PowhegBox Validation

THIS PAGE IS A WORK IN PROGRESS AND WILL BE UPDATED WITH ALL SETTINGS, INPUT FILES AND PLOTS USED/PRODUCED IN THE STUDY

Introduction

The aim of this work was to validate events produced by the PowhegBox by comparing them to Powheg-style events produced by Herwig++.

Validation Approach

The benefit of using Shower Monte Carlo programs written in C++ is that every stage of the event generation procedure can, in principle, be precisely controlled, and it is easy to control which stages or turned on or off. For example, events can be produce with the parton shower and multiple parton interactions (MPI) turned on, but Hadronization turned off.

Thus, it was my goal to generate events one 'level' at a time (see table below) and attempt to understand and fix any differences seen between the sets of events, before moving on to the next level.

Level DescriptionSorted ascending
Complete Event Everything on. (Truncated shower on and off)
MPI Off Hard process + parton shower + hadronization. No MPI.
Hadronization Off Hard process + parton shower + MPI. No hadronization.
Parton level Hard process + parton shower. No hadronization, MPI etc.
Matrix Element Hard process only. No parton shower, hadronization etc

General Generator setup

PowhegBox

Herwig++(NLO)

PDFs
PDFs need to be set for the hard process and underlying event separately.
  • Hard process (as described in the Herwig++ manual):
  • cd /Herwig/Partons
    create ThePEG::LHAPDF myPDFset ThePEGLHAPDF.so
    set myPDFset:PDFName CT10.LHgrid
    set myPDFset:RemnantHandler HadronRemnants
    set /Herwig/Particles/p+:PDF myPDFset
    set /Herwig/Particles/pbar-:PDF myPDFset
    
  • Underlying event (not in the manual)
  • cd /Herwig/Partons
    cp QCDExtractor MPIExtractor
    set MPIExtractor:FirstPDF MRST
    set MPIExtractor:SecondPDF MRST
    cd /Herwig/UnderlyingEvent
    set FastQCD:PartonExtractor /Herwig/Partons/MPIExtractor
    
Tune
Tune parameters taken from http://projects.hepforge.org/herwig/trac/wiki/MB_UE_tunes

PowhegBox + Herwig++

PDFs
  • Hard process PDF should match what was used to generate the events in PowhegBox:
  • set myReader:InitPDFs 0
    cd /Herwig/Partons
    create ThePEG::LHAPDF myPDFset ThePEGLHAPDF.so
    set myPDFset:PDFName CT10.LHgrid
    set myPDFset:RemnantHandler HadronRemnants
    set /Herwig/EventHandlers/myReader:PDFA myPDFset
    set /Herwig/EventHandlers/myReader:PDFB myPDFset
    
  • Underlying event automatically uses the correct (LO**) PDF, but it probably doesn't hurt to repeat the lines from above.
Tune
As above, see http://projects.hepforge.org/herwig/trac/wiki/MB_UE_tunes

PowhegBox + Pythia8

PDFs
To compare like with like, the same set of PDFs were used when showering the PowhegBox events with Pythia8.
  • Hard process:
  • pythia.readString("PDF:useHard = on");
    pythia.readString("PDF:useHardLHAPDF = on");
    pythia.readString("PDF:hardLHAPDFset = CT10.LHgrid");
    
  • Underlying event
  • pythia.readString("PDF:pSet = 4");
    
Tune
Tune:pp = 4
See http://home.thep.lu.se/~torbjorn/php8150/Tunes.php

Validation

Matrix element level

Level-specific setup:

Herwig++(NLO)

  • Complete input file: LINK
At matrix element level we are only interested in the hard process partons. The parton shower, hadronization, underlying event should be turned off. However, the Powheg emission in Herwig++(NLO) is produced as the first step of the parton shower. So we need to leave the parton shower turned on but use the HardOnly flag:
cd /Herwig/Shower
set Evolver:HardEmissionMode POWHEG
set Evolver:HardOnly Yes 
Hadronization, MPI and final-state Decays can then be turned off as follows:
set /Herwig/EventHandlers/LHCHandler:HadronizationHandler NULL
set /Herwig/EventHandlers/LHCHandler:DecayHandler NULL
set /Herwig/Shower/ShowerHandler:MPIHandler NULL

Parton level

Hadronization Only

MPI Only

Complete Event

Truncated Shower

-- KiranJoshi - 31-May-2011

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Topic revision: r4 - 2011-06-02 - KiranJoshi
 
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