Transcript MC Generation for your own Model: FeynRule

MC GENERATION FOR YOUR
OWN MODEL: FEYNRULE
INTRODUCTION
Simon Lin (Shan Lin)
UW Spring Final Report
Jun. 16th, 2014
OUTLINE

Introduction

About FeynRules

A simple example
Installation
 Writing model files
 Generate Feynman rules and UFO output


Example model files are available after the talk
INTRODUCTION
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The natural language of high energy physics is
Quantum Field Theory (QFT)
From Lagrangian one derives a series of
Feynman rules, and used it to calculate the
scattering result.
Since this process is totally unambiguous, many
computing tools are developed to handle such
process to obtain the events.
FLOW SHEET
Lagrangian
FeynRules
MadGraph
Pythia

FeynRules – Feynman rules calculator
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MadGraph – matrix element generator
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Pythia – refragmentization/ hadronization
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Delphes – fast dector simulation
Delphes
Events
ABOUT FEYNRULES
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Mathematica-based packge
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Many pre-written models (e.g. SM, HEFT, 2HDM)
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Specify Lagrangian from assigning fields of
various spins/charges/masses/etc…
Calculate Feynman rules for given Lagrangian
In our case, generate UFO(Universal FeynRules
Output) model files for MG5.
INSTALLATION
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Latest version of FeynRules can be downloaded
at http://feynrules.irmp.ucl.ac.be.
There’s no need for compilation, just untar it
under your preferred directory.
We’ll need SM model file for later example. Find
it at feynrules-current/Models/SM/
A SIMPLE EXAMPLE
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Start from standard model, I’m going to add a
massive scalar particle X and let it couple to
quark pairs (uu~, dd~, tt~, etc.)
We have two external variables:
Mass of X: mx
 Coupling strength to quarks: gx


Let’s create a new model file X.fr in a new folder
(to keep a clean copy of SM model files), and copy
the SM files to that folder.
ADD MODEL DESCRIPTION

First, let’s add model description.
M$ModelName = "FeynRules Demostration UW Final Report 2014";
M$Information = {Authors -> {"Simon Lin"},
Version -> "0.1",
Date -> "6. 16. 2014",
Institutions -> {"University of Washington"},
Emails -> {"[email protected]"}
};
IMPLEMENT PARAMETERS

The declaration of the parameters must lie in
M$Parameters{}
M$Parameters = {
mx == {
ParameterType -> External,
Value -> 200,
Description -> "Mass of X"
},
gx == {
ParameterType -> External,
Value -> 1e-3,
InteractionOrder -> (NP, 1),
Description -> "X-qq~ Coupling strength"
}
}
IMPLEMENT FIELD
We need a scalar field X to describe our new
particle.
 FeynRules use S[ ] to label scalar fields, other
labels like F[ ] (fermions), V[ ] (vector bosons)are
also available.

S[4] == {
ClassName
-> X,
SelfConjugate -> True,
Mass -> {mx, 200},
ParticleName -> "X",
FullName
-> "X"
}
INPLEMENT LAGRANGIANS
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The new Lagrangian consists of two parts:
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The free field Lagrangian(describe a free particle):
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The interaction Lagrangian(describe a vertex):
(This Lagrangian describes a constant vertex factor gx)
INPLEMENT LAGRANGIANS (2)
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Putting these altogether:
Lnew := 1/2 del[X, mu] del[X, mu] - 1/2 mx^2 X^2 + gx X uq uqbar + gx X dq dqbar;
Ltotal := LSM + Lnew;
LOAD MODEL FILES IN FEYNRULES
REFERENCES
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From Lagrangians to Events: Computer Tutorial
at the MC4BSM-2012 Workshop
(arXiv:1209.0297v1)
FeynRules 2.0 - A complete toolbox for tree-level
phenomenology (arXiv:1310.1921)