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First look at W gg P. Bell Université de Genève Informal W g /Z g meeting 30th June 2011

First look at W gg P. Bell Université de Genève Informal W g /Z g meeting 30th June 2011. W + 2 photon production at LHC. Sensitive to Anomalous Quartic Coupling of form WW gg Generic (non-ATLAS) study in arXiv: 0907.5299 : Compared /cross-checked LO predictions of Baur and

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First look at W gg P. Bell Université de Genève Informal W g /Z g meeting 30th June 2011

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  1. First look at WggP. BellUniversité de GenèveInformal Wg/Zg meeting30th June 2011

  2. W + 2 photon production at LHC • Sensitive to Anomalous Quartic Coupling of form WWgg • Generic (non-ATLAS) study in arXiv:0907.5299: • Compared/cross-checked LO predictions of Baur and • Eboli/LiettiMC generators • - Assumed: • √s = 14 TeV • 25GeV min. charged lepton pT, • 15GeV min. photon pT • 25Gev min. MET • |eta|<2.5 for charged leptons and photons • DR = 0.4 between the three objects • Detector response (smearing and efficiency) modelled by “PGS” package • accepted signal cross section 8.6fb (electron and muon channels combined) • now is the time to start this analysis! • NB: More recent MC studies (arxiv.org/abs/1103.4613, arxiv.org/abs/1001.2688) show large NLO corrections (k-factor > 3, non-uniform)

  3. First look at data (electron channel) • Started to slim and look for events in EGAMMA and later SMENU D3PDs (to period H) • Cutflow not refined: essentially took the most certain of the Wg selection and asked for two photons: • Egamma GRL + e20medium trigger • At least one tight electron with cl_pT > 25GeV inside |eta| < 2.47, crack region • removed; no further medium electrons; el oq flag applied • At least two tight photons with cl_pT > 15GeV inside |eta| < 2.47, crack region • removed; ph oq flag applied • MET_RefFinal > 15GeV • deltaR between the three EM objects > 0.4 • Results appearing at https://twiki.cern.ch/twiki/bin/view/AtlasProtected/WGammaGamma • Found 12 candidate events in B-G2 – might be interesting to examine each • Cross-check of results would be good

  4. MC review • Existing datasets: • Based on MadGraph (SM couplings) e.g. from • MC10.118615.Pythia_MadGraph_Wminushadgammagamma.py • MC10.118616.Pythia_MadGraph_Wminuslepgammagamma.py • MC10.118617.Pythia_MadGraph_Wplushadgammagamma.py • MC10.118618.Pythia_MadGraph_Wpluslepgammagamma.py • Existing LO generators: • “W2Pho” (http://projects.hepforge.org/w2pho/) based on Lietti MC, cross-checked with • BaurMC, has AQGC vertex and Les Houchesoutput, could be used to generate official • samples • ALPGEN can do W + 1 or 2 photons • At NLO: • arxiv.org/abs/1103.4613 and • arxiv.org/abs/1001.2688 • => Both refer to private MC generators, have sent a request to obtain them. SM only.

  5. Summary • Expect ~9 Wgg events in 1fb-1 according to LO predictions + simple detector simulation • Started to look for candidate events in electron channel (found 12 in period B-G2) • need to refine and cross-check cut flow • need to start on muon channel • Which generator to use? • – For eventually measuring a cross-section and ultimately setting QGC limits • Need to understand NLO effect - can we make do with existing LO generators? • Ideally want NLO predictions with AQGC terms • Should demonstrate to the theorists we are serious • Not forgetting backgrounds – misidentified jets passing the photon selection expected to dominate – hope to use a “fake rate” method currently being developed for Z’ analysis. • Rough plans • could make an “observation” and first total cross-section measurement soon • better to use differential distributions (Mgg) for probing AQGCs

  6. Backup

  7. Nomenclature Two lowest dimension effectiveLagrangianterms that give rise to purely quartic couplings involving at least two photons are: Both terms generate Anomalous QGCs of the form WWgg and ZZgg The coupling parameters b0 and bc (units GeV-2) are distinguished for the W and Z vertices and studied separately NB: in LEP (and other) analyses the same couplings are expressed a0/L2 and ac/L2 All 4 parameters are zero in the SM The anomalous couplings scale with the square of the photon energies, so a substantial improvement in the sensitivity can be expected at the LHC over the results from LEP.

  8. Form factors Any deviation of the parameters aW,Z0,c/L2 from zero value will eventually violate unitarity so the cross section rise has to be regulated by a form factor (Not necessary in the LEP analyses) Usual convention: (i.e., for values of Mgg above LFF the AQGCs get suppressed) If we work in a unitarity safe region, meaning up to some limit of Mgg, no need to apply FF and so can compare limits on bare couplings

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