# Presentation: 009124

## HERWIG and PYTHIA

- Peter Richardson
- IPPP, Durham University

**Durham University**

## Summary

- Introduction
- Basics of Monte Carlo Simulations
- Processes inside the Generators
- The Les Houches Accord
- Future

## Introduction

- Monte Carlo event generators are programs which starting with some fundamental process predict the stable particles which will interact with a detector.
- There are a number of Monte Carlo event generators in common use
- PYTHIA
- HERWIG
- SHERPA
- They all split the event generation up into the same pieces.
- The models and approximations they use for the different pieces are of course different.

## A Monte Carlo Event

- Initial and Final State parton showers resum the large QCD logs.

- Hard Perturbative scattering:
- Usually calculated at leading order in QCD, electroweak theory or some BSM model.

- Perturbative Decays calculated in QCD, EW or some BSM theory.

- Multiple perturbative scattering.

- Non-perturbative modelling of the hadronization process.

- Modelling of the soft underlying event

- Finally the unstable hadrons are decayed.

## Monte Carlo Event Generators

- For BSM physics the main pieces of the event generators are
- Hard Process
- New intermediate particles
- New particles produced
- Changes to SM distributions
- Decays
- Decays of new particles produced in the hard process or previous decays.

## Built In Models

- Traditionally models of new physics are built into the event generator.
- This will often include hard processes and decays.
- Relatively few models have been implemented and the sophistication of the simulation varies.

## Built In Models

|  | HERWIG | PYTHIA |
| --- | --- | --- |
| SUSY |  |  |
| SUSY+RPV |  |  |
| RS Gravitons |  |  |
| Z’/W’  |  |  |
| Technicolor |  |  |
| Left-Right Models |  |  |
| Compositeness |  |  |
| Excited fermions |  |  |
| Leptoquarks |  |  |
| Fourth generation |  |  |

## Built In Models

- In general a lot more effort has gone into the simulation of SUSY than everything else put together.
- In general the simulation of SUSY is very sophisticated including simulation of the hard process, matrix elements for the decays and (in HERWIG) spin correlations between the production and decay.

## Spin Correlations

## Spin Correlations

## Les Houches Accord

- In 2001 there was a paradigm change in the way new processes were implemented in event generators.
- The old procedure
- Email author of program to ask for a new process to be included
- Author thinks about it
- Maybe gets implemented.

## Les Houches Accord

- Had many problems
- Only authors of the programs could add new processes.
- Large demand on a small number of people.
- Often processes people wanted (needed?) did not get implemented.

## Les Houches Accord

- At the 2001 Les Houches meeting an agreement was reached on a method of passing information between programs generating partonic processes and general purpose event generators.
- Based on a common block.

## Les Houches Accord

- Basic idea was that the Matrix Element Generator (MEG) provides
- Momenta and types of the particles.
- A option for the cross section calculation.
- The colour flow for the process.
- Optionally information on intermediate particles which need special treatment.

## Les Houches Accord

- Most of the rest is fairly self explanatory but colour flow isn’t.
- If we take the large number of colours limit we can assign a colour flow.
- Needed by the event generator to get the simulation of QCD radiation and hadronization right.

## Les Houches Accord

- New paradigm
- If asked to implement a new process a generator author will say do it yourself using the Les Houches accord.
- We have provided a simple tutorial to help with this
- _[http://www.ippp.dur.ac.uk/montecarlo/leshouches/tutorial/](http://www.ippp.dur.ac.uk/montecarlo/leshouches/tutorial/)_

## Les Houches Accord

- Implemented in both HERWIG and PYTHIA
- Some problems
- Particles involved need to be known to the generator.
- HERWIG not good at handling coloured non-SM particles.
- But has proven very successful.

## Les Houches Accord

- Many of the matrix element generators use it
- MADGRAPH
- ALPGEN
- CALCHEP
- Together with some BSM packages (e.g. CHARYBDIS)
- Also used by many private programs to pass information into the general purpose event generators.

## Future

- It may be a good idea to have a simple way of tell generators about new particles and their decays, perhaps based on what already exists for SUSY.
- In general this will be good enough for most cases.
- However there are some situations where sophisticated simulation is needed.

## Future

- It order to decide whether the spin of SUSY particles can be measured need spin correlations.
- In this case the event generator has to perform all the simulation (otherwise it’s almost impossible to get it right.)
- This was done for the MSSM in HERWIG.
- However it was a lot of work and even simple extensions (NMSSM etc.) can’t be handled.

## Herwig++

- In designing the new Herwig++ generator we decide to go with a different approach.
- A C++ helicity library based on the HELAS formalism is used for all matrix element and decay calculations.
- This makes coding new matrix elements for both production and decay much easier.
- Also easy to do the spin correlations as we have access to the spin unaveraged matrix elements.

## Herwig++

- In principle we could write a matrix element generator using this as the basis.
- However a lot of work and there are all ready many good programs available.
- Settle for a more limited aim.
- Code the hard            matrix elements based on the spin structures (there’s less possibilities than you might think).
- Code the          and           decays in the same way.

## Herwig++

- For most models then only need to supply the Vertices , i.e. the Feynman rules for a new model.
- C++ inheritance should help even there with say the NMSSM inheriting from the MSSM and only having to implement the new features.
- May be supplemented with special treatment for some models.

## Conclusions

- The existing HERWIG and PYTHIA programs will remain the workhorses of event simulation in the near future.
- Unlikely to be any new models implemented in them directly.
- New processes should use the Les Houches.
- The simulation in the new C++ generators will be different and hopefully allow more models to be studied.