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12 June 2007 Warning using ATLAS Fast Simulation Julien MOREL CSC note – diphoton dilepton LPSC - Grenoble Julien MOREL Warning using ATLFast 1 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 AtlFast v12.0.4 FullSim v12.0.31 M Z ' (GeV ) M Z ' (GeV ) 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. LPSC - Grenoble Julien MOREL Warning using ATLFast 2 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 compute efficiency with a good statistic, 60100 « non physical » events of Z’ e+e- at diffents masses : Dilepton invariante mass spectrum Mll > cut (TeV) Z’ Mass (TeV) # event 2 3 100 3 4 10k 4 5 10k 6 7 10k 7 8 10k 8 9 10k 9 10 10k Electron energy sectrum Mll(GeV) LPSC - Grenoble Julien MOREL Warning using ATLFast Eele (GeV) 3 Results obtained using the ATLFast default isolation cuts Z’ Selection efficiency 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) LPSC - Grenoble Julien MOREL Warning using ATLFast 4 Waiting for results without 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%. LPSC - Grenoble Julien MOREL Warning using ATLFast 5 Results obtained using 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’ Selection efficiency hele Mll (GeV) The behavior versus Mll and h become more reasonable LPSC - Grenoble Julien MOREL Warning using ATLFast 6 Results obtained using ATLFast without energy smearing I run ATLFast without smearing the energy, and obtain the same result LPSC - Grenoble Julien MOREL Warning using ATLFast 7