Collider Event Generation

​NumS-HEP 2011 school, NCTU, Taiwan, 17-23 Jan 2011

Authors

• Johan Alwall (lecturer)

Lectures

Find the pdf of the 3 x 1.5 hours lectures:

• NumS-HEP_lectures.pdf​

References

• ​QCD and Collider Physics by Keith Ellis, James Stirling, Bryan Webber (Cambridge Monographs, 1996).
• Perspectives on LHC physics, Ed. by G. Kane and A. Pierce.
• ​Pythia 6 Physics and Manual (excellent introduction to event generation, parton showers and hadronization).

Basic exercises 1: Familiarize with MadGraph

• Logon to the MadGraph web site and register: ​http://madgraph.hep.uiuc.edu.
• Register with a valid e-mail address
• Feynman diagrams. Generate a few processes (with different QED + QCD couplings) trying to guess which diagrams appear:
  • uu~>tt~
  • gg>tt~
  • gg>tt~h (in SM and HEFT)
  • uu~>tt~bb~
• Subprocess identification. List all subprocesses contributing to:
  • pp>l+vljj
  • pp>tt~bb~
  • pp>tt~jj
• Look at the new physics models and check the particle and interactions content.
• Advanced: Download the MG/ME package and play with the standalone version.

Basic exercises 2: Calculate cross sections and generate events

• Generate events for a few selected processes (work in groups, pick one each), for Tevatron and LHC energies. Does the cross section scale as you expect?
• Compare cross sections with the processes of your neighbors. Do they differ as you would expect?
• Look at the generated plots. Are the distributions as you would expect? Discuss with your neighbors.

Example processes:

• ttbar production: pp > tt~, QED=0
• W production: pp > l+vl
• Z/gamma* production: pp > e+e-
• g g > (H > e+ ve mu- vm~) (in HEFT model)
• p p > (W+>e+ ve) (W- > mu- vm~) (important background to previous process)

Advanced exercises 1: The full simulation chain and advanced features

Try out the Analysis Tools:

• a Fortran-based package: MadAnalysis (​Download it here)
• a Root-based package: ExRootAnalysis (​Download it here)
• a Mathematica-based package: ​Chameleon

Study SM Higgs production at the LHC. Choose a channel and investigate signal and background, using either of the packages above:

1. The 2 lepton + missing Et final state: $ pp \to H\to W^{+ W}- \to e^{- \bar \nu_e \mu}+ \nu_\mu $
2. • Signal

Events: Parton Level​

Detector Level​

Plots: Parton Level​

Detector Level​

• Background

Events: Parton Level​

Detector Level​

Plots: Parton Level​

Detector Level​

1. Top associated production $pp \to t\bar tH$ with $H \to b \bar b$
   • Signal + Background samples:

Events: attachment:ttbb.lhe.gz​ Parton Level]

Detector Level​

Plots: Parton Level​

Detector Level​

Advanced exercises 2: Implementation of a new model in MadGraph or ​FeynRules + MadGraph

Note that for these exercises you need to download the ​MadGraph/MadEvent package (or ​MadGraph 5 package) and work locally. You need a Fortran compiler (and for MadGraph 5, Python v. 2.6 or 2.7).

For FeynRules download and information, see the ​FeynRules wiki

• Implement a (part of a) Standard Model extension into MadGraph, specific for studying a given process. Generate events, study relevant distributions.
• Determine most relevant Standard Model backgrounds with identical final states (“non-reducible”). If not too complicated, generate backgrounds with cuts determined from signal distributions.
• Which other (“reducible”) backgrounds might be important?
• Can the signal be seen at Tevatron? At the LHC?

Animations

• ​Interactive Flash version Note you may want to zoom in!
• ​Fast movie (.avi) of collision
• ​Guided movie (.mov) of collision
• ​John Ellis on Daily Show

-- Main.JohanAlwall - 2011-01-17