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

The [[http://arXiv.org/pdf/0709.2092][Powheg]] method is an alternative to [[http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/MCatNLO_man33.pdf][MC@NLO]] for interfacing Shower Monte Carlos (SMCs) with NLO QCD computations. The !PowhegBox is one program which makes use of this method and generates the hard processes of events, to be read into SMCs to be showered and hadronized. 

One advantage of using the Powheg method over !MC@NLO is that Powheg-style events can be read into _any_ SMC (Herwig++ and Pythia8 are considered here).

Herwig++ is also able to generate Powheg-style events, and correctly includes its [[http://xxx.lanl.gov/abs/0905.3072][truncated shower]].

---+ 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 steered; and it is easy to control which stages or turned on or off. For example, events can be produced with the parton shower and multiple parton interactions (MPI) turned on, but hadronization turned off.

Thus, we intended 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.
#TableOfLevels
| *Level* | *Description* |
| Matrix Element | Hard process only. No parton shower, hadronization etc |
| Parton level | Hard process + parton shower. No hadronization, MPI etc. |
| Hadronization Off | Hard process + parton shower + MPI. No hadronization. |
| MPI Off | Hard process + parton shower + hadronization. No MPI. |
| Complete Event | Everything on. (Truncated shower on and off) |

Powheg-style events generated and showered entirely in Herwig++ will be labelled *Herwig++(NLO)*. Events in which the hard process was generated by !PowhegBox and showering was done with Herwig++ will be labelled *PowhegBox+Herwig++*, and those showered with Pythia8 labelled *PowhegBox+Pythia8*.

---+ General Generator setup

---++ !PowhegBox
   * Version: SVN Revision 572 (http://powhegbox.mib.infn.it/)
   * PDF CT10
   * Input file: [[%ATTACHURL%/wp.e.7TeV-powheg.input][wp.e+.7TeV-powheg.input]]

---++ Herwig++(NLO)
   * Version: Herwig++ 2.5.0 (http://projects.hepforge.org/herwig/versions)
---+++++!! PDFs
PDFs need to be set for the hard process and underlying event separately.
   * Hard process (as described in the Herwig++ manual):
   * <verbatim>
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
</verbatim>
   * Underlying event (not in the manual)
   * <verbatim>
cd /Herwig/Partons
cp QCDExtractor MPIExtractor
set MPIExtractor:FirstPDF MRST
set MPIExtractor:SecondPDF MRST
cd /Herwig/UnderlyingEvent
set FastQCD:PartonExtractor /Herwig/Partons/MPIExtractor
</verbatim>
---+++++!! 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:
   * <verbatim>
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
</verbatim>
   * 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:
   * <verbatim>
pythia.readString("PDF:useHard = on");
pythia.readString("PDF:useHardLHAPDF = on");
pythia.readString("PDF:hardLHAPDFset = CT10.LHgrid");
</verbatim>
   * Underlying event
   * <verbatim>
pythia.readString("PDF:pSet = 4");
</verbatim>
---+++++!! Tune
<verbatim>
Tune:pp = 4
</verbatim>
See http://home.thep.lu.se/~torbjorn/php8150/Tunes.php

---+RIVET analysis
Description and code.

---+ Validation
---++ Matrix element level
---+++!! Level-specific setup:
---++++!! Herwig++(NLO)
   * Complete input file: [[%ATTACHURL%/HerwigNLO_Matrix-Element.in.txt][Herwig++(NLO)_Matrix-element.in]]
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:
<verbatim>
cd /Herwig/Shower
set Evolver:HardEmissionMode POWHEG
set Evolver:HardOnly Yes 
</verbatim>
Hadronization, MPI and final-state Decays can then be turned off as follows:
<verbatim>
set /Herwig/EventHandlers/LHCHandler:HadronizationHandler NULL
set /Herwig/EventHandlers/LHCHandler:DecayHandler NULL
set /Herwig/Shower/ShowerHandler:MPIHandler NULL
</verbatim>
---+++++!! Plots
   * [[MatixElement][All matrix element level plots]]

| %IMAGE{"me_W_ANALYSIS_leadingJetPtXs.png" type="thumb" href="%ATTACHURL%/me_W_ANALYSIS_leadingJetPtXs.pdf" caption="Leading jet pT"}% | %IMAGE{"me_W_ANALYSIS_WPtXs.png" type="thumb" href="%ATTACHURL%/me_W_ANALYSIS_WPtXs.pdf" caption="W pT"}% | %IMAGE{"me_W_ANALYSIS_WMassXs.png" type="thumb" href="%ATTACHURL%/me_W_ANALYSIS_WMassXs.pdf" caption="W invariant mass"}% |

---+++++!! Notes
As is evident, even at this early stage there is a significant difference between all of the Herwig++(NLO) and !PowhegBox+Herwig++ distributions shown above. This was quite unexpected; ideally the two sets of Powheg events coming out of Herwig++ would be more-or-less identical.

The W invariant mass was taken as the invariant mass of the e+ & %$\nu_{e}$% in the final state, and the discrepancy between Herwig++(NLO) and !PowhegBox+Herwig++ turned out to be because of different default behaviours when including (or not including) final-state QED radiation.

By default, the Herwig++(NLO) events _had_ final state QED radiation, whereas the !PowhegBox+Herwig++ events _did not_. A simple way to confirm that this was the case was to explicitly turn off the generation of the QED radiation in the Herwig++(NLO) case, by adding the following line to the input file:
<verbatim>
set /Herwig/QEDRadiation/QEDRadiationHandler:DecayingParticles 2 NULL
</verbatim>
(the particle at position 2 of the decaying particles list is the W+)
The invariant mass distribution then becomes:
%IMAGE{"me_W_ANALYSIS_WMassXs_QEDOff.pdf" type="thumb" href="%ATTACHURL%/me_W_ANALYSIS_WMassXs_QEDOff.pdf" caption="W invariant mass, no QED radiation"}%

---++ Parton level
---++ Hadronization Only
---++ MPI Only
---++ Complete Event
---+++ Truncated Shower 



-- Main.KiranJoshi - 31-May-2011
   * [[%ATTACHURL%/wp.e.7TeV-powheg.input][wp.e.7TeV-powheg.input]]: powheg input file for W+ -> e+ nu_e
Topic revision: r6 - 2011-06-03 - KiranJoshi
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