Parton Showers, Event Generators and Resummation 2018

4 Jun 2018, 09:00 → 6 Jun 2018, 17:30 Europe/Zurich

Lundmarksalen (Department of Theoretical Physics and Astronomy, Lund University)

The aim of the PSR workshop is to bring together the QCD communities involved in analytic resummations and parton shower event generators. In particular, it will cover the following topics:

• Different resummation techniques and their relations
• Different parton-shower formalisms and their relations
• The connection between parton showers and resummation
• Matching of showers and resummations with fixed-order results

The workshop will continue the successful series of meetings at DESY Hamburg (2009, 2012), Manchester (2010), Durham (2013), Munster (2014), Krakow (2015) , Paris (2016) and Cambridge (2017).

Organisers: J. Andersen, M. Cacciari, M. Dasgupta, S. Jadach, A. Kulesza, E. Laenen, L. Lönnblad, A. Mitov, Z. Nagy, S. Plätzer, M. Schönherr, A. Siodmok, G. Soyez, B. Webber

Local organising committee: J. Bellm, L. Lönnblad, M. Sjödahl, T. Sjöstrand.

 

Monday, 4 June

09:00 Registration

1 Welcome

Speaker: Leif Lönnblad (Lund University (SE))

Overview PS / R

2 Overview Parton Shower

Speaker: Stefan Hoche (SLAC National Accelerator Laboratory (US))

lund_18-06.pdf

3 Overview Resummation

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

lund18.pdf

11:10 Coffee

Jets and Substructure

4 A study of jet mass distributions with grooming

We perform a phenomenological study of the invariant mass distribution of hadronic jets produced in pp collisions, in conjunction with a groomer (modified MassDrop Tagger and Soft Drop). Our calculation resums large logarithms of the jet mass and includes the full dependence on the groomer’s energy threshold parameter, and it is matched to fixed-order QCD matrix elements at next-to-leading order. We accounted for non-perturbative contributions by including a correction factor derived from Monte Carlo parton-shower simulations.

Speaker: Laís Sarem Schunk (IPhT, CEA - Saclay)

psr-2018-schunk.pdf

5 Exploring improvements to the fitting of the strong coupling constant through means of soft drop

Speaker: Vincent Theeuwes (University of Goettingen)

VTheeuwes-PSR2018.pdf

12:30 Lunch

Parton Showers

6 Logarithmic accuracy of parton showers

I present some first fundamental elements of an approach for assessing the logarithmic accuracy of parton-shower algorithms based
on two broad criteria: their ability to reproduce the singularity structure of multi-parton matrix elements, and their ability to reproduce logarithmic resummation results.
As a case study, I consider properties of two
transverse momentum ordered final-state showers, examining features up to second order in the strong coupling. In particular, it is possible to identify regions where they fail to reproduce the known singular limits of matrix elements. This inevitably affects the logarithmic resummation accuracies of the shower, both in terms of leading (double) logarithms at subleading N_C and next-to-leading (single) logarithms at leading N_C.

Speaker: Pier Francesco Monni (CERN)

PSR_Monni.pdf

7 Subleading colour corrections in Herwig

We investigate the effects of keeping the full color structure for parton emissions in parton showers for both an LEP and an LHC setup. This is done within the Herwig dipole shower by treating the subleading Nc terms as color matrix element corrections.

Speaker: Per Johan Hakan Thoren

PSR_johant.pdf

8 Soft gluon evolution and non-global logarithms

Speaker: Jeffrey Forshaw (University of Manchester)

Jeff.pdf

9 ResBos2 and Full Color Parton Showers

TBA

Speaker: Joshua Isaacson (Fermilab)

PSR2018_isaacson.pdf

15:30 Coffee

Resummation

10 Jet Vetoes with a Jet Rapidity Cut

Speaker: Frank Tackmann (Deutsches Elektronen-Synchrotron (DE))

2018-06-04_PSR_FT.pdf

11 Combined resummation of transverse momentum and beam thrust in Drell-Yan

The Drell-Yan cross-section differential in both transverse momentum and beam thrust contains large logarithms of both variables that require resummation. I will discuss a framework to jointly resum these logarithms in Soft-Collinear Effective Theory, and present obtained numerical results. In this talk, I will also address some technical aspects, such as the combination of relevant regions, the method of hybrid matching in momentum space and impact parameter space and the specific choices of (profile) scales.

Speaker: Mr Gillian Lustermans (University of Amsterdam/Nikhef)

Lustermans_PSR_2018.pdf

12 Non-global logarithms in jet and isolation cone cross sections

Starting from a factorization theorem in effective field theory, we derive a parton-shower equation for the resummation of non-global logarithms. We have implemented this shower and interfaced it with a tree-level event generator to obtain an automated framework to resum the leading logarithm of non-global observables in the large-N_c limit. Using this setup, we compute gap fractions for dijet processes and isolation cone cross sections relevant for photon production. We compare our results with fixed-order computations and LHC measurements. We find that naive exponentiation is often not adequate, especially when the vetoed region is small, since non-global contributions are enhanced due to their dependence on the veto-region size. Since our parton shower is derived from first principles and based on renormalization-group evolution, it is clear what ingredients will have to be included to perform resummations at subleading logarithmic accuracy in the future.

Speaker: Thomas Becher (University of Bern)

becher_PSR18.pdf

Tuesday, 5 June

Tops and Heavy Flavors

13 State of the art simulations for top quark production and decay at the LHC

I'll review recent progress in the simulation of top quark production at the LHC. In particular, I will focus on event generators that describe the decay of the top quark beyond the LO accuracy. I will then discuss the importance of the radiative corrections in the top decay for observables potentially relevant for top mass determinations.

Speaker: Tomas Jezo (University of Zurich)

TJezo.pdf

14 Cutoff dependence of the top quark mass parameter in angular ordered parton showers

We prove for the simple case of hemisphere masses in e+e- annihilation that using a transverse momentum cut $Q_0$ on the evolution of parton showers based on coherent branching implies that the heavy quark mass parameter of the algorithm is not the pole mass, but differs from the pole mass by an amount proportional to \alpha_s(Q_0)Q_0$. The basis of the proof is (a) an analytic solution of the PS evolution in the narrow width approximation, (b) that the infrared cut on the angular ordered shower can be implemented into corresponding analytic calculations in QCD factorization and (c) the dependence of the peak of the jet mass distribution on the shower cut. Numerical comparisons to the Herwig event generator confirm our analytic results. Observables based on reconstruction and the relevance and irrelevance of having NLO matched calculations to address
the issue are also discussed.

Speaker: Andre Hoang (University of Vienna)

Hoang-Topmass.pdf

15 NLO parton shower matching uncertainties in loop-induced processes

Speaker: Silvan Kuttimalai (SLAC)

kuttimalai-psr-18.pdf

16 NLO and offshell effects in top quark mass determinations

The top quark mass is one of the most important parameters of the Standard Model, which therefore has to be determined very precisely. The LHC experiments dedicate entire analysis programs to deal with this task. Among various strategies, the template fitting method is one successful approach to extract the top quark mass from LHC data relying on top quark mass sensitive observables such as the mass of the lepton-bjet pairs. Using different theoretical models ranging from fixed-order detailed descriptions to parton showers, a number of scenarios are studied to obtain an overview of physics effects and uncertainties that impact the determination of the top quark mass in the current experiments.

Speaker: Jan-Christopher Winter (Michigan State University (US))

tmd-psr.pdf

11:00 Coffee

Coloured Uncertainties

17 Bird Tracks for SU(N)

I review some results on Hermitian Young operators, gluon projectors, and multiplet bases for SU(N) colour space, employing the diagrammatic birdtrack notation.

Speaker: Stefan Keppler

2018_Lund_Keppeler.pdf

18 TBA

Speaker: Joey Huston (Michigan State University (US))

huston_PSR2018.pdf

huston_PSR2018.pptx

12:30 Uncertainty Lunch

TMD and SCET

19 Joint resummation of two angularities at NNLL

As a first step towards precise and differential predictions, we calculate the cross section for e+e− → 2 jets differential in the angularities eα and eβ. The logarithms of eα and eβ are jointly resummed to NNLL logarithmic accuracy, using the SCET+ framework we developed, and are matched with the NLO fixed-order prediction. We calculate the necessary ingredients, discuss scales and uncertainties, and obtain numerical predictions that we compare to Pythia. Predictions for the cross section differential in the ratio eα/eβ are also obtained. Furthermore, our results are validated at order αs2 by comparing to Event2.

Speaker: Lisa Zeune

Zeune_PSR_0618.pdf

20 Numerical resummation in SCET

Speaker: Christian Bauer

Numerical Resummation.pdf

21 TMD parton densities from Parton Branching method and applications

Transverse momentum dependent (TMD) parton densities at NLO from the Parton Branching method are obtained from fits to inclusive HERA DIS data. Collinear parton densities are also obtained using angular ordering constraints. The TMD densities are applied for predictions of processes at the LHC.

Speaker: Hannes Jung (Deutsches Elektronen-Synchrotron (DE))

TMD parton densities.pdf

22 TMD splitting functions in kT factorization and TMD parton showers

In this talk we present two results: 1) We calculate the transverse momentum dependent gluon-to-gluon splitting function within kT-factorization, generalizing the framework employed in the calculation of the quark splitting functions in [1205.1759, 1511.08439, 1607.01507] and demonstrate at the same time the consistency of the extended formalism with previous results. While existing versions of kT factorized evolution equations contain already a gluon-to-gluon splitting function i.e. the leading order BFKL kernel or the CCFM kernel, the obtained splitting function has the important property that it reduces both to the leading order BFKL kernel in the high energy limit, to the DGLAP gluon-to-gluon splitting function in the collinear limit as well as to the CCFM kernel in the soft limit. At the same time we demonstrate that this splitting kernel can be obtained from a direct calculation of the QCD Feynman diagrams, based on a combined implementation of the Curci-Furmanski-Petronzio formalism for the calculation of the collinear splitting functions and the framework of high energy factorization. 2) We show a new calculation using off-shell matrix elements with TMD parton densities supplemented with a newly developed initial state TMD parton shower. The calculation is based on the KaTie package for automated calculation of the partonic process in high-energy factorization, making use of TMD parton densities implemented in TMDlib. The partonic events are stored in an LHE file, similar to the conventional LHE files, but now containing the transverse momenta of the initial partons. The LHE files are read in by the Cascade package for the full TMD parton shower, final state shower and hadronization from Pythia where events in HEPMC format are produced. We have determined a full set of TMD parton densities and developed an initial state TMD parton shower, including all flavors following the TMD distribution. As an example of application we have calculated the azimuthal de-correlation of high pt dijets as measured at the LHC and found very good agreement with the measurement when including initial state TMD parton showers together with conventional final state parton showers and hadronization.

Speaker: Aleksander Kusina (IFJ PAN)

kusina_PSR2018.pdf

15:30 Excursion

19:00 Dinner

Wednesday, 6 June

QCD+EW

23 WW Production With a Jet Veto Made Simple

I will present a new method to compute the cross section for WW production with a jet veto, fully exclusive in the decay products of the WW pair.

Speaker: Luke Arpino (University of Sussex)

arpino-psr2018.pdf

24 Electroweak logarithms in inclusive processes

I will present a framework to resum electroweak logarithms in inclusive processes, that we recently developed in arXiv:1802.08687 and for which we calculated all ingredients at NLL order. If only the invariant mass of the final-state is measured, all electroweak logarithms can be resummed by the PDF evolution. However, simply identifying e.g. a lepton in the final state requires the corresponding fragmentation function and introduces an angular dependence through the exchange of soft gauge bosons. I will also discuss a few extensions, including jets and how to calculate the EW logarithms when one is fully exclusive in the central (detector) region and fully inclusive in the forward (beam) regions.

Speaker: Wouter Waalewijn (University of Amsterdam)

EW_resummation.pdf

25 Electroweak corrections in event generation

Speaker: Marek Schoenherr (CERN)

PSR18_MarekSchoenherr.pdf

26 Global Recoil in Initial-Final Antennae

Speaker: Rob Verheyen

Rob_Verheyen_Lund.pdf

10:55 Coffee

PS and R

27 Jets and threshold summation in Deductor

I use the shower generator Deductor to explore the effects of threshold summation and out of cone energy on the one jet cross section. I also examine the two jet cross section with a rapidity gap.

Speaker: Davison Soper (University of Oregon)

Soper.pdf

28 Merging High Energy with Soft and Collinear Logarithms using HEJ and PYTHIA

Abstract We present a method to combine the all-order treatment of the High Energy Jets exclusive partonic Monte Carlo (HEJ) with the parton shower of Pythia8, while retaining the logarithmic accuracy of both. This procedure enables the generation of fully realistic and hadronised events with HEJ. Furthermore, the combination of the two all-order treatments leads to improvements in the quality of the description of observables, in particular for regions with disparate transverse scales.

Speaker: Helen Brooks (IPPP/Durham University)

Brooks_PSR2018.pdf

29 Comparing parton showers and NLL resummation

A systematic study of differences between NLL resummation and parton showers is presented. While large logarithms arising from the IR structure of QCD can be resummed to higher logarithmic accuracy for specific observables, numerical calculations based on parton showers are formally correct to LL only. However, they incorporate additional effects like local momentum conservation, which are beyond this formal accuracy. To investigate the numerical size of these effects, a Markovian Monte-Carlo algorithm for resummation of additive observables in electron-positron annihilation is presented. The numerical effects of individual approximations intrinsic to the pure NLL result are studied and an overall comparison is made between the parton shower and pure NLL resummation.

Speaker: Daniel Reichelt

Lund_DReichelt.pdf

12:45 Lunch

PS vs. R

Conveners: Andrea Banfi (University of Sussex), Mike Seymour (University of Manchester (GB)), Stefan Hoeche (SLAC)

15:30 Coffee and close