Generating multi-jet events with MadGraph

Yoshitaro Takaesu (KEK)

July 24, 2016 1

Overview: Simulation for LHC

July 24, 2016 2

Parton Shower Generator

Matrix Element Generator

Event Generator

Detector Simulator

Importance of multi-jet at the LHC

Multi-jet signature appears in many New Physics models.

Event Generator should be able to generate > 4 jets.

July 24, 2016 3

Status of ME Generators

Model Alpgen HELAC Sherpa MadGraph

SM 6 jets 10 jets ? 6,7 jets 4,5 jets

MSSM ✕ ？ 5 jets 4,5 jets

Others ✕ ✕ 5 jets 4,5 jets

There is no ME generator which can simulate New Physics with > 5 jets.

July 24, 2016 4

What is MadGraph?

5

Matrix-Element & Event Generator

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Input

Event generator (MadEvent)

Output

Matrix-Element Generator (“MadGraph”)

+ Fortran Code

Model + process

Events

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MG

MSSM, ADD,

2HDM,etc

Feyn-Rules

Higher Dim. Op.

Spin-3/2

Spin-2

NLO

Interface

PYTHIA

Detector Simulator

ROOTLagrangian

Limitation of MG

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The file size of codes for QCD processes

Cannot be Compiled

Structure of QCD Amp

July 24, 2016 8Color flow

Color Factorcolor-ordered amp.

i: Color flow

How to overcomethe Limitation

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STRATEGY

Divide

• Speed up Amp Evaluation• Off-shell Recursive Relations

• Re-organize Color Summation1/Nc expansion

Speed up Amp Evaluation

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Off-shell Recursive Relations

Lv.1

Lv.2

Lv.3

Time performance

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@ 1 helicity & Phase pace point

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Model dependent Model independent

Recursive Relation Feynman Diagram

Hybrid

Alpgen (SM) COMIX (SM)HELAC (SM, MSSM?)

MadGraph, AMEGIC, compHEP

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Recursive Relations for gluonic sub-amplitude

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@ 1 helicity & Phase pace point

Re-organize Color Sum

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1/Nc expansion

Polynomial of Nc:

Multi-jet event generation

• Generate events with Leading Color Approximation

• For each event, include higher order corrections into its weight.

July 24, 2016 19

Leading color Event Generation

Color-flow sampling

( ~ 40,000 for gg -> 7g )

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Too LARGE

Event is a set of momenta, helicities and a color flow

Sample Results

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Total cross sections

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(Linear scale) (Log scale)

Conclusion• We proposed a method to generate multi-jet

events with MadGraph.

– Implemented gluonic off-shell recursive relations and generate color-ordered amps.

– Generated LO events by sampling color flows

– Shown results for gluonic processes for LO event generation.

July 24, 2016 27

OutLook

• Generate events with quarks

• Include higher order corrections in 1/Nc exp

• Detailed study for many distributions of Multi-jet events with our method

• Apply the framework to New Physics Pheno

July 24, 2016 28