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Exploring Anomalous Quartic Couplings in Wγγ Production at LHC

A study at Bell Université de Genève focusing on Wγγ production at the LHC, sensitive to quartic couplings. Initial data analysis done in the electron channel, findings shared on AtlasProtected Wiki. Plans for further analysis in muon channel and exploration of NLO effects using existing LO generators.

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Exploring Anomalous Quartic Couplings in Wγγ Production at LHC

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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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