Event%20Generator%20Status

1
Event Generator Status
Aspen Winter Conference, January 2006
Peter Skands

• Matching (whos doing it, how, and where?)
• New ideas and C

2
Traditional Event Generators

• Basic aim improve lowest order perturbation
  theory by including leading corrections ?
  exclusive event samples
• sequential resonance decays
• parton showers
• underlying event
• hadronization
• hadron and tau decays

Helper Ill try to avoid acronyms, but this list
will be on all slides and may help when I fail.
It is ordered alphabetically.
3
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
? Entering era of precision event generators for
hadron colliders
Better understanding of PS uncertainties À LA
ERROR PDFS? 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
4
Matching

• Matching of up to one hard additional jet
• PYTHIA-style (reweight shower)
• HERWIG-style (add separate events from ME weight
  ME-PS)
• MC_at_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_at_NLO (Nason)
• CKKW-style at NLO (Nagy, Soper)
• SCET approach (based on SCET Bauer, Schwarz,
  SEE BAUERS TALK ON FRIDAY!)
• VINCIA (based on QCD antennae Giele, Kosower,
  PS, THIS TALK)

5
ALPGEN

• MLM matching (Mangano)
• Simpler but similar in spirit to CKKW
• First generate events the stupid way
• WnME showering
• Wn1ME showering
• 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 inclusive
n1 inclusive
n2 inclusive
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SHERPA, ARIADNE, PATRIOT
S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP
0111 (2001) 063

• The CKKW algorithm
• Slices phase space
• ME for pT gt pTcut
• PS for pT lt pTcut
• WnME pTgtpTcut Wveto(pTcut)
  showeringpTltpTcut
• Wn1MEpTgtpTcut Wveto(pTcut)
  showeringpTltpTcut
• Wnmax MEpTgtpTcut showering
• Wveto lt 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
• Lönnblads 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. Lonnblad, JHEP05 (2002) 046
S. Mrenna, P. Richardson, JHEP 0405(2004)040
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MC_at_NLO
Frixione, Nason, Webber, JHEP 0206(2002)029 and
0308(2003)007
Nasons approach Generate 1st shower emission
separately ? easier matching Avoid negative
weights explicit study of ZZ production
JHEP 0411(2004)040
JHEP 0608(2006)077
8
VINCIA
virtual numerical collider with interlinked
antennae
Giele, Kosower, PS in progress
1

• 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)

Dipoles a dual description of QCD
2
3

• 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

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VINCIA-style matching
Giele, Kosower, PS in progress

• Subtraction method subtract showering off
  n-parton state from n1-parton Matrix Element ?
  subtracted (IR finite) matrix elements.
• Generate parton-level configurations, with
  weights given by the subtracted Matrix Elements
• Shower them using the subtraction function from
  step 1.
• Similar to HERWIG and MC_at_NLO approaches (with
  antenna subtraction instead of Catani-Seymour) ?
  matching at NLO
• Can simultaneously include arbitrarily many
  tree-level MEs ? multijet matching
• Combines virtue of CKKW (multijet matching) with
  that of MC_at_NLO (NLO matching)
• Avoids the vices of CKKW (dependence on
  unphysical clustering and cuts, LO) and MC_at_NLO
  (complicated, not applicable beyond one jet)

10
H?gg Example
Giele, Kosower, PS in progress
Antenna Function IR singularities plus arbitrary finite terms A(sa,sbs)2 (a.k.a. Radiation Function, a.k.a. Splitting Kernel)
Systematically improved by matching
First Branching first order in perturbation
theory
VINCIA 0.008 Unmatched soft A2
VINCIA 0.008 Matched soft A2
y23
y23
Expect public code and long writeup before summer
Next Quarks and ISR ? hadron collider event
generator
y23
y23
VINCIA 0.008 Unmatched hard A2
VINCIA 0.008 Matched hard A2
Later plug-in for Pythia 8 ?
y12
y12
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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

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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?
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The (QCD) Landscape
In reality, this all happens on top of each
other. (only possible exception long-lived
colour singlet)
D. B. Leinweber, hep-lat/0004025
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New physics in the Underlying Event?
Sandhoff PS, in Les Houches 05 SMH
Proceedings, hep-ph/0604120

• 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 ( gt10 ) 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

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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!

B. Allanach et al, in hep-ph/0602198
G. Bozzi, B. Fuks, M. Klasen, PS in progress
Ellwanger, Hugonie, Moretti, Pukhov, in
progress
PS et al, in hep-ph/0602198
http//www.ippp.dur.ac.uk/montecarlo/BSM
Summary of 1st MC4BSM J. Hubisz, PS,
FERMILAB-CONF-05-082-T
http//www.phys.ufl.edu/matchev/MC4BSM/
lots of material, e.g. for warped ED R. Contino
et al, hep-ph/0612180
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The Generator Outlook

• Generators in state of continuous development
• Better more user-friendly general-purpose ME
  calculatorsintegrators
• 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