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Status and Developmentsof Event Generators

Torbjorn Sjostrand

Theoretical Particle PhysicsDepartment of Astronomy and Theoretical Physics

Lund UniversitySolvegatan 14A, 223 62 Lund

LHCP 2016, Lund, 13 June 2016

Table of Contents

? Historical introduction from the Lund perspective ?? Generator overview + MCnet ?

? Herwig, Sherpa, PYTHIA news & plans ?? Match & merge overview ?? Some other programs ?? Summary and outlook ?

Further talks by Gavin Salam, Rikkert Frederix,Tomas Jezo, Marek Schonherr, Frank Tackmann, . . .. . . and several of the experimental presentations (?)

Torbjorn Sjostrand Status and Developments of Event Generators slide 2/28

1976: Lund QCD Phenomenology group

Created by Bo Andersson and Gosta Gustafson

space

timequarkantiquarkpair creation

Lund string model: ∼ like rubber bandthat is pulled apart and breaks into pieces,or like a magnet broken into smaller pieces.

Complete, consistent description of 2-jet events— but not necessarily perfect.

Torbjorn Sjostrand Status and Developments of Event Generators slide 3/28

1978: JETSET version 1

≈ 200 punched cards

Fortran code

Torbjorn Sjostrand Status and Developments of Event Generators slide 4/28

1980: string (colour coherence) effect

quark

antiquark

gluon

string motion in the event plane(without breakups)

Predicted unique event structure;inside & between jets.Confirmed first by JADE 1980.

Generator crucialto sell physics!

(today: PS, M&M, MPI, . . . )

Torbjorn Sjostrand Status and Developments of Event Generators slide 5/28

1980: string (colour coherence) effect

quark

antiquark

gluon

string motion in the event plane(without breakups)

Predicted unique event structure;inside & between jets.Confirmed first by JADE 1980.

Generator crucialto sell physics!

(today: PS, M&M, MPI, . . . )

Torbjorn Sjostrand Status and Developments of Event Generators slide 5/28

Lund contributions

Physics:

string fragmentation(& colour coherence)

dipole showers

backwards evolution (for ISR)

multiparton interactions (MPI)

colour reconnection (CR)

matching (POWHEG style)& merging (CKKW-L, . . . )

small-x evolution (CCFM, . . . )

interleaved evolution

heavy-ion collisions

QCD effects for BSM

Generators:

JETSETPYTHIAFritiofAriadneLDCDIPSY

LeptoVINCIADIRE

RapGapHIJING· · ·GEANT

Torbjorn Sjostrand Status and Developments of Event Generators slide 6/28

The workhorses

Herwig, PYTHIA and Sherpa offer convenient frameworksfor LHC physics studies, covering all aspects above,but with slightly different history/emphasis:

PYTHIA (successor to JETSET, begun in 1978):originated in hadronization studies,still special interest in soft physics.

Herwig (successor to EARWIG, begun in 1984):originated in coherent showers (angular ordering),cluster hadronization as simple complement.

Sherpa (APACIC++/AMEGIC++, begun in 2000):had own matrix-element calculator/generatororiginated with matching & merging issues.

Torbjorn Sjostrand Status and Developments of Event Generators slide 7/28

The structure of an event

All full-fledged generators need to address many issues:

Torbjorn Sjostrand Status and Developments of Event Generators slide 8/28

MCnet

HerwigPYTHIASherpaMadGraph

Plugin:AriadneDIPSYHEJ

CEDAR:RivetProfessorHepForgeLHAPDFHepMC

EU-funded 2007–10,2013–16, 2017–20

Generator development

Services to community

PhD student training

Common activities

Short-term studentships(3 - 6 months).Experimentalists welcome!

Summer schools2016: DESY (w. CTEQ)2017: Lund, 3 - 7 July

Send your students!

Nodes:ManchesterCERNDurhamGlasgowGottingenHeidelbergKarlsruheUC LondonLouvainLund

Monash (Au)SLAC (US)

Torbjorn Sjostrand Status and Developments of Event Generators slide 9/28

MCnet

HerwigPYTHIASherpaMadGraph

Plugin:AriadneDIPSYHEJ

CEDAR:RivetProfessorHepForgeLHAPDFHepMC

EU-funded 2007–10,2013–16, 2017–20

Generator development

Services to community

PhD student training

Common activities

Short-term studentships(3 - 6 months).Experimentalists welcome!

Summer schools2016: DESY (w. CTEQ)2017: Lund, 3 - 7 July

Send your students!

Nodes:ManchesterCERNDurhamGlasgowGottingenHeidelbergKarlsruheUC LondonLouvainLund

Monash (Au)SLAC (US)

Torbjorn Sjostrand Status and Developments of Event Generators slide 9/28

Herwig 7.0 news

Herwig++ 3.0 ⇒ Herwig 7.0 (December 2015).Concludes 16 years effort to replace Fortran Herwig 6.

NLO matched to parton showers default for hard process.

Fully automated: no external codes to run,no intermediate event files.Choice of subtractive (MC@NLO type)or multiplicative (PowHeg type) matching.

Two showers: angular ordered or dipole.Spin correlations and QED radiation in the former.

Facilities for parton-shower uncertainties.

New tunes, including MB/UE.

Vastly improved documentation, usage and installation.

Several parallelization options.

Torbjorn Sjostrand Status and Developments of Event Generators slide 10/28

Matchbox in Herwig 7Herwig 7.0 – Under the Hood

Matchbox

MadGraph

ColorFull

GoSam

NJet

OpenLoops

VBFNLO

HJets++

CVolver

Recola

QTildeShower

DipoleShower

Cluster Hadronization

Decays

Eikonal MPI

Matching subtractions

ME corrections

Built-in MEBSM & UFO

Simon Platzer (IP3 Durham & Manchester) Status of Herwig 7 4 / 13

script downloads & sets up external libraries (above + more)

(figure by S. Platzer)

Torbjorn Sjostrand Status and Developments of Event Generators slide 11/28

Herwig 7.0 news

Herwig++ 3.0 ⇒ Herwig 7.0 (December 2015).Concludes 16 years effort to replace Fortran Herwig 6.

NLO matched to parton showers default for hard process.

Fully automated: no external codes to run,no intermediate event files.Choice of subtractive (MC@NLO type)or multiplicative (PowHeg type) matching.

Two showers: angular ordered or dipole.Spin correlations and QED radiation in the former.

Facilities for parton-shower uncertainties.

New tunes, including MB/UE.

Vastly improved documentation, usage and installation.

Several parallelization options.

Torbjorn Sjostrand Status and Developments of Event Generators slide 12/28

Herwig 7 examplesHerwig 7.0 – Few Examples

Herwig 7MadGraph / ColorFull / OpenLoops

ALEPH DataHerwig++ 2.7 LO ⌦ PSLO ⌦ PSNLO � PSNLO ⌦ PSNLO � Dipoles

10�2

10�1

1

10 1

1-Thrust, 1 � T (charged)

1/N

dN

/d(1

�T)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

0.6

0.8

1

1.2

1.4

1 � T

MC

/Dat

a

Herwig 7MadGraph / ColorFull / OpenLoops

ALEPH DataHerwig++ 2.7 LO ⌦ PSLO ⌦ PSQCD ⌦ QED ⌦ PSNLO � PS

10�3

10�2

10�1

1

10 1

10 2

10 3Photon Fragmentation in 3-jet events with ycut = 0.1

1/s

ds(3

�je

t)/

dz⇥

103

0.7 0.75 0.8 0.85 0.9 0.95 1.0-3 s-2 s-1 s0 s1 s2 s3 s

zg

(MC�

dat

a)

Herwig 7.0 at LEP – new tune available with the release.Several improvements to angular ordered shower.

Tons of plots using all combinations at: https://herwig.hepforge.org/plots/herwig7.0/

Simon Platzer (IP3 Durham & Manchester) Status of Herwig 7 7 / 13

Herwig 7.0 – Few Examples

Herwig 7MadGraph / ColorFull / OpenLoops

CMS DataLO � PSNLO � PSNLO ⌦ PSNLO � Dipoles

10�4

10�3

10�2

10�1

1

Df(Z, J1),p

s = 7 TeV

1 sds df

0 0.5 1 1.5 2 2.5 3

0.6

0.8

1

1.2

1.4

Df(Z, J1) [rad]

MC

/Dat

a

Herwig 7MadGraph / ColorFull / OpenLoops

ATLAS DataHerwig++ 2.7 LO ⌦ PSLO � PSNLO � PSNLO ⌦ PS

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1.0

Gap fraction vs. Qsum for veto region: |y| < 2.1

f gap

50 100 150 200 250 300 350 400

0.96

0.98

1.0

1.02

1.04

Qsum [GeV]

MC

/Dat

a

Z+jet events from CMS and top pairs from ATLAS.Matchbox using MadGraph, ColorFull and OpenLoops.

Tons of plots using all combinations at: https://herwig.hepforge.org/plots/herwig7.0/

Simon Platzer (IP3 Durham & Manchester) Status of Herwig 7 8 / 13

LO → NLO ⇒ major improvements in e+e− and pp alike.Subtractive or multiplicative matching less important.Ditto angular-ordered or dipole shower.

Torbjorn Sjostrand Status and Developments of Event Generators slide 13/28

Future of Herwig 7

Herwig 7.1 later this year:

NLO multijet merging (unitarized merging ideas).

Loop-induced processes.

Extended UFO-model support.

Extended reweighting: weight vectors in HepMC files.

Improved top decay in dipole shower.

Interface to HEJ.

Soft interactions and diffraction.

In the longer run:

Code now 500k lines ⇒ need for significant restructuring.

Amplitude-based parton showers.

Torbjorn Sjostrand Status and Developments of Event Generators slide 14/28

Sherpa 2.2 news and activities

Recent news:

DIRE shower (see below).

UNNLOPS - first results on NNLO merging.

On-the-fly scale variations of NLO ME + PS.ME observables through interpolating grids(ApplGrid, FastNLO, MCgrid, . . . ).

Electroweak NLO corrections, together with OpenLoops.

Merging for loop-induced processes.

Ongoing work and plans:

Full NNLO QCD + NLO EW (for 2→ 1, 2→ 2).

Higher-order shower(one-loop splitting functions, sub-leading colour).

Automated N-jettiness slicing.

Torbjorn Sjostrand Status and Developments of Event Generators slide 15/28

Sherpa NNLO QCD with parton showersNew: NNLO QCD with parton showers

W production @ NNLO+PS with SHERPA +BLACKHAT

[Hoche et al. arXiv:1507.05325]

Sher

paM

C

pjet? > 20 GeV

ATLAS dataarXiv:1201.1276UN2LOPSmln/2 < µR/F < 2 mlnmln/2 < µQ < 2 mln

10 1

10 2

10 3

10 4Inclusive Jet Multiplicity

s(W

+�

Nje

tje

ts)

[pb]

0 1 2 3 4 50

0.5

1

1.5

2

Njet

MC

/Dat

a

,! fully di↵erential hadron-level NNLO+PS simulationinclusive (born-like) distribution NNLO accurate0-jet bin NNLO, 1-jet bin NLO, 2-jet bin LO, � 3-jets shower accuracy

,! small corrections away from Born kinematics

14/27

Torbjorn Sjostrand Status and Developments of Event Generators slide 16/28

Sherpa 2.2 news and activities

Recent news:

DIRE shower (see below).

UNNLOPS - first results on NNLO merging.

On-the-fly scale variations of NLO ME + PS.ME observables through interpolating grids(ApplGrid, FastNLO, MCgrid, . . . ).

Electroweak NLO corrections, together with OpenLoops.

Merging for loop-induced processes.

Ongoing work and plans:

Full NNLO QCD + NLO EW (for 2→ 1, 2→ 2).

Higher-order shower(one-loop splitting functions, sub-leading colour).

Automated N-jettiness slicing.

Torbjorn Sjostrand Status and Developments of Event Generators slide 17/28

Sherpa QCD coherence test

Study events with two hard and one further softer third jets.Angular distribution of third around second probes colour coherence:

Pheno: QCD color coherence

Coherence e↵ects in 3-jet events (CMS) [Chatrchyan et al. Eur. Phys. J. C 74 (2014) 2901]

presented comparison to shower MCs

PYTHIA 6 & 8, HERWIG++ 2.3

2 ! 2, 3jet LO merging from

MADGRAPH + PYTHIA 6

; sizeable deviations observed

; HERWIG yields best modelling

; LO merging somewhat improves purePYTHIA 6, but di↵erent tune used

; switching o↵ coherence in PYTHIA 6shower worsens agreement

β0.5 1 1.5 2 2.5 3

)β( 2ηF

0.03

0.04

0.05

0.06

0.07

0.08

DataPYTHIA6 Z2PYTHIA8 4CHERWIG++ 2.3MADGRAPH + PYTHIA6 D6TSystematic uncertainty

0.8≤| 2η|

= 7 TeVs-1CMS, L = 36 pb

β0.5 1 1.5 2 2.5 3

0.03

0.04

0.05

0.06

0.07

0.08

DataPYTHIA6 Z2PYTHIA8 4CHERWIG++ 2.3MADGRAPH + PYTHIA6 D6TSystematic uncertainty

2.5≤| 2η0.8 < |

= 7 TeVs-1CMS, L = 36 pb

β

0.5 1 1.5 2 2.5 3

MC/Data

0.7

0.8

0.9

1

1.1

1.2

PYTHIA6 Z2PYTHIA8 4CHERWIG++ 2.3MADGRAPH + PYTHIA6 D6TStatistical uncertaintySystematic uncertaintyStat.+ Sys. uncertainty

0.8≤| 2η|

= 7 TeVs-1CMS, L = 36 pb

β

0.5 1 1.5 2 2.5 3

0.7

0.8

0.9

1

1.1

1.2

PYTHIA6 Z2PYTHIA8 4CHERWIG++ 2.3MADGRAPH + PYTHIA6 D6TStatistical uncertaintySystematic uncertaintyStat.+ Sys. uncertainty

= 7 TeVs-1CMS, L = 36 pb

2.5≤| 2η0.8 < |

⌘2 central ⌘2 forward

25/27

Pheno: QCD color coherence

Coherence e↵ects in 3-jet eventsanalysis meanwhile available in RIVET

comparison against SHERPA dipole shower and 2 ! 2, 3, 4jets MEPS@LO

default hadronisation & underlying event tune; yields satisfactory agreement with data

CMS dataSherpa-2.1.1 (shower)Sherpa-2.1.1 (MEPS@LO)

0.03

0.04

0.05

0.06

0.07

0.08

CMS,ps = 7 TeV, central jet 2–3 correlation, |h2| < 0.8

Fh2(b

)

0.5 1 1.5 2 2.5 3

0.70.80.91.01.11.2

b

MC/Data

CMS dataSherpa-2.1.1 (shower)=Sherpa-2.1.1 (MEPS@LO)

0.03

0.04

0.05

0.06

0.07

0.08

CMS,ps = 7 TeV, jet 2–3 correlation, 0.8 < |h2| < 2.5

Fh2(b

)

0.5 1 1.5 2 2.5 3

0.70.80.91.01.11.2

b

MC/Data

,! measurement di↵erentiates MC models, e.g. pQCD/npQCD interplay,! would be interesting to check for generator tunes & settings, possibly

gradually move to standard (inclusive) 3-jet selection26/27

PYTHIA/Herwig does not quitedescribe data, whereas Sherpafares much better.

Torbjorn Sjostrand Status and Developments of Event Generators slide 18/28

Sherpa 2.2 news and activities

Recent news:

DIRE shower (see below).

UNNLOPS - first results on NNLO merging.

On-the-fly scale variations of NLO ME + PS.ME observables through interpolating grids(ApplGrid, FastNLO, MCgrid, . . . ).

Electroweak NLO corrections, together with OpenLoops.

Merging for loop-induced processes.

Ongoing work and plans:

Full NNLO QCD + NLO EW (for 2→ 1, 2→ 2).

Higher-order shower(one-loop splitting functions, sub-leading colour).

Automated N-jettiness slicing.

Torbjorn Sjostrand Status and Developments of Event Generators slide 19/28

PYTHIA 8.2 news

New match&merge schemes(now 8) and options.

Weak showers: q→ qZ0,q→ q′W± (also merged).

Allow reweighting ofrare shower branchings.

Automated parton-showeruncertainty bands.

Extended interface forexternal shower plugins.

Z/W + jets results

The Pythia distributions arenormalized such that firstbin fit the data.

The shower starting scale iss for Drell-Yanand p? for QCD 2 ! 2.

ATLAS dataDrell-Yan productionRadiationCombined

10�2

10�1

1

10 1

10 2s(� Njet), Z ! µ+µ�, p?(jet) > 30 GeV, |yjet < 4.4|

s(Z

+µ�+

�N

jet)

[pb

]

1 2 3 4 5 6 7

0.6

0.8

1

1.2

1.4

NjetM

C/

dat

a

JRC (Lund) CR and Weak Showers November 5, Lund 12 / 30

Complete LHEF v3 support.

Can run Madgraph5 aMC@NLO and POWHEG BOXfrom within PYTHIA.

Complete Python interface.

Torbjorn Sjostrand Status and Developments of Event Generators slide 20/28

PYTHIA 8.2 news

New match&merge schemes(now 8) and options.

Weak showers: q→ qZ0,q→ q′W± (also merged).

Allow reweighting ofrare shower branchings.

Automated parton-showeruncertainty bands.

Extended interface forexternal shower plugins.

Z/W + jets results

The Pythia distributions arenormalized such that firstbin fit the data.

The shower starting scale iss for Drell-Yanand p? for QCD 2 ! 2.

ATLAS dataDrell-Yan productionRadiationCombined

10�2

10�1

1

10 1

10 2s(� Njet), Z ! µ+µ�, p?(jet) > 30 GeV, |yjet < 4.4|

s(Z

+µ�+

�N

jet)

[pb

]

1 2 3 4 5 6 7

0.6

0.8

1

1.2

1.4

NjetM

C/

dat

a

JRC (Lund) CR and Weak Showers November 5, Lund 12 / 30

Complete LHEF v3 support.

Can run Madgraph5 aMC@NLO and POWHEG BOXfrom within PYTHIA.

Complete Python interface.

Torbjorn Sjostrand Status and Developments of Event Generators slide 20/28

PYTHIA 8.2 news

CR off

default

forced random

100 120 140 160 180 200 220 2400

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

Reconstructed top mass, mW ∈ [75, 85] GeV, pT(jets) > 40 GeV

mtop [GeV]

1/NdN/dmtop[GeV

−1]

Tests - ⇤/Ks and ⌅/⇤

DataPythiaPythia new CR

0

0.2

0.4

0.6

0.8

1

1.2

L/K0S versus transverse momentum at

ps = 7 TeV

N(L

)/

N(K

0 S)

0 2 4 6 8 10

0.6

0.8

1

1.2

1.4

pT [GeV/c]

MC

/Dat

a

DataPythiaPythia new CR

0

0.05

0.1

0.15

0.2

0.25

0.3

X�/L versus transverse momentum atp

s = 7 TeV

N(X

�)

/N

(L)

0 1 2 3 4 5 6

0.6

0.8

1

1.2

1.4

pT [GeV/c]

MC

/Dat

a

⇤/KS is better described by the model (overall yield is tuned)

(No rate change in e+e�)

⌅/⇤ is the same as old model - no strangeness enhancement

JRC (Lund) CR and Weak Showers November 5, Lund 22 / 30

Many new colourreconnection models.

Double onium production.New model for hard diffraction.Several new tunes; Monash new default.

Ongoing work and plans:

γγ, γp and ep.Total, elastic and diffractive cross sections.Improved showers and hadronization.

Torbjorn Sjostrand Status and Developments of Event Generators slide 21/28

PYTHIA 8.2 news

CR off

default

forced random

100 120 140 160 180 200 220 2400

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

Reconstructed top mass, mW ∈ [75, 85] GeV, pT(jets) > 40 GeV

mtop [GeV]

1/NdN/dmtop[GeV

−1]

Tests - ⇤/Ks and ⌅/⇤

DataPythiaPythia new CR

0

0.2

0.4

0.6

0.8

1

1.2

L/K0S versus transverse momentum at

ps = 7 TeV

N(L

)/

N(K

0 S)

0 2 4 6 8 10

0.6

0.8

1

1.2

1.4

pT [GeV/c]

MC

/Dat

a

DataPythiaPythia new CR

0

0.05

0.1

0.15

0.2

0.25

0.3

X�/L versus transverse momentum atp

s = 7 TeV

N(X

�)

/N

(L)

0 1 2 3 4 5 6

0.6

0.8

1

1.2

1.4

pT [GeV/c]

MC

/Dat

a

⇤/KS is better described by the model (overall yield is tuned)

(No rate change in e+e�)

⌅/⇤ is the same as old model - no strangeness enhancement

JRC (Lund) CR and Weak Showers November 5, Lund 22 / 30

Many new colourreconnection models.Double onium production.New model for hard diffraction.Several new tunes; Monash new default.

Ongoing work and plans:

γγ, γp and ep.Total, elastic and diffractive cross sections.Improved showers and hadronization.

Torbjorn Sjostrand Status and Developments of Event Generators slide 21/28

Match and merge strategies

Input from:Madgraph5 aMC@NLOPOWHEG BOXALPGENCOMIX/SherpaNLOJET++JETRADHJETS++BlackHatGoSamHelacOpenLoopsVBFNLOCalpHEP/CompHEP. . .

legsn +1 +2 +3 +4

loops

0

1

2

legsn +1 +2 +3 +4

loops

0

1

2

legsn +1 +2 +3 +4

loops

0

1CKKWCKKW-LMLMUMEPSMC@NLOPOWHEGMENLOPSMEPS@NLONL3

UNLOPSFxFxNNLOPSMiNLOUN2LOPSMIN2LOPS

Intense activity, no “final word”.

Torbjorn Sjostrand Status and Developments of Event Generators slide 22/28

Combination strategies

Big flexibility, but different baseline “world view”:

MadGraph:

ME gen

M&M

Event gen

Herwig, Sherpa:

ME gen

M&M

Event gen

Pythia:

M&MEvent gen

ME gen

Les Houches

Event gen = ISR + FSR + MPI + BBR + CR + hadronization + . . .6= “hadronizer”

ME and Event Generators both indispensable

Torbjorn Sjostrand Status and Developments of Event Generators slide 23/28

VINCIA: an Interleaved Antennae shower

Markovian process: no memory of path to reach current state.

Based on antenna factorization of amplitudes and phase space.

Smooth ordering fillswhole phase space.

Step-by-step reweightingto new matrix elements:Z → Zj → Zjj → Zjjj(also Sudakov), e.g.

W =|MZj|2∑i ai |MZ|2i

Replaces PYTHIAnormal showers;recent release.

A Result

Predictions made withpublicly availableVINCIA 2.0.01(vincia.hepforge.org)+ PYTHIA 8+ MADGRAPH 4

CMS dataPhys. Lett. B 722 (2013) 238no MECsMECs O(a1

s )MECs O(a2

s )MECs O(a3

s )

10�2

10�1

1

CMS, Df(Z, J1),p

s = 7 TeV

1 sds df

0 0.5 1 1.5 2 2.5 3

0.6

0.8

1

1.2

1.4

Df(Z, J1) [rad]

MC

/Dat

a

Showeronly

$&%

c

c

18/21

Torbjorn Sjostrand Status and Developments of Event Generators slide 24/28

DIRE: a Dipole Resummation shower

Joint Sherpa/PYTHIA development,but separate implementations,means technically well tested.

“Midpoint between dipole andparton shower”,dipole with emitter & spectator,but not quite CS ones:unified initial–initial, initial–final,final–initial, final–final.

Soft term of kernels in alldipole types is less singular

1

1− z→ 1− z

(1− z)2 + p2⊥/M2

The midpoint between dipole and parton showers

Sher

paM

C

⇥10�1

⇥10�2

⇥10�3

⇥10�4

⇥10�5

⇥10�6

⇥10�7

⇥10�8

ATLAS dataPhys.Rev.Lett. 106 (2011) 172002Dire

0.4 0.5 0.6 0.7 0.8 0.9 1.0

10�9

10�8

10�7

10�6

10�5

10�4

10�3

10�2

10�1

1

10 1Dijet azimuthal decorrelations

Df [rad/p]

1/s

ds

/d

Df

[p/r

ad]

110 < pmax? /GeV < 160

0.60.8

11.21.4

?

MC

/Dat

a

160 < pmax? /GeV < 210

0.60.8

11.21.4

?

MC

/Dat

a

210 < pmax? /GeV < 260

0.60.8

11.21.4

?

MC

/Dat

a

260 < pmax? /GeV < 310

0.60.8

11.21.4

?

MC

/Dat

a

310 < pmax? /GeV < 400

0.60.8

11.21.4

?

MC

/Dat

a

400 < pmax? /GeV < 500

0.60.8

11.21.4

?

MC

/Dat

a

500 < pmax? /GeV < 600

0.60.8

11.21.4

?

MC

/Dat

a

600 < pmax? /GeV < 800

0.60.8

11.21.4

?

MC

/Dat

a

pmax? /GeV > 800

0.5 0.6 0.7 0.8 0.9 1.00.60.8

11.21.4

?

Df [rad/p]

MC

/Dat

a

14Torbjorn Sjostrand Status and Developments of Event Generators slide 25/28

Apologies: will not cover

High Energy Jets (HEJ): BFKL-inspired description ofwell-separated multijets, with approximate matrix elementsand virtual corrections.

Deductor: improved handling of colour, partitioned dipoles,all final partons share recoil, q2/E evolution variable.

Geneva: Soft Collinear Effective Theory resummed (exclusive)n-jet rates as starting point for showers.

Ariadne: first dipole parton shower program.

DIPSY: evolution and collision of dipoles in transverse space.

revived Fritiof: overlayed modified pp collisions to model pA.

EPOS: pp/pA/AA, MPI + strings, saturation, thermalizedcore separate from corona, hydrodynamical evolution.

DPMJET, QGSJET, SIBYLL: other pp/pA/AA/cosmic ray.

. . .

Torbjorn Sjostrand Status and Developments of Event Generators slide 26/28

Summary and Outlook

Increased ME calculational capability: legs and loops.

Match and merge approaches still steadily developing.Generators typically offer several options.Spread between approaches one measure of uncertainty.

Continued/increased interest in parton shower development,with each generator offering several options.

Automated uncertainty bands for scale choices etc.

Many challenges remaining in soft physics, pA, AA:diffraction, colour reconnection, collective effects, . . .

Generators have gone from fringe activity for a few

to a mainstream part of phenomenology research.

Torbjorn Sjostrand Status and Developments of Event Generators slide 27/28

Thank You!

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