Herwig

1
Herwig
S. Gieseke, D. Grellscheid, K. Hamilton, A.
Ribon, PR, P. Stephens, M.H. Seymour, B.R. Webber
M. Baehr, M. Gigg, S. Latunde-Dada, S. Plaetzer,
A. Sherstnev, J. Tully

• Peter Richardson
• IPPP, Durham University

2
Introduction

• Monte Carlo event generators are an essential
  part of most experimental analyses.
• The HERWIG program was highly successful during
  at LEP/HERA and the Tevatron.
• However our understanding of the physics involved
  has improved over the last 20 years.
• It has been possible to improve and extend the
  program a great deal but it has reached the end
  of its life.

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Introduction

• In order to include all the new theoretical ideas
  from the last 5-10 years for the LHC major
  changes were needed.
• The Herwig project was to write a new
  generator, using the same physics philosophy as
  HERWIG, but including new developments wherever
  possible.
• The initial plan was
• Recode the cluster hadronization model making
  minor improvements to fix problems related to the
  number of excited mesons included.
• Write a new angular-ordered parton shower with
  better
• treatment of mass effects
• Lorentz invariance properties.

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Shower Improvements

• Most of the recent progress in Monte Carlo
  simulations has been in better simulation of hard
  radiation.
• While there are a number of different ideas all
  of them rely on being able to understand what the
  shower does.
• To make these improvements we often need an
  analytic understanding on what the shower is
  doing.
• Also some of the approximations which we made in
  the past were shown to be poor.

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Dead-Cone

• For massive particles radiation with angle less
  than m/E is suppressed, the dead-cone.
• However in order to implement this we had to make
  an extreme approximation which leads to problems
  in physical distributions.

Soft radiation pattern from a top quark with 1
TeV energy.
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Shower Improvements

• In the FORTRAN program the shower implemented
  angular ordering using
• and the DGLAP splitting functions.

• The major change with the new algorithm were to
  generalise the evolution variable
• and use the quasi-collinear splitting functions
  of S. Catani et. al. Phys.Lett.B500149-160,2001
• Also changed the definition of z to give
  invariance under boosts along the jet direction.

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Shower Improvements

• The main aim was to allow evolution down to zero
  pT for radiation from massive particles and to
  avoid the dead-cone approximation used in the
  FORTRAN program.

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Status

• After the first release which only did ee-
  collisions we decided to make a number of further
  improvements
• The extension to hadron collisions
• Developments to make implementing Standard Model
  scattering processes and Beyond the Standard
  Model physics easier
• Improvements to the simulation of QED radiation
• Improvements to the hadron decays.

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pT of the Z compared with CDF data
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Top Shower

• In the FORTRAN algorithm the simulation of QCD
  radiation in the decay of heavy particles was
  performed in the rest frame of the decaying
  particle.
• Within the formalism the decaying particle did
  not radiate in this process.
• However if we consider tgbW the radiation from
  the bottom quark did not fill all the soft region.

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Top Shower and Matrix Element Correction in tgbWg

• In the new formalism there is radiation from the
  top quark in the decay ensuring that the soft
  region is filled.
• However the soft matrix element is required to
  give smooth coverage in the soft region

Keith Hamilton, Peter Richardson, hep-ph/0612236,
JHEP0702069,2007
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Simulation of QED Radiation

• In the FORTRAN there was no simulation of QED
  radiation. This is important for leptonic W and Z
  decay.
• Also in some hadronic decay processes.
• In the C we have a new simulation of QED
  radiation based on the YFS formalism.
• This is a formalism for simulating soft
  electromagnetic radiation which can be
  systematically improved by including higher-order
  corrections and collinear emission.

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Simulation of QED Radiation
K. Hamilton and PR hep-ph/0603034, JHEP 0607010,
2006.
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Status

• The current release 2.0, S. Gieseke et. al.
  hep-ph/0609306 includes
• Initial-State showers
• Top Decay Shower
• UA5 Soft Underlying event model
• QED Radiation
• Many important hadron-hadron matrix elements.
• This version can be used for hadron collider
  physics.

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Status

• After this version there are a number of features
  which we still need to include
• JIMMY multiple scattering model for the
  underlying event
• different kinematic reconstruction procedures for
  the shower
• BSM Physics
• new hadron decay model
• spin correlations throughout the simulation.
• In order for the simulation to be as good as, or
  better than, the FORTRAN for everything.

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Hard Processes and New Physics

• In the FORTRAN each hard process and decay matrix
  element was typed in by hand.
• Isnt a good use of time.
• Meant that models of new physics were very hard
  to include.
• In the C we have used an entirely different
  philosophy.
• A C helicity library based on the HELAS
  formalism is used for all matrix element and
  decay calculations.
• Code the hard 2g2 matrix elements based on the
  spin structures.
• Code the 1g2 decays in the same way and use phase
  space for the 1g3 decays to start with.
• Easy to include spin correlations as we have
  access to the spin unaveraged matrix elements.

M. Gigg and PR hep-ph/0703199.
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New Physics
M. Gigg and PR hep-ph/0703199.
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New Physics
Unpolarised
M. Gigg and PR hep-ph/0703199.
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Tau Decays
Left Handed stau
Right Handed stau
Fraction of visible energy carried by the charged
pion
M. Gigg and PR hep-ph/0703199.
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Tau Decays
H,Agtt-gpnp-n
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UED

• The idea was that with the new approach to
  simulating BSM models other than SUSY should be
  easier.
• The first test of that is the inclusion of the
  Universal Extra Dimensions (UED) model.
• This is a model where all the particle propagate
  in an extra dimension.
• Gives a similar spectrum of new particles to SUSY
  but the new particles have the same spin as their
  Standard Model partners.
• It is a useful straw-man model to decide if the
  spins of new particles can be measured

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UED
Look at the decay
e- near
e- far
e far
qL
e near
e far
J. Smillie, B. Webber JHEP 0510069,2005,
hep-ph/0507170
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Hadron Decays

• The simulation includes detailed modelling of
  many decays.

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Future Shower Improvements

• In addition to the other features one of the main
  reasons for going to C was to allow
  improvements to the shower algorithm.
• CKKW matrix element matching
• The multi-scale shower
• MC_at_NLO
• The Nason approach to MC_at_NLO

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Nason Approach to MC_at_NLO
hep-ph/0612281 Oluseyi Latunde-Dada, Stefan
Gieseke, Bryan Webber
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Summary

• Herwig is now ready for hadron collisions.
• Many further improvements are planned.
• There will be a new release in the summer which
  will include the new simulation of BSM physics,
  hadron decays and some shower improvements.
• In the near future the C simulation will
  replace the FORTRAN.