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POWHEG Adaptation of the Baur Zγ Generator. Lindsey Gray University of Wisconsin at Madison Weekly Meeting 3 June, 2009. Action Items. Show plots of pythia based CMSSW analysis, scaled by appropriate factors. First results of multivariate analysis

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Powheg adaptation of the baur z generator

POWHEG Adaptation of the Baur Zγ Generator

Lindsey Gray

University of Wisconsin at Madison

Weekly Meeting

3 June, 2009

Action items
Action Items

  • Show plots of pythia based CMSSW analysis, scaled by appropriate factors.

    • First results of multivariate analysis

  • Resurrect and show plots from RCT Calibration Routines

  • Status of POWHEG adaptation of Baur’s MC Generator

    • Draft for Multi-boson group talk in 2 weeks!

Lindsey Gray, UW Madison

Getting back to z analysis
Getting Back to Zγ Analysis

  • Original analysis had a flaw in normalization

    • Calculate event yield was actually too small!

  • To show:

    • Results of using production Zγ sample (Pythia 6)

      • Use same set of fiducial, photon id and kinematics cuts, but with proper normalization

    • Initial results from using Mike’s multivariate photon id

Lindsey Gray, UW Madison

Summary of signal background
Summary of Signal & Background




Lindsey Gray, UW Madison

Output of tmva based photon id
Output of TMVA-based Photon ID

  • Fisher discriminant works properly

    • Good separation between signal and background

  • Small cross section of Zγ signal implies tight cut on Fisher output

    • Fisher > .2

Lindsey Gray, UW Madison

Draft of multi boson talk

Draft of Multi-boson talk

2 weeks from now


  • Overview of POWHEG

    • Motivation & Definition

  • Current Results of Adapting Baur’s MC

    • Development Strategy & Status

    • Born Level Results & Comparison to Baur’s MC

      • Including Anomalous Couplings

  • Conclusions & Next Steps

Lindsey Gray, UW Madison

Shower mcs nlo generators
Shower MCs & NLO Generators

Shower Monte Carlo

NLO MC Generator

Take 2 -> n process

Determine virtual and collinear corrections


Calculate full 2 -> n+1 cross section


Sum of all contributions is finite!

Accurate normalizations but bad shapes at low pT

  • Take 2 -> n process and determine probability for no QCD radiation

    • Sudakov form factor

    • Collinear factorization

  • Reconstruct n+1 body process in collinear limit

  • Accept or reject radiation

    • Iterate until emission energy is below some threshold

  • Normalizations wrong, bad shapes at high pT

Lindsey Gray, UW Madison

Combining shower mc with nlo
Combining Shower MC with NLO

  • It is natural to want to combine NLO calculations with Shower MCs (SMC)!

    • However there are issues:

      • Double counting

        • Simply interfacing an NLO generator directly to a SMC will cause the SMC to generate events the NLO generator has already produced!

      • Matching NLO matrix elements to Shower MC emissions

        • Make sure QCD emission has correct probability in hard, soft and collinear regimes!

  • Many solutions exist:

    • [email protected], POWHEG, CKKW, MLM

Lindsey Gray, UW Madison

The powheg method
The POWHEG Method

  • POsitive Weight Hardest Event Generator

    • One of the many solutions to interfacing a NLO event generator with a shower monte carlo.

    • Independent of the Shower Monte Carlo used for hadronization

      • Hence independent of CMSSW

      • Do not allow SMC to generate radiation harder than the radiation from the POWHEG event.

    • As the name says, it generates events with all positive weights.

      • Produce unweighted events through well established methods.

Lindsey Gray, UW Madison

Applying powheg to baur s mc
Applying POWHEG to Baur’s MC

  • To implement any process using the POWHEG method you need:

    • The Born level phase space

    • A list of Born and Real subprocesses

    • Born squared amplitude

    • Real squared amplitude

    • Finite part of the virtual amplitude contribution

  • All of these are available in Baur’s Zγ cross section calculation (though not explicitly labeled!)

    • Plan to extract and adapt necessary pieces from Baur’s code into existing POWHEG W/Z production code

Lindsey Gray, UW Madison

What s in baur s mc
What’s in Baur’s MC

  • Baur’s MC really consists of three MCs where the results of each are summed

    • 1 MC for pp -> l+l-γ process to generate Born, finite virtual and collinear pieces

      • Soft/Collinear IR divergence is regulated by cutoffs

    • 1 MC for pp -> l+l-γ+jet process

      • “Real” process, regulated by the same cutoffs

    • 1 MC for pp -> l+l-jet+γ

      • Contribution from Z+jet events where the jet brems

      • Can be safely ignored if ΔRγ,jet cut is applied

        • 1% Effect for ΔR > .7 confirmed with Dr. Baur

Lindsey Gray, UW Madison

What s different in powheg
What’s Different in POWHEG

  • Baur’s cutoff regulator is replaced with Catani-Seymour or FKS regulator

    • Physical observables must independent of the regulator chosen

    • Collinear and Real contributions will change, total and differential cross sections should be unaffected

      • Collinear contribution ~ to Born level result

        • Just “paste” Born level Zγ result into POWHEG’s collinear contribution calculation

  • Baur’s code uses narrow width approximation, POWHEG code has finite Z width

Lindsey Gray, UW Madison

Current status of development
Current Status of Development

  • All necessary portions of Baur’s NLO calculation have been extracted

  • Testing is in progress:

    • Born matrix element result is fully tested

      • 2.5% normalization difference relative to Baur LO

      • Shapes match

      • Results to be shown

    • Collinear & Virtual corrections implemented

      • Currently debugging & testing

    • Real matrix element result is implemented

      • Currently debugging & testing

Lindsey Gray, UW Madison

Shape comparison diboson mass lepton p t
Shape Comparison: Diboson Mass & Lepton PT

  • All plots are area normalized.

    • 2.5% normalization difference

  • All Born level shapes agree!

    • With the exception of the diboson mass.

    • Known effect since Baur’s code uses narrow width approx. and POWHEG code has finite Z width.

    • Using narrow width approx. in POWHEG code doesn’t fix 2.5% normalization difference.



Lindsey Gray, UW Madison

Shape comparison photon p t lepton photon
Shape Comparison:Photon PT, Lepton η & Photon η



Lindsey Gray, UW Madison

Shape comparison photon p t anomalous coupling
Shape Comparison:Photon PT + Anomalous Coupling

  • ACs are automatically included in the routines from Baur’s code.

  • Scaling is the same between Baur MC and POWHEG.

Lindsey Gray, UW Madison

Conclusions and next steps
Conclusions and Next Steps

  • All of the necessary components for NLO Zγ cross section calculation are in place in POWHEG

    • What remains now is extensive testing, debugging and comparison to Baur’s NLO code

    • Determine source of 2.5% normalization difference

  • Once NLO calculation is fully ported and tested

    • Test event generation

      • Compare unweighted event kinematics distributions to NLO differential cross sections

      • Run trials interfacing POWHEG lhe-event file to Pythia, Herwig, et al. and cross check results.

Lindsey Gray, UW Madison