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Warning using ATLAS Fast Simulation

Warning using ATLAS Fast Simulation. 12 June 2007. Julien MOREL CSC note – diphoton dilepton. FullSim / AtlFast comparison for the Z’ selection efficiency.

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Warning using ATLAS Fast Simulation

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  1. Warning using ATLAS Fast Simulation 12 June 2007 Julien MOREL CSC note – diphoton dilepton

  2. FullSim / AtlFast comparison for the Z’ selection efficiency For an exotic study, we need fast simulation for hight mass Z’e+e- and we find a surprising low Z’ reconstruction efficiency with AtlFast FullSim v12.0.31 AtlFast v12.0.4 We find out that this low efficiency is due to the cell isolation cut in ATLFast but the paper is not so clear : “ In order to define isolated electrons, the following criteria are applied : the difference in the energy in a cone of DR=0.2 around the electron direction and the smeared electron energy needs to be below 10 GeV. In addition, there should be no further clusters reconstructed within a cone of DR=0.4 around the electron direction. ” [ATL-COM-Soft-2006-020, page 6] “ If the sum of the transverse energie of all cells within DR<0.2 around the true particle exceeds the energy of the true particle by more than 10 GeV the electron is treated as non-isolated. ” [Same note, page 10] No isolated electrons are not present in the ATLFast_Electron_container.

  3. ATLFast version and event use for inverstigation • I use : • ATLAS Fast Simulation V12.0.4 • Official JobOption : DC3.005605.Zprime_ee_pythia_chi1000.py • In order to computeefficiencywith a good statistic, 60100 «  non physical» events of Z’ e+e- atdiffents masses : Dilepton invariante mass spectrum Electron energy sectrum Eele (GeV) Mll(GeV)

  4. Resultsobtainedusing the ATLFast default isolation cuts Z’ Selectionefficiency hele Mll (GeV) • Why such an efficiency ? • As ATLFast only smears the energy of the true particles, does the radiated particles (FSR) make the « main » electron non isolated ? • Does ATLFast compare the cone energy with the truth energy instead of the smeared one? (0.7% of 1.5TeV electron is larger than 10 GeV)

  5. Waiting for resultswithout ISR / FSR Radiated electron/photon (which are reconstructed as the electron by the cone algorithm in the full simulation)may be considered as “other” particles in the FastSim. Theses radiations are closed to the true electron but are not reconstructed in the same cell (as they are not the same particle) so the cell is not isolated. This can explain the strange eta dependence as the large eta cell are smaller than the small eta ones. (the radiated electrons are not in the cluster DR<0.4 cone) hele I can’t manage to run the same Z’e+e- jobs without ISR/FSR but I try with single electrons without ISR/FSR and it seems that the efficiency is near 100%.

  6. Resultsobtainedusing an energy relative cut Obviously, a fixed cut at 10 GeV is not appropriate for hight pT electron. We try with a relative cut : 20% of the smeared energy of the electron. Z’ Selectionefficiency hele Mll (GeV) The behavior versus Mll and h become more reasonable

  7. ResultsobtainedusingATLFastwithoutenergysmearing I run ATLFast without smearing the energy, and obtain the same result

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