Workshop on Photon Physics and Simulation at Hadron Colliders

Bertrand Laforge (LPNHE - Paris) , Giovanni Marchiori (LPNHE - Paris) , Lydia Roos (LPNHE - Paris) , Marco Delmastro (CERN) , Matteo Cacciari (LPTHE - Paris) , Nicolas Chanon (ETH - Zurich)
A one-day workshop on prompt photon production at hadron colliders will be held at the beginning of 2012 in Paris, inside the campus of Université Pierre et Marie Curie.
The workshop will focus on the tools used to obtain the NLO theoretical predictions and the experimental problematics connected with the prompt photon cross section measurements.
In particular, the following items of discussions are envisaged:
  • survey of properties and characteristics of NLO prompt photon generators on the market
  • treatment of photon production (including fragmentation) and isolation in LO and NLO generators
  • experimental isolation prescription and connection with the theoretical predictions
The workshop is organised with the financial support of LPNHE and FRIF.
  • Bertrand Laforge
  • Bibhuti Parida
  • Costas Vellidis
  • Csaba Balazs
  • Daniel de Florian
  • Darko Mekterovic
  • David d'Enterria
  • Eric Pilon
  • Federico Ferri
  • Frank Siegert
  • François Arleo
  • Giovanni Marchiori
  • Gouranga Kole
  • Gudrun Heinrich
  • Heberth TORRES
  • Irena Nikolic
  • Jana Schaarschmidt
  • Jean-Philippe Guillet
  • Julie Malcles
  • Kadir Ocalan
  • Leandro Cieri
  • Leonardo Carminati
  • leveque jessica
  • Louis FAYARD
  • louis sgandurra
  • Lydia Roos
  • Marcello Fanti
  • Marco Delmastro
  • Marco Peruzzi
  • Marine Kuna
  • Mark Stockton
  • Massimiliano Grazzini
  • Matteo Cacciari
  • Mauro Donega
  • Mayuko KATAOKA
  • michel fontannaz
  • Mohamed Ouchrif
  • Nicolas Chanon
  • Paloma Quiroga Arias
  • Paolo Meridiani
  • Pavel Nadolsky
  • Philippe Gras
  • Philippe SCHWEMLING
  • roberta arcidiacono
  • Sandrine Laplace
  • Shigeru Odaka
  • Stephan LINN
  • Yoshimasa Kurihara
  • Yuji Enari
  • Zhao Li
- Introduction by M. Fontannaz.
- Overview of photon theory and results
        - Log(R) divergences: avoid cone size R<~0.2
        - Nominal scale choice: try to find saddle point (xsection stationary wrt scales). If not, stick to conventions (pT(gamma) or M(gamma gamma) for single/double photons). Vary scale by factor 1/2, 2.
- Last slide : work in progress, enhanced direct-fragmentation component at NLO in DIPHOX (enhance low delta phi xsection by ~1.6)? (very preliminary)
- Tevatron results :
- New photon + b/c results CDF / D0 with ~full Tevatron dataset (arXiv:1203.5865). More results on full dataset by the summer. 
           - Pythia and NLO differ from data by up factor ~2
           - Sherpa in good agreement with data
           - Pythia with increased gluon splitting to heavy quarks agrees better with data
- Update on diphoton production by CDF with 9.5fb-1. Comparison with many generators : Pythia, Resbos, Diphox, MCFM, Sherpa
           - good agreement NLO/data for Mgg>30 GeV
           - resummation important for dphi>2.2. pT>20 GeV
           - Sherpa better than PYTHIA and MCFM, though still a bit low at very low dphi
        - Comparison of previous diphoton CDF analysis (5.4 fb-1) and 2gNNLO => very good (impressive) agreement in almost the whole kinematical range
- ATLAS results :
- Inclusive photon and diphoton shown with 37 pb-1.
- New result on gamma+jet with 37b-1 was presented. Data is still lower than Jetphox at low ptgamma. Comparison with Jetphox, Pythia born, Pythia born+frag, Sherpa. Some discrepancies (data/Jetphox) for very forward jets, although not significant. Best agreement with full sim is provided by Sherpa. 
- CMS results :
- Inclusive and diphoton presented with 36pb-1.
- Some studies on the differential cross section for different isolation criteria by Philippe : up to 15% difference in some kinematical regions when changing parton-level isolation cut by +- 2GeV .
- Gluon pdf :
        - isolated photon measurements from colliders: 35 measurements (~400 data points) at LHC/TeV/SppS/RHIC
- Reduction of the gluon pdf uncertainty by 20% by inclusion of photon measurements (and thus the Higgs gluon fusion cross-section uncertainty). Inclusion of photon data in global fit would be useful.
        - More constraining data on gluon pdf: LHC
        - Chi2~1 for LHC data => experimental uncertainties well estimated
        - Comment by Fontannaz : it could be possible to find the best theory scales 
trying to fit the scale to all these data.
- Generators :
- Jetphox/Diphox presentation. Both can now output ROOT ntuples and provide PDF error band in one run. Future version of Diphox to contain improved calculation of direct-fragmentation component mentioned by Michel
- Resbos : more flexible than in the past (different pdf available, different isolation criteria). Should be easier now to get grids for PDF uncertainties. Xsection matched to NLO at high pT => ~10% uncertainty?
- Sherpa : advocating for measurements with the democratic approach (photons inside jets)
- Grappa : working on a NLO+PS with consistent photon fragmentation. Box is not included yet. Needs further development
- 2gNNLO : NNLO parton-level calculation. Uses MSTW2008 NNLO PDFs. Remarkable agreement with Tevatron and LHC results. Still using smooth Frixione isolation. Showing that isolation criterion does not matter much for the total cross-section if the cut is tight enough. Not yet available for user (work in progress). Plans also to include the fixed cones and fragmentation function in (later) future.
- Issues in matching theoretical predictions with data in photon measurements :
- discussion about PU subtraction with Fastjet : OK as long as correcting data with data and observable is clearly defined (also at truth-particle level)
- discussion about the difficulty of matching isolation prescription at parton-level and detector level => UE correction : still keep the same methodology (ratio between PS MC with/without UE+hadronization. Use different UE tunes). ATLAS: apply fast-jet technique to remove/reduce UE both at experimental level (reconstructed soft jets) and truth-particle level (truth-particle jets)
- Recent progress from Les Houches :
- Testing Frixione isolation at NLO and in Pythia to kill fragmentation contribution
- Able to kill fragmentation contribution at NLO, but don't know the direct photon efficiency (too tight cuts => total cross-section not reliable)
- Difficult to kill fragmentation in parton-shower with high signal efficiency. Is it due to UE/hadronization? Definition of fragmentation in Pythia?
- Possible new measurements : 
  - non-isolated xsections, differential cross section versus isolation cut => sensitive to fragmentation function
- photon + b/c at LHC => intrinsic b/c; b/c PDF
- Ratios 8/7 TeV ? would be not interesting for gluon pdf but would reduce the scale uncertainties
- democratic approach jet vs photon
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