Introduction to Event Generators 4

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Introduction to Event Generators 4

Torbj¨ orn Sj¨ ostrand

Theoretical Particle Physics

Department of Astronomy and Theoretical Physics Lund University

S¨olvegatan 14A, SE-223 62 Lund, Sweden

CTEQ/MCnet School, DESY, 12 July 2016

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Hadronization

Hadronization/confinement is nonperturbative ⇒ only models.

Main contenders: string and cluster fragmentation.

Begin with e⁺e⁻→ γ^∗/Z⁰→ qq and e⁺e⁻→ γ^∗/Z⁰ → qqg:

Y

Z X

Y

Z X

Torbj¨orn Sj¨ostrand Event Generators 4 slide 2/38

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The QCD potential – 1

In QCD, for large charge separation, field lines are believed to be compressed to tubelike region(s) ⇒ string(s)

Gives force/potential between a q and a q:

F (r ) ≈ const = κ ⇐⇒ V (r ) ≈ κr

κ ≈ 1 GeV/fm≈ potential energy gain lifting a 16 ton truck.

Flux tube parametrized by center location as a function of time

⇒ simple description as a 1+1-dimensional object – a string .

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The QCD potential – 2

Linear confinement confirmed e.g. by lattice QCD calculation of gluon field between a static colour and anticolour charge pair:

At short distances also Coulomb potential, important for internal structure of hadrons, but not for particle production (?).

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The QCD potential – 3

Full QCD = gluonic field between charges (“quenched QCD”) plus virtual fluctuations g → qq (→ g)

=⇒ nonperturbative string breakings gg . . . → qq

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String motion

The Lund Model: starting point Use only linear potential V (r ) ≈ κr to trace string motion, and let string fragment by repeated qq breaks.

Assume negligibly small quark masses.

Then linearity between space–time and energy–momentum gives

dE dz

=

dp_z dz

=

dE dt

=

dp_z dt

= κ

(c = 1) for a qq pair flying apart along the ±z axis.

But signs relevant: the q moving in the +z direction has dz/dt = +1 but dp_z/dt = −κ.

B. Andersson et a!., Patton fragmentation and string dynamics 41

____

^-L/2 ^L12 ^X

____

^-p ^p~

<V

Fig. 2.1. The motion of q and ~ in the CM frame. The hatched areas Fig. 2.2. The motion of q and ~ in a Lorentz frame boosted relative to

show where the field is nonvanishing. the CM frame.

M2. In fig. 2.2 the same motion is shown after a Lorentz boost /3. The maximum relative distance has been contracted to

L’ = Ly(1 ^—

/3)

^L

e~and the time period dilated to

T’ = TI’y = T

cosh(y) where y

is the rapidity difference between the two frames.

In this model the “field” corresponding to the potential energy carries no momentum, which is a consequence of the fact that in 1

⁺

1 dimensions there is no Poynting vector. Thus all the momentum is carried by the endpoint quarks. This is possible since the turning points, where q and 4 have zero momentum, are simultaneous only in the CM frame. In fact, for a fast-moving q4 system the q4-pair will most of the time move forward with a small, constant relative distance (see fig. 2.2).

In the following we will use this kind of yo-yo modes as representations both of our original q4 jet system and of the final state hadrons formed when the system breaks up. It is for the subsequent work necessary to know the level spectrum of the yo-yo modes. A precise calculation would need a knowledge of the quantization of the massless relativistic string but for our purposes it is sufficient to use semi-classical considerations well-known from the investigations of Schrodinger operator spectra.

We consider the Hamiltonian of eq. (2.14) in the CM frame with

q =

x

1

^—

x2

H=IpI+KIql

(2.18)

and we note that our problem is to find the dependence on n of the nth energy level

E~.

If the spatial size of the state is given by 5~then the momentum size of such a state with n

^—

1 nodes is

IpI=nI& (2.19)

and the energy eigenvalue

E~

corresponds according to variational principles to a minimum of

H(6~)=

n/&,

+ Kô~

(2.20)

i.e.

2Vttn. (2.21)

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The Lund Model

Combine yo-yo-style string motion with string breakings!

Motion ofquarksandantiquarks with intermediatestring pieces:

space time

quark antiquark pair creation

Aqfrom one string break combines with aqfrom an adjacent one.

Gives simple but powerful picture of hadron production.

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Where does the string break?

Fragmentation starts in the middle and spreads outwards:

Corresponds to roughly same invariant time of all breaks, τ²= t²− z² ∼ constant,

with breaks separated by hadronic area m²_⊥= m²+ p_⊥². Hadrons at outskirts are more boosted.

Approximately flat rapidity distribution, dn/dy ≈ constant

⇒ total hadron multiplicity in a jet grows like ln E_jet.

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How does the string break?

String breaking modelled by tunneling:

P ∝ exp −πm²_⊥q κ

!

= exp −πp_⊥q² κ

!

exp −πm_q² κ

!

• Common Gaussian p_⊥ spectrum, hp⊥i ≈ 0.4 GeV.

• Suppression of heavy quarks,

uu : dd : ss : cc ≈ 1 : 1 : 0.3 : 10⁻¹¹.

• Diquark ∼ antiquark ⇒ simple model for baryon production.

String model unpredictive in understanding of hadron mass effects

⇒ many parameters, 10–20 depending on how you count.

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How does the string break?

String breaking modelled by tunneling:

P ∝ exp −πm²_⊥q κ

!

= exp −πp_⊥q² κ

!

exp −πm_q² κ

!

• Common Gaussian p_⊥ spectrum, hp⊥i ≈ 0.4 GeV.

• Suppression of heavy quarks,

uu : dd : ss : cc ≈ 1 : 1 : 0.3 : 10⁻¹¹.

• Diquark ∼ antiquark ⇒ simple model for baryon production.

String model unpredictive in understanding of hadron mass effects

⇒ many parameters, 10–20 depending on how you count.

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The Lund gluon picture – 1

The most characteristic feature of the Lund model:

quark

antiquark gluon

string motion in the event plane (without breakups)

Gluon = kink on string Force ratio gluon/ quark = 2,

cf. QCD NC/CF = 9/4, → 2 for NC → ∞ No new parameters introduced for gluon jets!

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The Lund gluon picture – 2

Energy sharing between two strings makes hadrons in gluon jets softer, more and broader in angle:

Jetset 7.4 Herwig 5.8 Ariadne 4.06 Cojets 6.23

OPAL

(1/Nevent ) dnch. /dxE

x_E uds jet gluon jet k_⊥ definition:

y_cut=0.02

0. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.

10^-3 10^-2 10^-1 1 10 10²

0.5 1.

1.5

Correction factors 0

0.05 0.1 0.15

0 5 10 15 20 25 30 35

g_incl. jets uds jets Jetset 7.4 Herwig 5.9 Ariadne 4.08 AR-2 AR-3

n_ch.

P(nch.)

(a) OPAL

|y| 2

0 0.05 0.1 0.15 0.2

0 5 10 15 20

gincl. jets uds jets Jetset 7.4 Herwig 5.9 Ariadne 4.08 AR-2 AR-3

n_ch.

P(nch.)

(b) OPAL

|y| 1

Jetset 7.4 Herwig 5.8 Ariadne 4.06 Cojets 6.23

0. 10. 20. 30. 40. 50. 60.

0.

0.02 0.04 0.06 0.08

OPAL

(1/Ejet) dEjet/dχ

χ (degrees) uds jet gluon jet

k_⊥ definition:

ycut=0.02 0.5

1.

1.5

Correction factors

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The Lund gluon picture – 3

Particle flow in the qqg event planedepleted in q–q region owing to boost of string pieces in q–g and g–q regions:

String fragmentation (SF) vs. independent fragmentation (IF), latter (nowadays) straw model of symmetric jet profile.

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The Herwig Cluster Model

1 Introduce forced g → qq branchings

2 Form colour singlet clusters

3 Clusters decay isotropically to 2 hadrons according to phase space weight ∼ (2s1+ 1)(2s2+ 1)(2p^∗/m)

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Cluster Model issues

1 Tail to very large-mass clusters (e.g. if no emission in shower);

if large-mass cluster → 2 hadrons then incorrect hadron momentum spectrum, crazy four-jet events

=⇒ split big cluster into 2 smaller along “string” direction;

daughter-mass spectrum ⇒ iterate if required;

∼ 15% of primary clusters are split, but give ∼ 50% of final hadrons 2 Isotropic baryon decay inside cluster

=⇒ splittings g → qq + qq 3 Too soft charm/bottom spectra

=⇒ anisotropic leading-cluster decay 4 Charge correlations still problematic

=⇒ all clusters anisotropic (?) 5 Sensitivity to particle content

=⇒ only include complete multiplets

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String vs. Cluster

program PYTHIA Herwig

model string cluster

energy–momentum picture powerful simple predictive unpredictive

parameters few many

flavour composition messy simple unpredictive in-between

parameters many few

“There ain’t no such thing as a parameter-free good description”

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Colour flow in hard processes – 1

One Feynman graph can correspond to several possible colour flows, e.g. for qg → qg:

while other qg → qg graphs only admit one colour flow:

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Colour flow in hard processes – 2

so nontrivial mix of kinematics variables (ˆs, ˆt) and colour flow topologies I, II:

|A(ˆs, ˆt)|² = |AI(ˆs, ˆt) + AII(ˆs, ˆt)|²

= |AI(ˆs, ˆt)|²+ |AII(ˆs, ˆt)|²+ 2 Re AI(ˆs, ˆt)A^∗_II(ˆs, ˆt) with Re AI(ˆs, ˆt)A^∗_II(ˆs, ˆt) 6= 0

⇒ indeterminate colour flow, while

• showers should know it (coherence),

• hadronization must know it (hadrons singlets).

Normal solution:

interference

total ∝ 1

N_C²− 1

so split I : II according to proportions in the NC→ ∞ limit, i.e.

|A(ˆs, ˆt)|² = |AI(ˆs, ˆt)|²_mod+ |AII(ˆs, ˆt)|²_mod

|AI(II)(ˆs, ˆt)|²_mod = |AI(ˆs, ˆt) + AII(ˆs, ˆt)|² |A_I(II)(ˆs, ˆt)|²

|AI(ˆs, ˆt)|²+ |AII(ˆs, ˆt)|²

!

NC→∞

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Colour Reconnection Revisited

Colour rearrangement well established e.g. in B decay.

Introduction

(V.A. Khoze & TS, PRL72 (1994) 28, ZPC62 (1994) 281, EPJC6 (1999) 271;

L. L ¨onnblad & TS, PLB351 (1995) 293, EPJC2 (1998) 165)

Γ_W, Γ_Z, Γ_t ≈ 2 GeV

Γ_h > 1.5 GeV for m_h > 200 GeV Γ_SUSY ∼ GeV (often)

τ = 1

Γ ≈ 0.2 GeV fm

2 GeV = 0.1 fm # rhad ≈ 1 fm

⇒ hadronic decay systems overlap, between pairs of resonances

⇒ cannot be considered separate systems!

Three main eras for interconnection:

1.

Perturbative: suppressed for ω > Γ by propaga- tors/timescales ⇒ only soft gluons.

2.

Nonperturbative, hadronization process:

colour rearrangement.

B⁰

d

b c

W⁻ c

s

!

"

!

"

B⁰

d b

c

W⁻

c gs ^!

"

K⁰_S

!

"

J/ψ

3.

Nonperturbative, hadronic phase:

Bose–Einstein.

Above topics among unsolved problems of strong in- teractions: confinement dynamics, 1/N

_C²

effects, QM interferences, . . . :

• opportunity to study dynamics of unstable parti- cles,

• opportunity to study QCD in new ways, but

• risk to limit/spoil precision mass measurements.

So far mainly studied for m

_W

at LEP2:

1. Perturbative: !δm

W

" ∼ 5 MeV.

^<

2. Colour rearrangement: many models, in general

!δm

W

" ∼ 40 MeV.

^<

e⁻ e⁺

W⁻ W⁺

q3

q4

q2

q1

!

"

!

"

π⁺ π⁺

#

$BE

3. Bose-Einstein: symmetrization of unknown am- plitude, wider spread 0–100 MeV among models, but realistically !δm

W

" ∼ 40 MeV.

^<

In sum: !δm

W

"

tot

< m

_π

, !δm

W

"

tot

/m

_W

∼ 0.1%; a

^<

small number that becomes of interest only because we aim for high accuracy.

At LEP 2 search for effects in e⁺e⁻→ W⁺W⁻→ q₁q₂q₃q₄: perturbativehδM_Wi . 5 MeV : negligible!

nonperturbativehδM_Wi ∼ 40 MeV :

favoured; no-effect option ruled out at 2.8σ.

Bose-Einstein hδM_Wi . 100 MeV : full effect ruled out (while models with ∼ 20 MeV barely acceptable).

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A top mass puzzle

Γ_t ≈ 1.5 GeV Γ_W ≈ 2 GeV ΓZ≈ 2.5 GeV







⇒ cτ ≈ 0.1 fm :

p “pancakes” have passed, MPI/ISR/FSR for p⊥≥ 2 GeV, inside hadronization colour fields.

t

t W b

2

Experiment mtop [GeV] Error due to CR Reference World comb. 173.34±0.76 310 MeV (40%) arXiv:1403.4427

CMS 172.22±0.73 150 MeV (20%) CMS-PAS-TOP-14-001 D0 174.98±0.76 100 MeV (13%) arXiv:1405.1756

1. Great job in reducing the errors

2. CR is one of the dominant systematics

3. Why is the CR uncertainty going down when there are

-no advances on the theoretical understanding

-no measurements to constrain it

A puzzle about mtop

(S. Argyropoulos) 1. Great job in reducing the errors.

2. CR is one of the dominant systematics.

3. Why is the CR uncertainty going down when there are

• no advances in theoretical understanding, and

• no measurements to constrain it?

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Effects on top mass before tuning

CR off default forced random

100 120 140 160 180 200 220 240

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045

Reconstructed top mass, m_W∈[75, 85]GeV, p_T(jets) >40 GeV

mtop[GeV]

1/NdN/dmtop[GeV−1]

∆mtop relative to no CR:

model ∆mtop ∆mtop

[GeV] rescaled default (late) −0.415 +0.209 default early +0.381 +0.285 forced random −6.970 −6.508

.

Asymmetric spread:

∆mtop < 0 easy,

∆m_top > 0 difficult.

Parton showers already prefer minimal λ.

Main effect from jet broadening, some from jet–jet angles.

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Effects on top mass after tuning

No publicly available measurements of UE in top events.

• Afterburner models tuned to ATLAS jet shapes in tt events

⇒ high CR strengths disfavoured.

• Early-decay models tuned to ATLAS minimum bias data

⇒ maximal CR strengths required to (almost) match hp_⊥i(n_ch).

model ∆mtop

rescaled default (late) +0.239 forced random −0.524

swap +0.273

∆m_top relative to no CR

Excluding most extreme (unrealistic) models

m^max_top − m^min_top ≈ 0.50 GeV

(in line with Sandhoff, Skands & Wicke) New: ∆mtop ≈ 0 in QCD-based model .

Studies of top events could help constrain models:

• jet profiles and jet pull (skewness)

• underlying event

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Probing reconnection through the top mass

DependenceofTopMassonEvent

Kinema2cs*

10*

!  First#top#mass#measurement#binned#in#kinema3c#observables.#

!  Addi2onal*valida2on*for*the*top*mass*measurements.**

!  With*the*current*precision,*no*mis^modelling*eﬀect*due*to*

"  color*reconnec2on,*ISR/FSR,**b^quark*kinema2cs,*diﬀerence*

between*pole*or*MS~*masses.*

color*recon.*

ISR/FSR*

b^quark*kin.*

Global*χ²/ndf*=*0.9*based*on*

mt1D*and*JES*which*are*

independent*(comparing*data*

and*MadGraph)*neglec2ng*

correla2ons*between*

observables.*

CMS^PAS^TOP^12^029*

NEW*

0 50 100 150 200 250 300

> [GeV]2Dt - <m

2D tm

-4 -2 0 2 4

-1) Data (5.0 fb MG, Pythia Z2 MG, Pythia P11 MG, Pythia P11noCR MC@NLO, Herwig = 7 TeV, lepton+jets s

CMS preliminary,

[GeV]

T,t,had p

0 50 100 150 200 250 300

data - MG Z2

-5 0 5

E. Yazgan

(Moriond 2013)

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BSM at the LHC

BSM particles usually short-lived, or weakly interacting (like DM).

Then visible final state consists of hadrons, leptons and photons, just like ordinary processes.

dummy text

p

q

g

q⇒ jet q⇒ jet q⇒ jet e

\p_⊥ BSM

dummy text

As easy to model as SM processes.

Original structure hidden, but traces of it may be left in terms of invariant masses and angular distributions. Discovery requires detailed understanding of

rare signals and huge backgrounds.

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BSM at the LHC

BSM particles usually short-lived, or weakly interacting (like DM).

Then visible final state consists of hadrons, leptons and photons, just like ordinary processes.

p

q

g

q⇒ jet q⇒ jet q⇒ jet e

\p_⊥ BSM

q

g

˜ q

˜

g q

˜ q

q

˜

q q

W! ˜e

e

˜γ

νe

˜γ

As easy to model as SM processes.

Original structure hidden, but traces of it may be left in terms of invariant masses and angular distributions.

Discovery requires detailed understanding of rare signals and huge backgrounds.

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BSM and QCD at the LHC

LHC is a QCD machine:

hard processes initiated by quarks and gluons, final state almost always dominated by hadrons,

underlying event by QCD mechanisms (showers, MPIs, . . . ), even in scenarios for physics Beyond the Standard Model (BSM) production of new coloured states often favoured (squarks, KK gluons, excited quarks, leptoquarks, . . . ).

In addition, BSM physics can raise “new”, specific QCD aspects:

new production mechanisms new parton-shower aspects new decay channels

new hadronization phenomena

new correlations with rest of the event

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Examples of nontrivial BSM physics

BNV ⇒ junction topology

⇒ special handling of showers and hadronization

Hidden valleys:

showers potentially interleaved with normal ones;

hadronization in hidden sector; decays back to normal sector R-hadron formation

Squark fragmenting to meson or baryon

Gluino fragmenting to baryon or glueball

Most hadronization properties by analogy with normal string fragmentation, but

glueball formation new aspect, assumed⇠ 10% of time(or less).

Torbj¨orn Sj¨ostrand QCD Aspects of BSM Physics slide 12/18

R-hadrons: long-lived ˜g or ˜q;

new: hadronization of massive object “inside” the string

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Examples of nontrivial BSM physics

Hidden valleys:

hadronization in hidden sector;

decays back to normal sector

R-hadron formation

Torbj¨orn Sj¨ostrand QCD Aspects of BSM Physics slide 12/18

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Examples of nontrivial BSM physics

Hidden valleys:

hadronization in hidden sector;

decays back to normal sector R-hadron formation

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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.

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Matchbox in Herwig 7

Herwig 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 ME BSM & UFO

Simon Pl¨atzer (IP³Durham & Manchester) Status of Herwig 7 4 / 13

script downloads & sets up external libraries (above + more) (figure by S. Pl¨atzer)

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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.

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Herwig 7 examples

Herwig 7.0 – Few Examples

Herwig 7

MadGraph / ColorFull / OpenLoops

ALEPH Data Herwig++ 2.7 LO ⌦ PS LO ⌦ PS NLO PS NLO ⌦ PS NLO Dipoles 10²

10¹ 1 10¹

1-Thrust, 1 T (charged)

1/NdN/d(1T)

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/Data

Herwig 7

ALEPH Data Herwig++ 2.7 LO ⌦ PS LO ⌦ PS QCD ⌦ QED ⌦ PS

NLO PS

10³ 10² 10¹ 1 10¹ 10²

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

1/sds(3jet)/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

(MCdata)

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/

Herwig 7.0 – Few Examples

Herwig 7

CMS Data

LO PS

NLO PS

NLO ⌦ PS NLO Dipoles

10 ⁴ 10 ³ 10 ² 10 ¹ 1

Df(Z, J1),ps = 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/Data

Herwig 7

ATLAS Data Herwig++ 2.7 LO ⌦ PS

LO PS

NLO PS

NLO ⌦ PS

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0

Gap fraction vs. Qsumfor veto region: |y| < 2.1 fgap

50 100 150 200 250 300 350 400

0.96 0.98 1.0 1.02 1.04

Qsum[GeV]

MC/Data

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/

LO → NLO ⇒ major improvements in e⁺e⁻ and pp alike.

Subtractive or multiplicative matching less important.

Ditto angular-ordered or dipole shower.

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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.

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Sherpa 2.2 news and activities

Sherpa NNLO QCD with parton showers New: NNLO QCD with parton showers

W production @ NNLO+PS with SHERPA +BLACKHAT

[H¨oche et al. arXiv:1507.05325]

SherpaMC

p^jet_?>20 GeV

ATLAS data arXiv:1201.1276 UN²LOPS mln/2 < µ_R/F<2 m_ln mln/2 < µQ<2 mln

10¹ 10² 10³

10⁴Inclusive Jet Multiplicity

s(W+Njetjets)[pb]

0 1 2 3 4 5

0 0.5 1 1.5 2

Njet

MC/Data

,! fully di↵erential hadron-level NNLO+PS simulation inclusive (born-like) distribution NNLO accurate

0-jet bin NNLO, 1-jet bin NLO, 2-jet bin LO, 3-jets shower accuracy ,! small corrections away from Born kinematics

14/27

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Sherpa 2.2 news and activities

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 PYTHIA6 & 8, HERWIG++ 2.3 2! 2, 3jet LO merging from MADGRAPH+ PYTHIA6

; sizeable deviations observed

; H^ERWIGyields best modelling

; LO merging somewhat improves pure P^YTHIA6, but di↵erent tune used

; switching o↵ coherence in PYTHIA6 shower worsens agreement

β

0.5 1 1.5 2 2.5 3

)β(2ηF

0.03 0.04 0.05 0.06 0.07 0.08

Data PYTHIA6 Z2 PYTHIA8 4C HERWIG++ 2.3 MADGRAPH + PYTHIA6 D6T Systematic uncertainty

≤ 0.8 2|

|η

= 7 TeV -1 s CMS, L = 36 pb

β

0.5 1 1.5 2 2.5 3

0.03 0.04 0.05 0.06 0.07 0.08

Data PYTHIA6 Z2 PYTHIA8 4C HERWIG++ 2.3 MADGRAPH + PYTHIA6 D6T Systematic uncertainty

≤ 2.5 2| 0.8 < |η

= 7 TeV -1 s CMS, 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 Z2 PYTHIA8 4C HERWIG++ 2.3 MADGRAPH + PYTHIA6 D6T Statistical uncertainty Systematic uncertainty Stat.+ Sys. uncertainty

≤ 0.8 2|

|η

= 7 TeV -1 s CMS, 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 Z2 PYTHIA8 4C HERWIG++ 2.3 MADGRAPH + PYTHIA6 D6T Statistical uncertainty Systematic uncertainty Stat.+ Sys. uncertainty = 7 TeV -1 s

CMS, L = 36 pb

≤ 2.5 2| 0.8 < |η

⌘₂ central ⌘₂forward

25/27

Pheno: QCD color coherence

Coherence e↵ects in 3-jet events analysis meanwhile available in RIVET

comparison against SHERPAdipole shower and 2! 2, 3, 4jets M^EPS@LO

default hadronisation & underlying event tune

; yields satisfactory agreement with data

CMS data Sherpa-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

0.70.8 0.9 1.0 1.1 1.2

b

MC/Data

CMS data Sherpa-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

0.70.8 0.9 1.0 1.1 1.2

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 selection

26/27

PYTHIA/Herwig does not quite describe data, whereas Sherpa fares much better.

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Sherpa 2.2 news and activities

PYTHIA 8.2 news

Newmatch&merge schemes (now 8) and options.

Weak showers: q → qZ⁰, q → q⁰W^± (also merged).

Allow reweighting of rare shower branchings. Automatedparton-shower uncertainty bands. Extended interface for external shower plugins, like VINCIA and DIRE.

Z/W + jets results

The Pythia distributions are normalized such that first bin fit the data.

The shower starting scale is ˆs for Drell-Yan

and p

_?

for QCD 2 ! 2.

ATLAS data Drell-Yan production Radiation Combined

10² 10¹ 1 10¹ 10²

s( Njet), Z! µ⁺µ , p_?(jet) > 30 GeV,|yjet< 4.4|

s(Z!µ+µ+Njet)[pb]

1 2 3 4 5 6 7

0.6 0.8 1 1.2 1.4

Njet

MC/data

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

Complete LHEF v3 support.

Can run Madgraph5 aMC@NLO and POWHEG BOX from within PYTHIA.

Complete Python interface.

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PYTHIA 8.2 news

Newmatch&merge schemes (now 8) and options.

Weak showers: q → qZ⁰, q → q⁰W^± (also merged).

Allow reweighting of rare shower branchings.

Automatedparton-shower uncertainty bands.

Extended interface for external shower plugins, like VINCIA and DIRE.

Z/W + jets results

The Pythia distributions are normalized such that first bin fit the data.

The shower starting scale is ˆs for Drell-Yan

and p

_?

for QCD 2 ! 2.

ATLAS data Drell-Yan production Radiation Combined

10² 10¹ 1 10¹ 10²

s( Njet), Z! µ⁺µ , p_?(jet) > 30 GeV,|yjet< 4.4|

s(Z!µ+µ+Njet)[pb]

1 2 3 4 5 6 7

0.6 0.8 1 1.2 1.4

Njet

MC/data

Complete LHEF v3 support.

Can run Madgraph5 aMC@NLO and POWHEG BOX from within PYTHIA.

Complete Python interface.

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PYTHIA 8.2 news

Many newcolour reconnection models.

Double onium production.

New model for hard diffraction.

Several new tunes;

Monashnew default.

Tests - ⇤/K

_s

and ⌅/⇤

Data Pythia Pythia new CR

0 0.2 0.4 0.6 0.8 1 1.2

L/K⁰Sversus transverse momentum atps = 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/Data

Data Pythia Pythia new CR

0 0.05 0.1 0.15 0.2 0.25 0.3

X /L versus transverse momentum atps = 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/Data

⇤/K_S 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

γγ, γp and ep.

Total, elastic and diffractive cross sections.

Improved showers (including VINCIA and DIRE).

New approaches to hadronization, in response to pp/pA/AA similarities.

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Summary and Outlook

Increased ME calculational capability: legs and loops.

Match and merge approaches still steadily developing.

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

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.

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Summary and Outlook

Increased ME calculational capability: legs and loops.

Match and merge approaches still steadily developing.

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

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.

Introduction to Event Generators 4