Transcript Overview of IPPP Monte Carlo Tools Peter Richardson IPPP, Durham University IPPP 26th July.

Overview of IPPP Monte Carlo
Tools
Peter Richardson
IPPP, Durham University
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Summary
• Introduction
• Basics of Event Generation
• Matrix Elements and Perturbative
Decays
• Parton Shower and Matching
• Hadronization and Tuning
• Underlying Event
• Summary
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Introduction
• Monte Carlo event generators are essential both
experimental analyses of real data and studies and
predictions for future experiments.
• As such the programs and their authors are a vital
interface between experimental and theoretical
communities.
• Often they are the only theoretical calculations
used in an experimental analyse and it is therefore
vital that they are as accurate as possible and
include the latest theoretical advances.
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A Monte Carlo Event
Hard Perturbative scattering:
Modelling of the
soft underlying
event
Multiple perturbative
scattering.
Usually calculated at leading order
in QCD, electroweak theory or
some BSM model.
Perturbative Decays
calculated
QCD,State
EW or
Initial andinFinal
parton showers resum the
Finally the unstable hadrons are
some
theory.
largeBSM
QCD
logs. of the
Non-perturbative
modelling
decayed.
hadronization process.
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Monte Carlo Projects
• At the IPPP we are involved with two of the
major simulation packages
– HERWIG
– SHERPA
Richardson, Grellscheid,
Hamilton, Gigg, Tully.
Krauss, Hoeche
• We play a major part in both projects and are
involved in many different aspects of the
physics.
• We are also involved in the CEDAR project
(Buckley, Whalley)
• and the EU MCnet network.
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COMIX, a new Matrix element generator
• Several independent analyses show clear advantages of
Berends-Giele type recursive relations for numerical
evaluation of tree-level Matrix Elements
• Can be generalised to the full Standard Model, not just
QCD.
• Employed in new Matrix Element generator Comix, together
with a new approach for combined colour sampling.
• Current checks in
randomly selected QED
processes.
• Shows clear advantages
over commonly used
helicity methods
(AMEGIC++, MadGraph)
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BSM physics in Herwig++
New method of including
new physics in
Herwig++, including spin
correlations,
e- near
e- far
M. Gigg, P. Richardson
hep-ph/0703199
q
e+ near
e+ far
e- near
q*L
e+ far
Z*
e*R
g*
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Herwig++ Shower
• Following the initial release
for e+e- collisions most
work has been on
– Initial-state shower in
hadron collisions
– Radiation in the decays of
unstable particles
– Various matching
approaches.
Herwig++ with intrinsic pT
compared to CDF data
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Top Shower and Matrix Element
Correction in tgbW+g
JHEP 0702:069, 2007, Keith Hamilton, Peter Richardson.
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Nason Approach
• One recent idea is an approach to NLO
Monte Carlo simulations with only
positive weights events, P. Nason JHEP
0411:040, 2004.
• We (K. Hamilton, J. Tully, PR) are
looking at this approach.
• In particular the treatment of soft
radiation (truncated shower) for both
NLO simulations and its application in
other situations.
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SHERPA Shower and CKKW
• The main SHERPA shower, APACIC++,
has been available for some time.
• Recently most work has been on
– CKKW matching
– Catani-Seymour Shower
– Colour Dipole Shower
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SHERPA Colour Dipole Shower
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SHERPA Colour Dipole Shower
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SHERPA Catani-Seymour Shower
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SHERPA Colour Dipole Shower
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Tuning of Event Generators
• Historically the event generators were
tuned to LEP data by the LEP
collaborations, obviously isn’t going to
happen for the new generation of
programs.
• However it is vital that the new programs
are tuned to both LEP, HERA and
Tevatron data.
• We are involved with this both as part of
CEDAR and MCnet and with a new SPG.
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SHERPA kT based UE Model
• Recently method proposed to
describe high-energy jet
production in kT-factorization
(IPPP/07/20)
• LO BFKL kernel matched to
unintegrated parton distributions
• Quark production modelled by
including LO DGLAP kernels in
the soft limit.
• Encouraging results for jet
production at the Tevatron.
• Will be employed for MEgeneration of hard secondary
interactions in new Multiple
Interaction model for Sherpa.
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Hadron Decays and QED
Radiation
• Traditionally the general purpose event
generators have relied on external packages
for the simulation of
– Tau Decays (TAUOLA)
– Hadron Decays (QQ, EvtGen)
– QED Radiation (PHOTOS)
• However this can cause problems with
interfaces.
• Both Herwig++ and SHERPA are moving to
sophisticated simulations for all of these.
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Tau Decays, tgrnt
Herwig++
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SHERPA
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Bgpen
Herwig++
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SHERPA
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B mixing in SHERPA
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bgsg in HERWIG
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YFS
• Both HERWIG and SHERPA have moved to
an approach based on the YFS formalism for
the simulation of QED radiation in particle
decays.
• This has the advantage that it is
systematically improvable by adding the
higher order corrections and has proved
highly successful in, for example e+ecollisions.
• In both cases any number of soft photons are
described together with a correction for the
hardest emission.
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YFS
Herwig++ K. Hamilton and P. Richardson
JHEP 0607:010, 2006.
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SHERPA M. Schoenherr
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Herwig++ Support and Maintenance
• On the Herwig++ side we have one long
term (core) post, D. Grellscheid, to provide
– Core user support
– Technical C++ and software expertise to
support the design, code management and
testing
• This will give improved support to the
experiments for the LHC and allow other
members of the collaboration to
concentrate on physics issues.
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Workshops and Schools
• We have had many Monte Carlo related
workshops and schools since the start of
the IPPP
– Physics and Techniques of Event Generators:
1st MCnet school. (2007)
– YETI 06 SM and BSM
– C++ Monte Carlo Developers Forum (2005)
– YETI 05
– Monte Carlos at Hadron Colliders (2003)
– Matrix Elements and Parton Showers (2000)
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Future
• Things are moving fast on both Herwig++ and
SHERPA.
• For Herwig++
– JIMMY underlying event model (Aug)
– BSM physics (Aug)
– Hadron decays, tau/mesons (Aug), baryons (Dec)
• For SHERPA
–
–
–
–
2 new shower models (Oct)
MHV techniques, Berends-Giele subtraction terms (Oct)
Cluster Hadronization (Dec)
kT underlying event model (Dec)
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Summary
• Since 2003 we have built a world-leading Monte Carlo
group.
• The IPPP’s Monte Carlo activities are a vital part of the IPPP
programme and provide an essential link to the experimental
community.
• We are involved in a wide range of event simulation physics
and projects using our simulation expertise for physics
studies.
• In the past few years most of the work has concentrated on
C++ and the development of the core physics modules.
• Work is now progressing on the physics developments and
studies.
• Our strength in this area will help us to support the U.K.
experimentalists during the LHC.
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