Event Generator Status

Aspen Winter Conference, January 2006

Peter Skands

• Matching (who’s doing it, how, and where?)Matching (who’s doing it, how, and where?)

• New ideas and C++New ideas and C++

Event Generator Status

Peter Skands Event Generator Status 2

BR: Beam Remnant

CR: Colour Reconnection

FSR: Final-State Radiation

ISR: Initial-State Radiation

Matching: Combining PS & ME consistently (e.g. CKKW, MLM)

ME: Matrix Element

MI: Multiple parton-partonInteractions(not pile-up)

PS: Parton Shower

PT: Perturbation Theory

Tune: A set of generator parameters

UE: Underlying Event

Traditional Event Generators

► Basic aim: improve lowest order perturbation theory by including leading corrections exclusive event samples1. sequential resonance decays

2. parton showers

3. underlying event

4. hadronization

5. hadron and tau decays

Helper: I’ll try to avoid acronyms, but this list will be on all slides and may help when I fail. It is ordered alphabetically.


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




Modern Event Generators► Specialized tools for calculating higher fixed orders (and

BSM processes) plus matching techniques

hard subprocess (and to some extent resonance decays) increasingly handled by separate codes (LO … NnLO)

Need universal interfaces and standards [e.g. the Les Houches Accords (Les Houches 2007: Jun 11-29, France) ]

► Beyond fixed order

MC4LHC `06: “A standard format for Les Houches Event Files” - hep-ph/0609017

Better understanding of PS uncertainties – À LA ERROR PDF’S?

Improved PS formulations – MORE CONSISTENT, MATCHING TO NnLO, RESUMMATION OF HIGHER LOGS & SMALL-X EFFECTS (BFKL), …

Better understanding of the underlying event and non-perturbative effects - ESPECIALLY IN THE BUSY ENVIRONMENT OFFERED BY LHC

Entering era of precision event generators for hadron colliders


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




Matching► Matching of up to one hard additional jet

• PYTHIA-style (reweight shower)

• HERWIG-style (add separate events from ME: weight = ME-PS)

• MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)

► Matching of generic (multijet) topologies:

• ALPGEN-style (MLM)

• SHERPA-style (CKKW)

• ARIADNE-style (Lönnblad-CKKW)

• PATRIOT-style (Mrenna & Richardson)

► Brand new approaches (still in the oven)

• Refinements of MC@NLO (Nason)

• CKKW-style at NLO (Nagy, Soper)

• SCET approach (based on SCET – Bauer, Schwarz, SEE BAUER’S TALK ON FRIDAY!)

• VINCIA (based on QCD antennae – Giele, Kosower, PS, THIS TALK)

Evolution


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




ALPGEN► “MLM” matching (Mangano)

• Simpler but similar in spirit to CKKW

► First generate events the “stupid” way:

1. [Wn]ME + showering

2. [Wn+1]ME + showering

3. …

► a set of fully showered events, with double counting. To get rid of the excess, accept/reject each event based on:

• (cone-)cluster showered event njets

• match partons from the ME to the clustered jets

• If all partons are matched, keep event. Else discard it.

► Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed

n inclusiven inclusive

n+1 inclusiven+1 inclusive


n exclusiven exclusive

n+1 exclusiven+1 exclusive



BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063SHERPA, ARIADNE, PATRIOT

► The CKKW algorithm

• Slices phase space :• ME for pT > pTcut

• PS for pT < pTcut

1. [Wn]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut

2. [Wn+1]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut

3. … [Wnmax] ME|pT>pTcut + showering

• Wveto < 1 is the probability that no parton shower emission happened above pTcut. Computed using clustered ‘parton shower histories’ on the ME final states. SHERPA uses an approximate analytical formula

• Lönnblad’s ARIADNE-style uses a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut

► Gets rid of double counting: those events that would have caused it are precisely those which do branch above pTcut

► A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step PATRIOT database (and HERWIG++?)

L. L¨onnblad, JHEP05 (2002) 046S. Mrenna, P. Richardson, JHEP 0405(2004)040


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




MC@NLO

Nason’s approach:

Generate 1st shower emission separately easier matching

Avoid negative weights + explicit study of ZZ production

Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007

JHEP 0411(2004)040

JHEP 0608(2006)077


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




VINCIA

► VINCIA Dipole shower

• C++ code for gluon showers – running

• Can evolve in either of 2 different shower evolution variables:

• pT-ordering (~ ARIADNE)

• Virtuality-ordering (~ Pythia 6.2 & SHERPA)

• + not hard to generalize to arbitrary IR safe

• For each evolution variable: • an infinite family of radiation functions

implemented, all with correct collinear and soft behaviour (= “antenna functions”)

► First parton shower with systematic possibility of variation of shower variable and shower functon control uncertainties

• (Future plans include also variation of kinematic map)

► To any fixed order, these variations can be absorbed by a new type of matching to matrix elements

Dipoles – a dual description of QCD

1

3

2

virtual numerical collider with interlinked antennae

Giele, Kosower, PS : in progress


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




► Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element subtracted (IR finite) matrix elements.

1. Generate parton-level configurations, with weights given by the subtracted Matrix Elements

2. Shower them using the subtraction function from step 1.

► Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour) matching at NLO

► Can simultaneously include arbitrarily many tree-level ME’s multijet matching

• Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching)

• Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet)

VINCIA-style matchingGiele, Kosower, PS : in progress


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




Hgg: Example

VINCIA 0.008

Unmatched

“soft” |A|2

VINCIA 0.008

Unmatched

“hard” |A|2

First Branching ~ first order in perturbation theoryVINCIA 0.008

Matched

“soft” |A|2

VINCIA 0.008

Matched

“hard” |A|2

Antenna Function

IR singularities plus arbitrary finite terms

|A(sa,sb;s)|2

(a.k.a. Radiation Function, a.k.a. Splitting Kernel)

Systematically improved by matching

Expect public Expect public code and long code and long writeup before writeup before

summersummer

Giele, Kosower, PS : in progress

y12

y23

y23

y23

y23

Later: plug-in for Later: plug-in for Pythia 8 ?Pythia 8 ?

Next: Quarks Next: Quarks and ISR and ISR

hadron collider hadron collider event generatorevent generator

y12


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




C++ Players► HERWIG++: complete reimplementation

• Improved PS and decay algorithms

• Eventually to include CKKW-style matching ?

• B.R. Webber; S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. Seymour, P. Stephens, . . .

► SHERPA: complete implementation, has CKKW• ME generator + wrappers to / adaptations of PYTHIA,

HERWIG

• F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter

► PYTHIA8: selective reimplementation• Improved PS and UE, limited number of hard subprocesses

• Many obsolete features not carried over simpler, less parameters

• T. Sjöstrand, S. Mrenna, P. Skands


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




PYTHIA 8

Basic generator already there

Includes a few processes (+ full Pythia6 library), new pT-ordered showers, new UE, Les Houches interfaces, and more

You are invited to try it out

Click /future/ on the Pythia homepage, download pythia8070.tgz, follow instructions in readme (./configure, ./make, and have fun)

Still not advised for production runs

If you have suggestions, now is the time!

Timeline:

Spring 2007: QED showers, LHAPDF, interleaved FSR, beam remnants, colour reconnections useful

Fall-Winter 2007: resonance decays, GUI, official release?


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower



UE: Underlying Event D. B. Leinweber, hep-lat/0004025

Anti-Triplet

Triplet

pbar beam remnant

p beam remnant

bbar

from

tbar

dec

ay

b fro

m t

deca

yq

bar fro

m W

q from W

hadronization

?

q from W

In reality, this all happens on top of each other.

(only possible exception: long-lived colour singlet)

The (QCD) Landscape


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




New physics in the Underlying Event?

►Given the lack of analytical solutions impacts on hadronic precision observables poorly known

►Example: sensitivity of Tevatron top mass analysis (simplified!) to variations of:

• Colour reconnections • string-string interactions?

• string-vacuum (re)interactions? “colour annealing” model large strength ( >10% ) required

for acceptable fits to min-bias

• Parton showers• Pythia: mass-ordered vs pT-ordered

• Underlying-event parameters• E.g. Tune A vs Tune DW, etc

ΔPS ~ 0.75 GeV

<Δ>UE/CR ~ 0.4 GeV

PS, D. Wicke : preliminary

Δmtop

Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




Beyond the Standard Model► SUSY Les Houches Accord being extended to RPV,

NMFV, CPV, NMSSM, … SLHA2

• Apologies for slow progress. A skeleton exists, but so far only small number of practical implementations to test viability

• General CPV and NMFV: MSSM implementation for PYTHIA8

• NMSSM:

► Les Houches BSM Tools Repository:

► Recent years more focus on non-SUSY

• 2nd MC4BSM: Princeton, March 21-24 2007 (in conjunction with LHC Olympics)

• Teams of “writers” + teams of “testers”: sign up for the fun!

G. Bozzi, B. Fuks, M. Klasen, PS : in progress

Ellwanger, Hugonie, Moretti, Pukhov, … : in progress

http://www.ippp.dur.ac.uk/montecarlo/BSM

http://www.phys.ufl.edu/~matchev/MC4BSM/

B. Allanach et al, in hep-ph/0602198

Summary of 1st MC4BSM: J. Hubisz, PS, FERMILAB-CONF-05-082-T

PS et al, in hep-ph/0602198

lots of material, e.g. for warped ED: R. Contino et al, hep-ph/0612180


BR: Beam Remnant





ME: Matrix Element


PS: Parton Shower




The Generator Outlook► Generators in state of continuous development:

► Better & more user-friendly general-purpose ME calculators+integrators

► New libraries of physics processes, also to NLO

► Improved parton showers

► Better matching of matrix elements to showers

► Improved models for underlying events / minimum bias

► Upgrades of hadronization and decays

► Moving to C++

more precise, more reliable theoretical predictions

Ultimately, the interesting talk is the experimental one, how good is a good calculation? Compare to LEP, Tevatron, and RHIC data constraints. Absolutely vital for high precision + often uncovers defects, and even hints of new phenomena…

Event Generator Status

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