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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

Signal

Background

200pb-1

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

outline
Outline
  • 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

Divergent!

Calculate full 2 -> n+1 cross section

Divergent!

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:

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.

Baur

POWHEG

Lindsey Gray, UW Madison

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

Baur

POWHEG

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

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