymour subtraction. Distortions of resonance line shapes are avoided by providing the full resonance cascade information on an event-by-event basis to the parton shower and by adapting the matching procedure through the introduction of dedicated counterterms.

Speaker: Daniele Lombardi (Università di Torino)

DL_PSR_2026.pdf

Thu, July 9

Talks: Thursday 1

Convener: Jack Helliwell (Monash University (AU))

12 PanScales Parton Showers Beyond NLL Accuracy

In this talk I will discuss recent progress to ameliorate the accuracy of the PanScales showers beyond NLL for massless final states.
In particular, I will present the inclusion of quark-mass effects and matching to fixed-order calculations. Matching is also paramout to obtain NNDL accuracy for event shapes and, combined with an improved treatment of soft radiation beyond the standard NLL approximation, it is essential for achieving NNLL and NSL accuracy for many collider observables.

Speaker: Silvia Ferrario Ravasio (Università degli Studi di Torino)

2026_07_09_PSR_PanScales_beyond_NLL.pdf

13 A first glimpse of matching beyond NLO with logarithmically accurate showers

Parton shower matching that retains logarithmic accuracy is a major challenge. We present first results beyond NLO using the PanScales framework. Focusing on $e^+e^-$ collisions, we construct a matching procedure that incorporates the full set of leading-colour double-real $\mathcal{O}(\alpha_s^2)$ contributions, in a manner consistent with the logarithmic structure of the showers. As a first phenomenological application, we present results for event shapes that receive non-zero contributions starting from four-parton configurations.

Speaker: Nicolas Francois Schalch (University of Oxford (GB))

manchester_psr.pdf

coffee break

Talks: Thursday 2

Convener: Jack Holguin (The University of Manchester)

14 Event Isotropy in perturbative QCD

It has recently been proposed that collider events can be equipped with a metric, the Energy Mover’s Distance (EMD), which allows one to rephrase multiple aspects of jet physics in a geometric language. Further, the EMD can be exploited to define new observables that measure the distance between a given event and an idealised one. For instance, event isotropy quantifies the resemblance of an event to a uniform energy distribution. We present the first field-theoretical description of the event isotropy distribution in electron-positron collisions. Our calculation includes the all-order resummation of soft and collinear logarithmic contributions at next-to-leading log accuracy, matched to next-to-leading order corrections at fixed order.

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

marzani-psr2026.pdf

15 Inclusive jet production at NNLO+NNLL and theory nuisance parameters

In this talk, I will present some recent calculations of the inclusive jet transverse momentum spectrum at the LHC, performed at next-to-next-to-leading order and including resummation of logarithms of the jet radius $R$ at next-to-next-to-leading-logarithmic accuracy. Estimates of missing-higher-order uncertainties obtained from scale variations are notoriously unreliable for inclusive jet production at the LHC. I will show how the method of theory nuisance parameters can be used to obtain robust theory uncertainties for this process, and demonstrate that such uncertainties paint a much more consistent picture than scale uncertainties, in light of the new NNLO+NNLL results.

Speaker: Terry Generet (University of Cambridge)

Generet.pdf

16 On Thrust Resummation Ambiguities in e+e− Annihilation into Hadrons

I present the results of a recent work, were different, yet legitimate, resummation prescriptions are considered for the calculation of the thrust distribution in e+e- collisions. In particular we consider differences in the resummation performed in the so called conjugate (Laplace transform) space with respect to the direct one, and differences that arises when certain commonly adopted approximations used in the conjugate space approach are avoided. We showed that these different approaches may lead to non-negligible numerical differences.

Speaker: Paolo Nason

PSR2026.pdf

lunch

Talks: Thursday 3

Convener: Emmet Byrne (The University of Manchester)

17 The Method of Regions: Recent Progress and Applications

I review recent progress in methodology and applications of the Method of Regions, the central approach to performing asymptotic expansions of Feynman integrals. In this method, general log-power expansions in singular kinematic limits are obtained through a sum over “region integrals”, each of which is defined by a Taylor expansion of the integrand under certain assumptions regarding the behaviour of the integration variables in the limit considered. In certain cases, there is a geometric algorithm in parameter space to find the complete set of regions. Recent progress, stemming from Landau singularity analysis, elucidated under what conditions the latter set is complete, and how to determine the remaining “hidden regions” otherwise. This analysis also led to a graph-theoretical momentum-space interpretation of both type of regions in certain expansions.
Turning to applications, a novel use of the geometric method is the recent computation of the three-loop soft anomalous dimension for scattering of a heavy particle and any number of massless ones. In this way the MoR contributes to our state-of-the-art knowledge of the multiloop IR structure of multileg amplitudes in general (wide-angle) kinematics. Next, I show that while in wide-angle kinematics hidden regions are only relevant for multiple hard scattering, in special kinematic limits, such as the (multi) Regge limit and the spacelike collinear limit, they are essential to understanding the dynamics, which is govern by Glauber effects.

Speaker: Einan Gardi

The_Method_of_Regions_PSR2026_short_Gardi.pdf

18 New results on small-x resummation for splitting functions

We revisit the basic steps necessary to obtain next-to-leading-logarithmic accurate small-x results for the DGLAP splitting functions, and their implementations within the HELL framework. We derive new analytical all-order results for the leading-logarithmic gg anomalous dimension, the qg and gg finite Green functions, and most importantly for the qg anomalous dimension, which allows us to arrive for the first time at a properly resummed qg splitting kernel. We use these results as cornerstones of a new implementation of small-x splitting-function resummation which is more solid and numerically better behaved with respect to those available thus far. All of these novelties are included in the upcoming 4.0 version of HELL.

Speaker: Giovanni Stagnitto (INFN e Universita Genova (IT))

smallx-upgrade-talk.pdf

19 The emerging importance of high energy logarithms for LHC studies

In this talk, I will discuss the growing importance of controlling high-energy logarithms in predictions for LHC processes. I will highlight recent directions pursued by the High-Energy Jets collaboration: Higgs boson plus quark-pair production as a background to di-Higgs searches, isolated photon plus jets, and Z plus dijets.

Speaker: Sebastian Eryk Jaskiewicz (University of Bern)

HEJ-Jaskiewicz-psr26-manchester.pdf

coffee break

Conference photo

NGL and SLLs anniversary

Convener: Mrinal Dasgupta (The University of Manchester (GB))

20 The discovery of non-global logarithms in QCD

Speaker: Gavin Salam (University of Oxford)

non-global-history.pdf

21 The discovery of super-leading logarithms in non-global observables in QCD

In 2006, Jeff Forshaw, Albrecht Kyrieleis and I published a paper in which we identified a tower of super-leading (relative to the single logarithmic series that was expected to be leading) logarithms in non-global observables and discussed their origin as a breakdown of QCD coherence, and possibly of factorization. In this historical talk, I will recall the work that led up to this paper, the reaction to it and some of the further work it stimulated.

Speaker: Mike Seymour (The University of Manchester (GB))

DiscoveryOfSLLs_no_presenter_notes.pdf

DiscoveryOfSLLs_no_presenter_notes.pptx

22 Recent progress and the modern perspective on NGLs and SLLs

After reviewing the development of effective-field-theory methods for the resummation of non-global and super-leading logarithms, I’ll discuss a recent milestone in detail, namely the resummation of subleading non-global logarithms, which was achieved using different, independent methods. I will compare these approaches, highlighting their similarities and differences, and explain some of the subtleties that arise at next-to-leading-logarithmic accuracy. I will then show a numerical comparison of results by GNOLE, MARZILI, and PanScales for the dijet gap fraction, before presenting new results for Higgs production at hadron colliders.

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

PSR2026_Becher.pdf

Conference Dinner

The conference dinner will be held at The Wharf in Castlefield. There are several ways to get there. The walk from Oxford Road Station to The Wharf is pleasant and takes around 20 minutes via the canal paths. You can either take a bus to Oxford Road Station or go by foot, which is an additional 15–20 minute walk. Alternatively, a taxi for the journey should cost around £10.

Fri, July 10

Talks: Friday 1

Convener: Jonathan Richard Gaunt (University of Cyprus (CY))

23 Factorisation, its breaking, and its restoration

Factorisation theorems underpin essentially all precision predictions at hadron colliders. Yet, for a broad class of observables, collinear factorisation is known to fail at the amplitude level. I review the current understanding of (soft-)collinear factorisation and the origin of its breaking, before discussing a recently identified mechanism through which PDF factorisation can nevertheless be restored. I place these results in the context of recent developments in splitting amplitudes, and conclude by discussing super-leading and coherence-violating logarithms and their implications for parton showers.

Speaker: Patrick Angus Hager

Hager_PSR26.pdf

24 Coherence violation in jet observables

When performing resummation of global rIRC safe observables, it is often assumed that gluons at wide angles feel the total colour charge of collinear particles. We show that this is not the case for one-jettiness, an observable which was believed to enjoy an all-order factorisation formula. In fact, due to the presence of Coulomb gluons, one-jettiness exhibits super-super-leading logarithms, a new class of coherence-violating contributions, one log higher than super-leading logarithms affecting gaps between jets.

Speaker: Andrea Banfi (University of Sussex)

banfi_PSR26.pdf

coffee break

Talks: Friday 2

Convener: Jonathan Richard Gaunt (University of Cyprus (CY))

25 Factorization Beyond Coherence

I derive a novel factorization theorem for N-jettiness at hadron colliders, which incorporates coherence-violating effects induced by Glauber gluons and several new momentum modes. Their interplay generates coherence-violating logarithms (CVLs) starting at four-loop (N≥1) or five-loop order (N=0). I calculate the anomalous dimensions required for the resummation of CVLs and establish a general framework for the systematic treatment of coherence violation. The findings imply that most existing factorization formulas for global LHC observables must be revised.

Speaker: Dominik Schwienbacher

PSR_2026_Dominik_Schwienbacher.pdf

26 New logarithmic structures in transverse-energy flow observables with azimuthal vetoes

In this talk, I will discuss a class of non-global observables in which one vetoes transverse-energy flow inside an azimuthal interval $\Delta \phi$ defined with respect to a suitable reference axis. Such vetoes are widely used in measurements involving missing transverse momentum at the LHC and probe a kinematic regime that has not previously been explored. We show that these observables exhibit novel logarithmic structures. In particular, both non-global logarithms (NGLs) and coherence-violating logarithms (CVLs) acquire an additional power of the large logarithm $L$, making them more singular than in standard non-global observables. For $pp$ collisions, we study the all-orders structure of these logarithms and perform an analytical resummation at next-to-double-logarithmic (NDL) accuracy, together with a numerical leading-logarithmic (LL) resummation in the large-$N_c$ limit. Furthermore, we calculate the leading contribution of the CVLs and show that they are enhanced from N$^{3}$DL to NNDL accuracy. This enhanced sensitivity makes the observable a particularly promising probe of collinear-factorization breaking at hadron colliders.

Speaker: Saad Nabeebaccus (University of Manchester)

nabeebaccus-PSR.pdf

27 Non-global Super-Leading Logarithms

We study subleading contributions to super-leading logarithms (SLLs) arising from two soft emission operators.
For the case of one Glauber insertion, we compute the full color structure and scale integrals, and characterize the corresponding angular integrals.
At this level of accuracy, for the first time, we are able to investigate the interplay of non-global and super-leading logarithms.
This work therefore represents a crucial step towards a deeper understanding of the origin and structure of SLLs.

Speaker: Josua Scholze

Nonglobal_SLLs_JosuaScholze.pdf

lunch

Talks: Friday 3

Convener: Simone Marzani (Università di Genova and INFN Sezione di Genova)

28 Collinear Factorization Violation and Reggeization

We demonstrate Reggeization associated with Glauber gluon exchanges in the strict collinear factorization violating pieces in the space-like splitting amplitudes featuring aspects of both forward  and hard scattering. We find that by exploiting contour deformation away from the Glauber region wherever possible the collinear subgraphs defined in the effective theory can be computed without requiring a non-analytic regulator.  Thus, a challenging two-loop computation is reduced to a handful of significantly simpler  one-loop subgraphs. Our work is aided with development of new software tools initiating a systematic study of processes involving Glauber exchanges at a multi-loop level.

Speaker: Dr Aditya Pathak (DESY)

29 Energy correlators, light-ray operators and new directions

Energy correlators admit a dual description as light-ray operators obeying an operator product expansion on the celestial sphere. I give a pedagogical introduction to the light-ray OPE and its misconceptions, then show that inclusive jet cross-sections are also light-ray measurements. Using the OPE, I derive the small-R jet factorisation theorem, pin down the scaling of hadronisation and higher order corrections, and outline what this framework opens up — including jets at strong coupling.

Speaker: Jack Holguin (The University of Manchester)

PSR_2026.pdf

30 Generalized detector correlators in the heavy-ion environment from the light-ray OPE

Correlation functions of generalized detectors provide a broad class of measurements relevant for heavy-ion jets, including the well-known energy–energy correlator (EEC), its energy-squared variant (E2E2C), and charged energy correlators. In this talk, we show that correlators of generalized detectors admit a universal theoretical description based on the light-ray operator product expansion (OPE) and track functions. Medium-driven modifications appear as an enhancement of higher-twist matrix elements in the light-ray OPE, making these observables directly sensitive to the heavy-ion environment. We validate this framework against CMS measurements of the EEC and E2E2C, revealing an enhanced quark-jet fraction in 0–10% central Pb–Pb collisions relative to p–p, which cannot be explained by nuclear parton distribution functions alone.

Speaker: Carlota Andres (CPHT, Ecole polytechnique)

andresc_psr.pdf

31 Towards Higher-Accuracy Resummation for Heavy-Flavour Jet Substructure

In this talk, I will present recent resummed and matched predictions at next-to-leading logarithmic (NLL) accuracy for heavy-flavour jet substructure. I will also discuss recent developments aimed at extending these calculations to higher logarithmic accuracy in the resummation framework.

Speaker: Andrea Ghira (TUM)

Ghira_PSR2026.pdf

coffee break

Talks: Friday 4

Convener: Saad Nabeebaccus (University of Manchester)

32 Soft functions in N-jettiness

Speaker: Luca Buonocore (CERN)

PSR2026_LB.pdf

33 Efficient Calculation of Soft functions

Over the past several years, a collection of techniques have enabled the efficient computation of new soft and beam functions at N$^2$LO. In this talk, I show how we have used some of those techniques to compute new soft functions at N$^2$LO, firstly for generalised event shapes at $e^+ e^-$ colliders, and secondly for the measurement of boson transverse momentum along with a rapidity-dependent jet veto at $pp$ colliders. Our focus is to make the calculation as simple as possible, and to make full use of existing results, in order to pave the way for higher-order calculations, and more differential observables.

Speaker: Emmet Byrne (The University of Manchester)

Emmet_PSR_2026.pdf

34 Automated Calculation of Soft Functions for Non-global Observables

The analytic, factorisation based, resummation of non-global logarithms requires ingredient functions of higher computational complexity than their global counterparts, due to the disparate treatment of different phase space regions, thus necessitating numerical evaluation. In this talk I will present progress towards automating the calculation of soft functions for non-global observables to second order in the strong coupling constant, with special focus on the implementation of rapidity regularisation, and the path towards a public code for the calculation of such soft functions.

Speaker: Rudi Rahn (University of Vienna)

PSR_Rahn.pdf

Closing

PSR-2026-concluding.pdf