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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