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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
+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
+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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