Fragmentation in the collider precision era

17 Mar 2025, 08:00 → 19 Mar 2025, 18:00 Europe/Zurich

KOL-H-317, KOL-E-18 (UZH Zentrum)

Leonardo Bonino (Universität Zürich), Matteo Cacciari (LPTHE Jussieu), Thomas Kurt Gehrmann (University of Zurich (CH)), Markus Löchner (Universität Zürich), Kay Schönwald (University of Zuerich), Giovanni Stagnitto (Universita & INFN, Milano-Bicocca (IT))

With the increase of the precision in theory calculations and experimental data, the study of the fragmentation of light and heavy quarks has received renewed attention in the past few years. The aim of this workshop is to gather the leading experts in the field to share results and ideas.

The workshop will cover a vast range of topics relevant to fragmentation, broadly listed in the following categories:

• fixed-order QCD calculations
• resummation and parton showers
• effective theories
• global fits of fragmentation functions
• experimental results at present colliders
• prospects for future experiments

 

It will be held at the University of Zürich from 17th to 19th of March 2025. Participation is upon invitation, but can also be requested via registration form.

The workshop is supported by the Pauli Center for Theoretical Studies.

Workshop Image 1

Workshop Image 2

Monday 17 March

08:30 Registration

1 Welcome

Speakers: Leonardo Bonino (Universität Zürich), Markus Löchner (Universität Zürich)

FFworkshop_intro.pdf

2 Introduction to fragmentation

I will start this introduction talk by some generalities about Fragmentation Functions (FFs). Then, I will move to FFs of partons into light hadrons, discussing the different reactions used to extract the non perturbative inputs and the available FFs. Lastly, I will present two cases where the FFs played an important role, namely the inclusive production of a heavy hadron and the di-photon production at hadronic colliders.

Speaker: Jean-Philippe GUILLET (LAPTh CNRS)

intro_ff.pdf

10:15 Coffee Break

3 NNLO QCD corrections to SIDIS

• e-Print: 2404.09959 [hep-ph]
• e-Print: 2312.17711 [hep-ph]

Speaker: Sven-Olaf Moch (Hamburg University (DE))

zrh-sidis.pdf

4 Open bottom production at NNLO+NNLL

In this talk, I will discuss the recent calculation of open bottom production at hadron colliders at next-to-next-to-leading order including the resummation of quasi-collinear logarithms through next-to-next-to-leading-logarithmic accuracy. This computation essentially extends the FONLL approach from NLO+NLL to NNLO+NNLL in a straightforward way, allowing for the computation of fully differential observables involving $b$-quarks, $B$-hadrons or $B$-hadron decay products. Aside from the technical details, I will also discuss comparisons to data, where the significantly improved predictions highlight a discrepancy in non-prompt $J/\psi$ production.

Speaker: Terry Generet (University of Cambridge)

Generet.pdf

12:15 Lunch Break

5 Overview of ALICE heavy-flavour measurements sensitive to fragmentation

Produced mainly in hard-scattering processes occurring in the initial stage of the collisions, heavy quarks offer a unique perspective to study quark fragmentation and hadronisation. The ALICE experiment at the LHC has performed several measurements in proton--proton collisions of the transverse-momentum ($p_\mathrm{T}$)-differential cross section of charm and beauty mesons and baryons and of the properties of the jets containing them. While prompt and non-prompt D-meson cross sections are well reproduced by theoretical models using fragmentation functions tuned on $\mathrm{e^{+}e^{-}}$-collision data, a large excess of $\mathrm{\Lambda_{\rm c}^{+}}$, $\mathrm{\Xi_{\rm c}^{0,+}}$, $\mathrm{ \Sigma_{\rm c}^{0,++}}$, $\mathrm{\Omega_{\rm c}^{0}}$, and non-prompt $\mathrm{\Lambda_{\rm c}^{+}}$ baryon production was found up to $p_\mathrm{T}\approx 10-20$ $\mathrm{{GeV}}/c$.
The effect, which was investigated also as a function of multiplicity, evidences a dependence of hadronisation on the collision system and questions the validity of the assumption of fragmentation universality, at least of its late non-perturbative phase, suggesting that other concurrent processes are relevant in hadronic collisions. Further information and constraints on the description of fragmentation in Monte Carlo event generators are provided by the measurements of the fraction of jet-momentum carried by D mesons and $\mathrm{\Lambda_{\rm c}^{+}}$, as well as by the study of the azimuthal correlations of charm hadrons with light particles.

In this talk, an overview of the main measurements related to heavy-quark fragmentation will be given. Comparison with light-flavour hadron results will be discussed when relevant.

Speaker: Andrea Rossi (INFN, Padova (IT))

RossiALICE_WSff_Zurich2025-8.pdf

6 Experimental results from LHCb

Speaker: Mick Mulder (University of Groningen (NL))

presentation_fragmentationworkshop_lhcb_20250317.pdf

15:15 Coffee Break

7 Fragmentation in GM-VFNS Open Heavy-Flavor Production

The open heavy-flavor-production process is of great importance for collider physics since it is sensitive to the gluon PDF at low $x$. We use an existing general-mass variable-flavor-number scheme (GM-VFNS) implementation of this process to study D- and B-meson production in proton-proton and proton-nucleus collisions, which we extend by an interface to the PineAPPL gridding library. The latter makes it possible to evaluate the predictions with different PDFs or FFs in only a fraction of the time needed by a full recalculation. Among others, we study the FF dependence and uncertainty of the predictions, where, for instance, we observe a small dependence on variations of the fragmentation scale around the central value. The study includes an analysis of the probed kinematic region of the predictions, including the fragmentation variable $z$. We plan to release the PDF- and FF-independent grids of the predictions produced with an upcoming update of the PineAPPL library. These grids will then be used in upcoming nCTEQ global nPDF fits.

Speaker: Jan Wissmann (Universität Münster)

wissmann_gmvfns.pdf

8 Bridging massive and massless schemes for soft gluon resummation in heavy-flavour production

Perturbative calculations for processes involving heavy flavours can be carried out using two approaches: the massive and the massless schemes. These schemes can also be combined to leverage their respective strengths. Additionally, both massive and massless frameworks can be supplemented by soft-gluon resummation. However, matching resummed calculations across the two schemes presents significant challenges, primarily due to the non-commutativity of the soft and small mass limits. In this talk we will focus on a consistent resummation of mass and soft logarithms at next-to-leading logarithmic (NLL) accuracy. Furthermore, we will extend this framework to achieve the so-called NLL′ accuracy, which accounts for finite terms in the soft and massless limit.

Speaker: Andrea Ghira (Università degli studi di Genova)

Zurich_AG.pdf

9 Heavy quark mass effects: jets and identified hadrons

I will discuss the role of power corrections arising from heavy-flavour quark masses, and study the impact they can have on some simple differential cross-section predictions that involve final-states with jets and identified hadrons.

Speaker: Rhorry Gauld (MPP Munich)

Gauld_FragJets.pdf

Tuesday 18 March

10 Experimental results from ATLAS

The ATLAS experiment has recently performed measurements of identified hadrons in $\sqrt{s}$ = 13 TeV proton-proton collisions, providing precision tests of QCD across a range of energy scales. This talk will present overview of results on the production cross-sections of J/ψ and ψ(2S) mesons, differential cross-section measurements of $D^{\pm}$ and $D^{\pm}_{s}$ mesons, W boson production in association with a charmed hadron, b-quark fragmentation properties in jets, jet track functions, and underlying-event studies with strange hadrons. The results are compared to latest QCD calculations, highlighting areas of agreement and discrepancies that motivate further theoretical developments.

Speaker: Dr Daniil Ponomarenko (Indiana University (US))

20250317_FCPE_Ponomarenko.pdf

11 Experimental results from CMS

Speaker: Valentina Mariani (Universita e INFN, Perugia (IT))

Mariani.pdf

10:15 Coffee Break

12 The NPC analysis on fragmentation functions of light hadrons

In this talk I will introduce a new framework on global analysis of fragmentation functions by the NPC collaboration. I will report our recent published results on FFs of light charged hadrons as well as several ongoing works on FFs of neutral hadrons and projections for future lepton colliders.

Speakers: Jun Gao (Deutsches Elektronen-Synchrotron (DE)), Jun Gao (Shanghai JiaoTong University)

zurich2025JG.pdf

13 Updates to BDSSV Fragmentation Functions

We revisit the BDSSV22 global QCD analysis of parton-to-pion fragmentation functions at next-to-next-to-leading order (NNLO) accuracy, in the light of the recently obtained NNLO coefficient functions for semi-inclusive deep-inelastic scattering (SIDIS). We explore the impact of these full NNLO corrections on the description of SIDIS datasets across various kinematic regimes and their implications for the resulting pion fragmentation functions.

Speaker: Ignacio Borsa (Tübingen University)

IB-ZURICH-FFs-25.pdf

12:15 Lunch Break

14 Semi-inclusive deep-inelastic scattering: phenomenological challenges and opportunities with Fragmentation Functions

I discuss some recent efforts into the phenomenological study of semi-inclusive deep-inelastic scattering performed by the MAP and NNPDF collaborations. In particular, I focus on three aspects. 1) The determination of the fragmentation functions of light hadrons from single-inclusive hadron production in electron-positron annihilation and deep-inelastic scattering, including next-to-next-to-leading order corrections in the strong coupling. 2) The implications of these determinations on the unpolarised and polarised parton distribution functions of the proton. 3) The extension of the PineAPPL framework to fast-interpolation tables that allow us to streamline all these studies.

Speaker: Emanuele Roberto Nocera

NOCERA_FFEICZURICH.pdf

15 TMD extractions using Drell-Yan and SIDIS data

Starting from TMD factorization of Drell-Yan and SIDIS it is possible to extract the nonperturbative TMD parton distribution for initial and final states. We are currently computing the fit of the TMD for both these processes. The fit of SIDIS data has resulted problematic to some groups. In our case we show that the form of fragmentation function can be crucial to understand data.

Speakers: IGNAZIO Scimemi (Universidad Complutense madrid), Ignazio Scimemi (Universidad Complutense (ES))

Zurich2025Scimemi.pdf

15:15 Coffee Break

16 Heavy Quark TMD FFs

I give an overview of recent progress in understanding the dynamics of heavy-quark fragmentation in the transverse plane. These advances include their all-order factorization in the large and small-mass limits using boosted Heavy-Quark Effective Theory (bHQET) or a twist expansion, respectively, explicit next-to-leading order calculations in bHQET and massive QCD (also relevant for extending fixed-order subtraction schemes to quasi-collinear limits), as well as powerful sum rules arising from heavy-quark spin symmetry that trim down the number of independent nonperturbative matrix elements. I close by presenting exciting prospects for applying tools from Quantum Information theory to TMD heavy-quark fragmentation to distinguish quantum and classical hadronization dynamics.

Speaker: Johannes Michel (MIT CTP)

2025-03-18_Johannes_Michel_heavy_quark_tmd_ffs.pdf

17 Hadronization in Monte Carlo Generators

We will briefly review the most common hadronization models in Monte Carlo event generators, namely the Lund string model and the cluster hadronization model. Some hints towards limitations of these models become apparent once they are confronted with recent data from the LHC. We will discuss these shortcomings and point out some newer developments that might help to alleviate these in the near future.

Speaker: Stefan Gieseke (Karlsruhe Institute of Technology)

zurich25.pdf

19:35 Social Dinner

Wednesday 19 March

18 Collinear fragmentation at NSL

We discuss critical analytical ingredients that are necessary to reproduce NSL terms of collinear origin in the context of a parton shower algorithm.
These ingredients are related to the resummation coefficient "B2" in an analytical resummation approach, which governs the intensity of collinear radiation from an energetic initial parton. They can also be viewed as defining the scheme of the strong coupling, beyond the known soft limit result given by the known coefficient A2.

We show how such ingredients may be calculated using specific integrals of the triple-collinear splitting functions and associated one-loop corrections. We also discuss new results ranging from analytic resummation for groomed jet shapes to NNLL event shape resummation in parton showers and achieving NSL non-singlet fragmentation in parton showers.

Speaker: Prof. Mrinal Dasgupta (The University of Manchester (GB))

md-zurich-fragmentation.pdf

10:15 Coffee Break

19 Beyond collinear fragmentation

Going beyond the inclusive hadron spectrum described by the collinear fragmentation functions, I want to discuss a range of topics:
(i) the effect of kinematic cuts on the fragmentation spectrum (featuring fragmenting jet functions),
(ii) transverse-momentum dependent (TMD) fragmentation,
(iii) TMD fragmentation with respect to recoil-free axes (and its connection to energy correlators),
(iv) generalized fragmentation functions that enter the description of fractal jet observables.

Speaker: Wouter Waalewijn (University of Amsterdam)

fragmentation_2025.pdf

20 Beyond DGLAP: Advancing Collinear Evolution and Energy Correlations

Renormalization group equations (RGEs) are essential for understanding scale dependence in QCD. While the DGLAP equations form the cornerstone of collider physics, modern studies of jet substructure and energy flow correlations demand a framework that extends beyond the standard DGLAP paradigm. We analytically derive a general RGE for collinear dynamics that incorporates correlations in fragmentation at next-to-leading order, with an approach extendable to higher orders. Connections to the evolution of single- and multi-hadron fragmentation functions are established, and a numerical implementation of the nonlinear evolution equation is developed. Applying this framework, we achieve the collinear NNLL resummation for energy-energy correlations of charged hadrons, providing precise predictions crucial for interpreting recent and future experimental measurements at colliders.

Speaker: Yibei Li

Yibei Li_slides_Zurich_Fragmentation_v3.pdf

12:15 Lunch Break