cteq ss 2002stephen mrenna fermilab 1 introduction to event generators stephen mrenna cd/simulations...

CTEQ SS 2002 Stephen MRENNA Fermilab 1

Introduction to Event Introduction to Event GeneratorsGenerators

Stephen MrennaCD/Simulations Group

FermilabEmail: [email protected]

mailto:[email protected]

mailto:[email protected]


MotivationMotivation Experiments rely on Monte Carlo

programs which calculate physical observables Correct for finite detector acceptance Find efficiency of isolation cuts Jet Energy (out of cone) corrections Connect particles to partons Determine promising signatures of “new”

physics Optimize cuts for discovery/limit Planning of future facilities . . .


Field Theory TrinityField Theory Trinity Many different calculational schemes

from same basic principles Tree level (lowest order)

Many partons All spin correlations Full color structure

NNLO Smaller theoretical errors More inclusive kinematics

“All” orders in towers of logarithms Leading Logarithm, NLL, … Analytic resummation (soft gluons integrated

out) Parton showers (soft gluons at leading log)

How to make sense of it all?

How to use the best parts of each?


Many computer programs CompHEP / Madgraph / Whizard / … MCFM / DYRad / JetRad / … Pythia / Herwig / Isajet / Ariadne / …

Often treated as Black Boxes Purpose of the Lectures: Open the Box

Separate the regions of validity Determine overlap Merge (use the best of each) Special role of Event Generators


An Important Topic!An Important Topic! Uncertainties in how events should be

generated are significant or most important errors for: Top mass determination Precision W-mass extraction

Together, a window to new physics

NNLO jet predictions with kT-algorithm

…

Our ignorance limits what we can learn about Nature


Event Generators: IntroductionEvent Generators: Introduction Most theorists make predictions about

Partons Valid to a specific order in perturbation theory The asymptotic states are not the physical ones

Quarks & gluons confined within hadrons Some predictions have peculiar properties

Slicing of phase space with cutoffs Negative weights cancelling positive ones

Experimentalists measure Objects in detector No distinction between Perturbative and Non-

Perturbative physics In- and Out-states are quasi-stable particles

Multiplicities can be large Observe (positive) integer number of events


Event Generators Connect “Theory” to Event Generators Connect “Theory” to “Experiment”“Experiment”

Describe the complicated Experimental Observable in terms of a chain of simpler, sequential processes Some components are perturbative

hard scattering, parton showering, some decays, … Others are non-perturbative and require modelling

hadronization, underlying event, kT smearing, …

models are not just arbitrary parametrizations, but have semi-classical, physical pictures

Sometimes as important as the perturbative pieces The Chain contains complicated integrals over

probability distributions Positive Definite Rely heavily on Monte Carlo techniques to choose a

history Final Output is E,p,x,t of stable and quasi-stable

particles Ready to Interface with Detector Simulations


Partial Event Partial Event DiagramDiagram

Hard ScatterFSRFSR

Resonance Decay

Remnant

“Underlying Event”

ISRISR

Hadronization

Particle Decay

Interconnection Bose-Einstein

σ̂)Q(x,f 2

i/p

)Qz,(P 2qq

)Qz,(D 2h/i

)Qz,(P 2qq


Hard ScatteringHard Scattering Characterizes the rest of the event

Sets a high energy scale Q Fixes a short time scale where partons are free

objects Allows use of perturbation theory External partons can be treated as on the mass-shell

Valid to 1/Q Properties at scales below Q are swept into PDFs

and fragmentation functions This is the Factorization Theorem

Sets flow of Quantum numbers (particularly Color) Note: Parton shower and hadronization models work

in 1/NC approximation

Gluon replaced by color-anticolor lines All color flows can be drawn on a piece of paper


Examples of color flowsExamples of color flows

gggg - WbbWttgg

Can influence:Can influence:

1.1. Pattern of additional soft gluon radiationPattern of additional soft gluon radiation

2.2. Fragmentation/HadronizationFragmentation/Hadronization


Tree Level Calculation of Hard Tree Level Calculation of Hard ScatterScatter Read Feynman rules from iLint

Use Wave Functions from Relativistic QM Propagators (Green functions) for internal lines

Specify initial and final states Track spins/colors/etc. if desired

Draw all valid graphs connecting them Tedious, but straight-forward

Algorithm can be coded in a computer program MadGraph / CompHEP / …

Calculate (Matrix Element)2

Evaluate Amplitudes, Add them, and Square (MadGraph) Symbolically Square, Evaluate (CompHEP) Do something trickier (Alpha)

(Monte Carlo) Integrate over Phase Space VEGAS …

Number of graphsgrows quicklywith number

of partons

Efficiency decreases with

number of internal lines


CompHEP Diagrams for CompHEP Diagrams for ududWW++bbbb

Vcb

Vub

Vcd

2sg

2sg

2ewg

2ewg2

ewg 2ewg

2ewg

2ewg

Vtd

2ewg

2ewg

Naïve application of rules leads to many

diagrams!


Higher Order = Higher TopologyHigher Order = Higher Topology

How sensitive is Mbb to additional gluon

radiation?

Both diagrams have 6 colored lines Amplitudes diverge in soft/collinear limit


Tree Level OverviewTree Level Overview

Leading order matrix element calculations describe explicit, many- particle topologies Well-separated

partons Full spin correlations Color flow

Many computer programs Different approaches

to the same problem Analytic vs Numeric Matrix Element vs

Phase Space

CompHep SM + MSSM + editable models Symbolic evaluation of squared matrix

element 2 4-6 processes with all QCD and

EW contributions color flow information outputs cross sections/plots/etc.

Grace similar to CompHep

Madgraph SM + MSSM helicity amplitudes “unlimited” external particles (12?) color flow information not much user interface (yet)

Alpha + O’Mega does not use Feynman diagrams gg10 g (5,348,843,500 diagrams)


Why Go Beyond Tree Level ?Why Go Beyond Tree Level ? Tree level (lowest order) prediction X has a large

dependence on the scale in couplings “the” hard scale is ambiguous Ideally dX/d=0, but not possible if X ~ g()N

More likely if X= a g()N + b g()N+1

No clear way to merge different topologies Some W/Z+N parton events will be reconstructed as

W/Z + N-1 jet events Some W/Z+N-1 parton events will be W+N-1 jet

events No way to avoid soft and/or collinear singularities

In fact, multiple gluon emission occurs in these kinematic configurations

No direct connection to hadronization


NLO: Looking Beyond the TreesNLO: Looking Beyond the TreesExample: hb+XExample: hb+X

ggss

ggss33

ggss22

)g2Re(gg g gg g~ 3sss

2s

bdy-2

23ss

bdy

2s

22

3

2σ


NLO: Some ImprovementNLO: Some Improvement Scale dependence greatly reduced Shape of some distributions similar up to

scale (K) factor But kinematics are inclusive Separation of different topologies depends on

cutoff Multiple, soft-collinear gluon emissions are not

included

Tevatron at

bbW


Wbb and ZbbWbb and Zbb


W+jj also to NLOW+jj also to NLO W+jj can still be used to normalize W+bb Overall scale dependence of W/Z+jj

reduced Program MCFMMCFM (Campbell, R.K. Ellis)


The need for even higher order…The need for even higher order…

As long as observables are inclusive enough, this is extremely important and useful

Beware of correlations between kinematics of different objects These can be

sensitive to multiple, soft gluon emission


Consider W production At LO in pQCD, the

rapidity Y and transverse momentum QT of the W are fixed by incoming partons

At NLO, single gluon emission occurs with QT>0

Cross sections at large QT or QT averaged are described well by fixed order in S

However, some observables are sensitive to region QT « Q For W/Z production, this

is most of the data! Solution: Reorganize

perturbative expansion N lnM(Q2/QT

2) Sums up infinite series

of soft gluon emissions kT dependent PDF’s

Resummation: Beyond Fixed Resummation: Beyond Fixed OrderOrder

...Q

Qlnαc c

Q Q

lnQα

dQσ̂d

:NLO

σ QδdQ

σ̂d

:LO

2T

22

S212T

2

2T

S2T

0T2T


Sudakov EffectSudakov Effect Solid: resummed

Superior at lower QT

Dot/Dash-dot: W+1j/W+2j Superior at high QT

Ln(Q/Q)=0

Multiple soft and collinear gluon emissions included, but

integrated out


Higher Order vs All OrderHigher Order vs All Order In the lower QT region, significantly

different predictions from NLO Decay products retain information about W

production Important for MW measurement in Run II

NLO prediction depends on cutoff


Review of “Hard Scattering”Review of “Hard Scattering” Tree Level Predictions

Pros Full spin and color

correlations “Easy” to calculate Good for large PT’s

and angular separations

Cons Rate not reliable Not clear how to

merge different topologies

(N)NLO Predictions Pros

Reduced scale dependence

Merging of topologies

Cons Also requires large PT’s

and angular separations

Resummation Pros

(N)NLO accuracy with all orders accuracy in kinematics

Cons No information on soft

gluons All of these approaches

ignore details of physics below hard scale Not yet connected to

hadronization Color must be

screened


Calculate the probability of a hard scattering at scale Q treating in and out partons as on-shell

Rest of the event can be described by positive probability distributions Prior to and after hard scatter, evolution of partons

is sensitive to quantum fluctuations below scale Q Cancellation of Virtual (-) and Real (+) effects occurs at

scales too small to resolve For color evolution, scale is typically QCD

In-partons evolved from some parents with P=1 Out-partons evolve into daughters with P=1 Final state partons hadronize with P=1 Beam particle remnant also hadronizes with P=1

The Factorization Theorem Factorization Theorem is essential for this to work

Monte Carlo Event GeneratorMonte Carlo Event Generator


““Monte Carlo” “Event Monte Carlo” “Event Generator”Generator” Improving the Physics complicates the Numerics

Difficult Integrand in Many dimensions Well-suited to Monte Carlo methods

Integrands are positive definite Normalize to be probability distributions

Hit-or-Miss Test integrand to find maximum weight WMAX (or just guess) Calculate weight W at some random point If W > r WMAX, then keep it, otherwise pick new W Sample enough points to keep error small

Can generate events like they will appear in an experiment N = [Xb] L[Xb-1]

NNLO QCD programs are not event generators Not positive definite (Cancellations between N and N+1) Superior method for calculating suitable observables

Tree level programs are not event generators Only limited topologies Can follow spins/color exactly


Parton ShowerParton Shower Hard Scattering sets scale Q Structure f(x,Q2) or fragmentation D(x,Q2) functions,

couplings S(Q2), etc. are evaluated at Q Asymptotic states have a scale Q0~1 GeV

Incoming/Outgoing partons are highly virtual How do incoming partons acquire mass2 ~ -Q2 ?

INITIAL STATE RADIATION (ISR) How do outgoing partons approach the mass shell ?

FINAL STATE RADIATION (FSR) Typically, resolving smaller scales generates many

partons with lower virtuality Virtualities on the order of QCD are expected for

partons bound in hadrons “traditional” calculations based on a small number

of Feynman diagrams are incomplete Parton Showering Monte Carlos are an approximation

to high-order, perturbative QCD


Parton Showering: More Parton Showering: More MotivationMotivation

Semi-classical description Accelerated charges radiate

22

)n(n c4

eddP

R

)n(nce

E βπΩ

β

Color is a charge, and thus quarks also radiate Gluon itself has charge (=q-q* pair to 1/Nc)

Field Theory Block and Nordsieck (QED)

Must include virtual and real (emission) corrections to obtain IR finite cross section Electron is ALWAYS accompanied by

cloud of quanta (photons)

nβ


2qqs

2qqs

0 Qu

(z)Pdzudu

2Qt

(z)Pdzt

dt2

g)q(qd

gqqee

π

α

π

ασσ

Example: gluon emission in Example: gluon emission in ** events events

Q2 st

u

gqgqqq

2

qq2

p2pu,p2pt,p2psz1z1

34

(z)P,Qs

z

t t 0 when gluon is 0 when gluon is Soft, collinear or bothSoft, collinear or both

zz1 when gluon is1 when gluon isSoft, collinear or bothSoft, collinear or both

Factorization of Mass Singularities

Probability of one additional soft emission proportional to rate without emission dN+1 = N S/2 dt/t dz P(z)


Tower of emissions described by Sudakov Form Tower of emissions described by Sudakov Form FactorFactor

Series of subsequent showers “exponentiate” Shower of resolvable emissions q*(p) q(zp) + g([1-z]p)

Emission RESOLVED if zC < z < 1 - zC

Sudakov built from Probability of no resolvable emission for small t

Prob(tmax,t) = S(tmax)/S(t) = random r Pick random r and solve for new t Resolvable emission at the end of “nothing”: dS/dt Continue picking new t’s down to tmin ~ QCD

Stop shower & begin hadronization

CC

0

t

tcb, bca

)(tz

)(tz

S'

cb, bca

(t)z

(t)z

S

z~z,z1~z

t),(t(z)P2(t)

dzdtexpS(t)

:emissions leirresolvab of numbers all over Sum

t(z)P2(t)

dz1

0

'

'

Δπ

α

δπ

α


Virtuality-Ordered PSVirtuality-Ordered PS

Highly virtual

Nearly on-shell

)ln(Q t

t t t2ii

321


Initial State RadiationInitial State Radiation Similar picture, but solving DGLAP for

PDFs

BRANCHINGS

"bbca

"abc

"

t

t

1

x

"

BRANCHING NO

aa )t(x/z,f (z)P̂2

)t(z,)(t)(t'

zdz

dt )(t' t)(x,f )t'(x,f'

π

α

Δ

ΔΔ

Increasing parton

virtuality

Parent has more momentum


Backwards ShoweringBackwards Showering

-Q02>-Q1

2>…>-Qn2

showering added after hard scatter with unit probability Something

happens, even if not resolvable

)t',(x'f z

(z)P̂2

t)(z, Prob(z) ;

)(t')t'(x,f

t)(x,f(t)

x/zx' ;)t'(x,f x)t',(x'f x'

(z)P̂2

)t'(z, dzdt' ln(S)-

abcaabca

b

t

t

z

z b

abca

abcMAX

-

π

α

Δ

Δ

π

α

Sjostrand

Marchesini/Webber

PRIMORDIAL KT

21Q 2

2Q2CQ2

CQ23Q


Parton Shower is a Parton Shower is a ResummationResummation Analytic Resummation

soft gluon emissions exponeniate into Sudakov form factor

kT conserved Total rate at (N)NLO

modified PDF's corrections for hard

emission soft gluons are

integrated out Predicts observables for

a theoretical W Needs modelling of non-

perturbative physics

Parton Showering DGLAP evolution generates

a shower of partons LL with some N-LL

Exact gluon kinematics LO event rates underestimates single,

hard emissions Explicit history of PS

More closely related to object identified with a W

natural transition to hadronization models Follow color flow down to

small scalesSimilar physics, but different approach with

different regimes of applicability


Comparison of PredictionsComparison of Predictions

Example of Matrix Element Corrections to

Parton Showering

Example of Treating

Kinematics differently in

shower

Analytic Resum


Color CoherenceColor Coherence In previous discussion of PS, interference effects

were ignored, but they can be relevant

Add a soft gluon to a shower of N almost collinear gluons incoherent emission:

couple to all gluons |M(N+1)|2 ~ N S NC

coherent emission: soft means long wavelength resolves only overall

color charge (that of initial gluon)

|M(N+1)|2 ~ 1 S NC


Angular-Ordered PSAngular-Ordered PS Showers should be Angular-Ordered

= pI • pJ / EI EJ = (1 - cosIJ) ~ IJ2/2

1 > 2 > 3 …

Running coupling depends on kT2 z(1-z)Q2

Dead Cone for Emissions Q2 = E2 < Q2

max

Q2max = z2 E2

< 1 [not 2] < /2

No emission in backwards hemisphere


Color Coherence in PracticeColor Coherence in Practice Emission is restricted inside cones

defined by the color flow

Partonic picture Large N picture

Enhanced

emissionBeam line


Essential to Describe DataEssential to Describe Data 3 Jet Distributions in Hadronic Collisions

Full Coherence

No Coherence

Partial Coherence

Pseudorapidity of Gluon Jet

Soft Soft emissions emissions

know about know about beam line beam line (large Y)(large Y)


The Programs (Pyt/Isa/Wig/Aria)The Programs (Pyt/Isa/Wig/Aria) ISAJET

Q2 ordering with no coherence large range of hard processes

PYTHIA Q2 ordering with veto of non-ordered emissions large range of hard processes

HERWIG complete color coherence & NLO evolution for

large x smaller range of hard processes

ARIADNE complete color dipole model (best fit to HERA

data) interfaced to PYTHIA/LEPTO for hard processes


Parton Shower SummaryParton Shower Summary Accelerated color charges radiate gluons

Gluons are also charged Showers of partons develop

IMPORTANT effect for experiments Showering is a Markov process and is added to

the hard scattering with P=1 Derived from factorization theorems of full gauge

theory Performed to LL and some sub-LL accuracy with

exact kinematics Color coherence leads to angular ordering

Modern PS models are very sophisticated implementations of perturbative QCD Still need hadronization modelshadronization models to connect with data Shower evolves virtualities of partons to a low

enough values where this connection is possible


ComparisonComparison Strings (Pythia)

PRODUCTION of HADRONS is non-perturbative, collective phenomena

Careful Modelling of non-perturbative dynamics

Improving data has meant successively refining perturbative phase of evolution

Clusters (Herwig) PERTURBATION

THEORY can be applied down to low scales if the coherence is treated correctly

There must be non-perturbative physics, but it should be very simple

Improving data has meant successively making non-pert phase more stringlikeSTRING model includes some non-STRING model includes some non-

perturbative aspect of color coherenceperturbative aspect of color coherence

See Bill Gary’s lectures


The ProgramsThe Programs ISAJET

Independent fragmentation & incoherent parton showers

JETSET (now PYTHIA) THETHE implementation of the Lund string model Excellent fit to e+ e- data

HERWIG THETHE implementation of the cluster model OK fit to data, but problems in several areas

String effect a consequence of full angular-ordering


W + Jet(s) at the TevatronW + Jet(s) at the Tevatron Good testing ground for

parton showers, LO,NLO Large Scale dependence

at LO

Good agreement with NLO


W + N jet RatesW + N jet Rates VECBOS for N hard partons HERWIG for additional gluon

radiation and hadronization

VECBOS for N-1 hard partons HERWIG for 1 “hard” parton

plus …


W + N jet ShapesW + N jet Shapes Start with W + N jets

from VECBOS + HERWIG

Start with W + (N-1) jets from VECBOS + HERWIG


When good Monte Carlos go badWhen good Monte Carlos go bad

#events >1 jet >2 jets >3 jets >4 jet

pT>10 GeV/c

Data 920 213 42 10

VECBOS + HERPRT (Q=<pT>)W + 1jet 920 178 21 1W + 2jet ----- 213 43 6W + 3jet ----- ----- 42 10

VECBOS+HERPRT(Q=mW)

W + 1jet 920 176 24 2W + 2jet ---- 213 46 6W + 3jet ---- ----- 42 7

CDF Run0 DataCDF Run0 Data

VECBOS starting point

More jets generated by HERWIG parton

shower

Normalize 1st bin to data

Results are cut dependent

PS only has collinear part of matrix element

PS has ordering in angles


Correcting the Parton ShowerCorrecting the Parton Shower PS is an accurate description for soft/collinear

kinematics Most of the data for a given process!

Underestimates wide angle emissions Also, no 1/NC suppressed color flows Tails of kinematic distributions are often most

interesting PS with a single emission can be reweighted to

behave like fixed-order result Correct all or hardest-so-far emissions this way

Populate kinematic regions not included in PS Delicate matching between different regions

Actual correction is generator dependent No attempt to generate NLO rate

Sjostrand/Miu/Seymour/Gorcella …


Parton Showering and Heavy Parton Showering and Heavy QuarksQuarks Heavy Quarks look like light quarks at

large angles but are sterile at small angles

Naïve -ordered shower has a cutoff > 0 = mQ/EQ = r Creates ‘dead cone’

Virtuality-ordered shower also needs a special treatment

2

2213

213

133

QQ133

3

332

32

2

31

1

r,0),(xd

)/Emr,,(xd

Epd

ppp

ppp

)q(qdg)q(qd

:emission gluon soft for expression Eikonal

θ

θ

θσ

θσ

σ

σ


Pythia Corrections to Top DecayPythia Corrections to Top Decay Relatively easy for Q2

ordered showering Rewrite Parton

Shower weight in terms of Matrix Element kinematics

Modify PS probability by WME / WPS

Angle btw. Quark and Gluon

Parton Level MtopHadron Level Mtop

Not Significant


Not Significant


Top with Hard EmissionsTop with Hard Emissions For top events with one very hard or two

hard jets, the PS description will not be valid Can estimate the relative importance of this

using COMPHEP

Rely on ME predictions plus parton showering Not the perfect solution Will not merge smoothly onto other predictions

NO ME Corrections

to PS in Pythia or

Herwig for Production


Correcting the NLO CalculationCorrecting the NLO Calculation Total rate is more stable to hard scale

variation Single wide-angle emission from onset

“models” soft-collinear region by single gluon with an inclusive subtraction of singularities

Desire: (N+1)xPS + (N)xPS – Overlap Overlap ~ (N)xPS (all orders) truncated to fixed

order Implementations

Phase space slicing with chosen to remove (N) Only one configuration to shower Positive definite weights Sensitivity to large logarithms of must arise in some

distributions Expansion to fixed order will not agree with NLO result

Baer/Reno/Dobbs/Potter/ …


Subtraction No dependence on a cutoff NLO calculation fixes initial kinematics for PS and

relative weights of (modified) N and N+1 body contributions

Suitably modified subtraction expands to NLO result

Negative weights are generated Can be treated practically using positive

definite integrands but keeping track of sign Relies on unmodified PS algorithm

Consistent? Power law dependence on some showering

parameters Should be small Frixione/Webber


Parton Shower and FactorizationParton Shower and Factorization Standard Result

Some freedom in defining PDF/Fragmentation function

Differences not observable for inclusive calculations Parton Shower needs a special treatment [Collins]

Exact kinematics throughout shower NLO predicts gauge boson at rapidity Y PS starts from Y0 and generates Y from emission/boost Mismatch involving evaluation of PDFs at different x Affects Overlap computed in previous approaches

PDFs for PS depend on process and showering algorithm Demonstrated large effect for small parton x

Same conclusion from considering the analytic resummation methods ala Collins/Soper/Sterman [Mrenna]


PS, Factorization, and PS, Factorization, and ResummationResummation b/QT-space resummation yields (N)NLO

rate and “all orders” kinematics

Y = Fixed order-Asymptotic = Overlap! Contains negative weights – just aesthetics?

1

x

2Q

QT

21T2T

2 12T

2

C(x/z)f(z)zdz

f)[x](C

BmQ

lnA mdm

Q),T(Q

H(Q) f)exp(-T)f)(C(C W~

Y)x,xQ,,(QW~

dQd

X)W h(hdydQdQ

d

2

2T

σ

Q)f(x,)Q,T(Q

21

exp

)Qf(x,Q),T(Q21

exp

dQd

dQd

LO, At

T0

T0

2T

02T

σσ

MW PS algorithm Sudakov determines PT

W

Soft gluon emissions are integrated out Sudakov contains soft

pieces not in DGLAP Total rate can be calculated

to any given order in S


Summary of NLO ShoweringSummary of NLO Showering Correct the PS for emissions that are not

soft/collinear enhanced Reweight by (ME2

PS)/(ME2Exact) [PYTHIA]

Fill out Dead Cone and correct shower [HERWIG] Still have to normalize to NLO rate

Add PS to (N)-(N+1) body configurations of full NLO calculation with suitable subtractions Phase Space Slicing with chosen to eliminate (N)

body configurations Approximation to kinematics sensitive to log()

Subtraction method with modifications to (N) and (N+1) body “Kernels” NLO predictions recovered upon expansion to O(2) Negative weight events are generated

Inconsistency in using “inclusive” definitions of PDFs/Fragmentation functions with exact kinematics


Summary (cont)Summary (cont) Process dependent PS

Factorization must be re-evaluated when exact kinematics are treated

Integration over KT and virtualities implicit to usual PDFs/Fragmentation functions

Feature also seen in analytic resummation (Collins/Soper/Sterman)

Analytic Resummation has essential ingredients Process dependent PDFs recover NLO rate and

weight Sudakov Y-piece subtraction corrects for soft-gluon

approximation inside Sudakov Introduces negative weights


Related IdeasRelated Ideas QCD Matrix Elements + Parton Showers

Catani/Kraus/Kuhn/Webber & Lönnblad Attempts to piece together many tree level

processes using PS Generate all ZN parton (tree level) processes

simultaneously For a given N, select a given topology Match topology to a PS history Reject if event would have been generated by

M<N with a parton shower attached Not clear how to match to higher-order rate

Fully numerical NLO in Coulomb gauge Soper and Kramer

Claims a more natural connection to PS


ConclusionsConclusions Great activity amongst relatively few

practitioners to develop NLO parton showering Several good ideas which need to be synthesized

Hadron collider phenomenology has greatest priority now Naively, effects will be larger with initial state

radiation Precision physics is sensitive to our

understanding of the parton shower Already a large “systematic” uncertainty to Run II

measurements (MW and mt) Perhaps a larger “systematic” at LEP than we think

b-quark asymmetries “4-jet” events …


Topics Not Topics Not DiscussedDiscussed

Hard ScatterFSRFSR

Resonance Decay

Remnant

“Underlying Event”

ISRISR

Hadronization

Particle Decay

Interconnection Bose-Einstein

σ̂)Q(x,f 2

i/p

)Qz,(P 2qq

)Qz,(D 2h/i

)Qz,(P 2qq


Topics Not CoveredTopics Not Covered

Some aspects of the event are beyond the scope of this introductory set of lectures [and my expertise], yet can be important when comparing to data and can impact new physics searches Treatment of the beam remnant may be relevant for

forward jet tagging studies Higgs production through WW fusion

Underlying event affects isolation and jet-energy corrections Observing Higgs in photons or jets

Interconnection and Bose-Einstein effects are relevant to precision EW measurements

Tau leptons and b-hadrons must be decayed correctly to understand polarization effects & tagging efficiency Is phase space enough?

The objects that experimentalists observe are not the same as the output of an event generator!


Overall SummaryOverall Summary Event Generators accumulate our knowledge and

intuition about the Standard Model into one package Apply perturbation theory whenever possible

hard scattering, parton showering, decays Rely on models or parametrizations when present

calculational methods fail hadronization, underlying event, beam remnants

Out of the box, they give reliable estimates of the full, complicated structure of an event Sophisticated users will find more flexibility & applications

And will avoid easy mistakes Understanding the output will lead to a broad

understanding of the Standard Model (and physics beyond)

cteq ss 2002stephen mrenna fermilab 1 introduction to event generators stephen mrenna cd/simulations...

Documents

important errors

physical onesquarks

perturbative piecesthe

nonperturbative physicsin

hadronssome predictions

physical observablescorrect

t of stable

quasistable particlesready