simulation of the h →zz (*) →4l channel in atlas

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A. Di SimoneA. Di Simone11, M. Moch, M. Moch22, A. Nisati, A. Nisati22, , D. RebuzziD. Rebuzzi33, S. Rosati, S. Rosati22,,

G. CorcellaG. Corcella44

Simulation of the HSimulation of the H→ZZ→ZZ(*)(*)→4l →4l Channel in ATLASChannel in ATLAS

Workshop sui MonteCarlo, la Fisica e le Simulazioni a LHC Frascati, 24 October 2006

1INFN-CNAF and CERN, 2INFN Roma, 3INFN Pavia and Pavia University, 4Roma University La Sapienza

Acknowledgements to G. Polesello, B. Mele and M. Grazzini

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1.Overview

2.Generators studies for H→ZZ(*)→4l

- Comparison Pythia-Herwig using the ATLAS defaults- Herwig-Pythia tuning- Matrix Element corrections to Herwig Parton Shower

3.Analysis of H→ZZ(*)→4l channel in the full ATLAS simulation (shortly)

- Selecting Higgs events- Analysis results (preliminary)- Trigger Aware Analysis

4.Summary

Outline

Overview Generator Studies H→ZZ(*)→4l Analysis Summary

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Event generators in ATLAS

Pythia, Herwig (+Jimmy), Hijing, Charbydis, TopRex, Tauola/Photos (both with Herwig and Pythia), Sherpa, Alpgen (+MLM matching), MC@NLO, AcerMC, MadEvent, CompHEP, Phojet, …


We try to use as many generators as reasonable

• MC Generators are incorporated in the common framework for the ATLAS offline software, athena, via interfaces

• Pythia Pythia and and HerwigHerwig can be runned directly, setting the generator parameters in the athena configuration script files

• All the other generators should be first run inAll the other generators should be first run in standalone modestandalone mode and the output file is passed to Herwig or Pythia for the showering/hadronization/etc.

• the events must be made with a generator version that is compatible, i.e. which supports the Les Houches interface

• Generated events are converted into a common format, HepMC, and made persistent for downstream use by the simulation, G4Atlas (geant4 ATLAS full simulation ) or Atlfast (ATLAS fast simulation)

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Generators for H→ZZ(*)→4l in ATLAS


Signal and most of the backgrounds evaluated at LO

• The generator choice, their settings, the configurations, the datasets, etc. are agreed among the collaboration e.g. ATLAS setting for Pythiae.g. ATLAS setting for Pythia

PDF choice for LO generators: PDF choice for LO generators: CTEQ6L1 (CTEQ6M1 for higher order)

Large backgrounds rates → need a good background understanding

→ first order running s ,no K factors, complex scenario + double gaussian matter distribution (UE tuning), longitudinal fragmentation function, etc.

[B. Mellado, G. Unal, S.L. Wu, ATLAS-COM-PHYS-2004-062]

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Need to know production cross sections and distributions of discriminating variables as precisely as possible

Higher Order QCD corrections for the Higgs generation process are available

• HO corrections do not reduce only to an overall factor to the cross section normalization..

- Rescaling by an overall K factor gives correct inclusive cross sections, but not necessarily correct kinematics and acceptance

- HO corrections imply non-trivial changes in the final state

Reweighing the LO results for the signal and the backgrounds of H→ZZ(*)→4l channel according to NLO calculation implemented in MC@NLO (full NLO QCD corrections + Parton Shower) which uses Herwig for the showering and the hadronization

PY-HW comparison for H→ZZ(*)→4l: motivation

The understanding of the differences between PY (used to generate the Higgs signal and backgrounds) and Herwig is crucial if we want to study the NLO effects with MC@NLO


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MonteCarlo versions

• Herwig (HW) 6.510 + Jimmy 4.0- Process (1)1600 gg/qqbar→H

- Process (1)1900 qq→q’q’WW/ZZ→q’q’H (H decay to ZZ forced – Z decay to /ee forced)

• Pythia (PY) 6.403 - Process 102 gg→H - Process 123-124 qq→q’q’WW/ZZ→q’q’H (H decay to ZZ forced – Z decay to /ee forced)

• ATLAS default settings

• (athena release 12.0.3)• LHAPDF version 5.0: CTEQ6L1 LO with LO S[J. Pumplin, D.R. Stump, J. Huston, H.L.Lai, P. Nadolsky, JHEP 0207 (2002)

012]


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Comparison of PY and HW - ATLAS defaults

• Cross section times branching ratios for the H→ZZ(*)→4l process at mH = 130 GeV (errors < 0.01 fb)

• Cross section for the Higgs productions at mH = 130 GeV (errors < 0.01 pb)

1. Cross sections

Clear difference both in the cross sections and in the branching ratios


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• Cross section for the Higgs ggF production (errors <1.5%)

Differences approximately constant over all the considered mass range..


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• Branching ratios for H→ZZ(*) in PY and HW as a function of mH, including results from HDECAY

2. Branching ratios

[A. Djouadi, J. Kalinowski, M. Spira, hep-ph/9704448]

Differences only in the mass range where one Z is off-shell


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Higgs and Z mass, pT and pseudorapidity distributions normalized to unitmH = 130 GeV - ggF


3. Differential distributions

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Higgs and Z mass, pT and pseudorapidity distributions normalized to unitmH = 130 GeV - VBF

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Understood differences: Higgs mass

• Higgs mass @130 GeV – ggF

1. The small peak at 130.2 GeV in HW is the contribution of the qqbar→H production which is included in the IPROC= 1600

2. The visible mass peak shift in HW is due to a FORTRAN bug in the precision in number manipulations

:

- through the computations from the parton momentum to the Higgs momentum and mass, precision can be lost and at the end one founds mH + some MeV

(4.326 MeV @ mH = 130 GeV)Temporary fixed by hand

Higgs mass plot after the fixes


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• PY and HW implements different calculations for the (H→ZZ*) HW uses the finite formula

PY uses a differential expression which needs an integration in the two Z mass space but allows also the decay in Z*Z*

HDECAY has an options to switch between the two formulas above

Understood differences: Z mass

Z masses distributions - mH = 130 GeV

• Differences HW-PY in the amount of events in the 40-80 GeV range

Investigation of the H→ZZ* decay

[R.N. Cahn, Rep. Prog. Phys 52 (1989) 389]

[W.Y. Keung, W.J. Marciano, Phys.Rev. D30 (1984) 248]


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Understood differences: Z mass

• Filtering for ZZ* by selecting events with one Z on-shell, i.e. in the range [-n, n ] around mZ – fraction of selected events

Here cut at 5

for both PY and HDECAY

To be understood why the three programs give different results for (H→ZZ*)

The region 40-80 GeV for HW is populated only by the tails, for PY they are also events with both the Zs off-shell


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PY-HW parameter tuning

• We would like to find a tuning for HW which reproduces the PY distributions

- in the follow we refer to “HW tuned” as HW configured using the above PY default parameters

the number of nominal resonance width above which the Breit-Wigner factor in the cross section is set to vanish


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PY-HW tuning of QCD parameters

Tuned on e+e- data

• for hadron-hadron collisions the PY default for S is overwritten by the S of the PDF set (default in athena 12.0.3, CTEQ6L1 LO with LO S)

This corresponds to S (mZ) = 0.118 when calculated at NLOBut PY default uses the LO calculation, which leads to 0.1298

the value of S fully accounts for the PY overestimation of the ggF cross section


• HIGLU configured with PY parameters and setting the values of S to the same values as PY or HW defaults

[M. Spira, hep-ph/9510347]

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HW tuning on PY

• if we want to reproduce the PY cross section predictions, i.e. the PY value for S, we have to configure HW with the following parameters

This set of parameters is leading to the same value of S as in PY (with a NLO computation)

cross sections of ggF Higgs (errors <1.5%)


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Comparisons of PY and HW tuned for VBF

• Cross section for the Higgs VBF production (errors <1.5%)

With HW tuned, differences are compatible within the statistical error


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PY tuning

• PY allows to configure the QCD parameters with large flexibility

Allow the user to configure QCD parameters

the default choice, MSTP(3) = 2, used in the Higgs production in ATLAS, causes the value to be set accordingly to the parton-distribution-function parameterizations, based on the PDF set selected

Second-order running S

value for hard interactions

value for space-like shower (ISR)

value for time-like shower (FSR)

value for resonance decay

• this set of parameters gives a value for S = 0.1198, more compatible to the world average cross sections of ggF Higgs (errors <1.5%)


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To be understood: branching ratios

tot of the Higgs, which enters into the BR calculation

Total width of the Higgs boson for PY and HW (not tuned) as a function of the Higgs mass, in comparison with HDECAY

- PY has two processes (H→gg and H→Z*) which are not implemented in HW

- Removing them, tot decrease to 4.15 MeV @ mH = 130 GeV


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• Both PY and HW have

i.e. resumming the LL (with LO S)

• But HW has also contributions NLO to tot and to mb(mH), depending on 1

- we expect that removing these NLO corrections yields the same part as PY

To be understood: branching ratios


• Under investigation also the contributions of the different part from the H decays

*HDECAY tuned - implements NNNLO (~ s

3) corrections to (H→bb) and NNLO terms for the running mass

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Matrix Element corrections to HW PS

• In HW, the parton shower emission is completely suppressed in the zone corresponding to hard and large-angle parton radiation (dead zone)- The HW cascades are supplemented by ME corrections for a full description of

the physical phase space

- The radiation in the dead zone can be generated according to exact first-order ME (neglecting multiple hard emissions)

• ME corrections for ggF Higgs production (not yet in the HW release)

-whenever the HW shower generates an emission which is the hardest so far, it is corrected according to the exact ME

• Comparison with HW default (tuned) and with PY, which implements ME correction to the first emission


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Comparison with HqT

[M. G

razzin

i, hep-p

h/0

51

20

25

]

[G. Bozzi, S. Catani, D. deFlorian, M. Grazzini, Phys.Lett.B 564(2003)65 and Nucl.Phys.B 737(2006)73]


Reproducing the same configuration as the right plot:S(mZ) =0.1120 (2 loop) - mH = 130 GeV – CTEQ6M1

• Integral of the resummed calculation

*MRST2004 s=0.1129

[C. Anastasiou, K. Melnikov, F. Petriello, hep-ph/0501130]

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Comparison with HqT (preliminary)


HqT tuned with PY parameters, to get a comparison with PY and HW tuned + ME

Discrepancies at high qT:

• the programs use different scale in S to generate events with pT > mH through the fixed order calculation

We have to set the same scale to get a comparison among the four programs

Logarithmic contributions to the Higgs pT

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Summary and Plans: Generator Studies

• We started studying the comparison between HW and PY for the H→ZZ(*)→4l channel

- Small fixes in HW for the Higgs mass shift and to remove qqbar→H contribution

- Using the ATLAS default for the two MC, differences in terms of cross sections, branching ratios, total Higgs width

• Large difference between PY and HW in the QCD parameter choices- PY default for hadron-hadron collisions (which we are using in ATLAS) leads to

an S value several far from the world average

- Tuning HW allows to reproduce the PY results in terms of cross sections

- Also a PY tuning (that may be further refined) is found, which allows to get an S in reasonable agreement with the PDG value and with HW value

• ME corrections to HW PS: we just started the comparison with PY and HqT• Plans:

- Comparison with HqT at large pT

- NLO with MC@NLO, comparison also on the Higgs pT

- Understanding the differences PY-HW in the Higgs partial widths and in the branching ratios

- Comparison HW-PY for the ZZ background


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Reconstruction

Event Generator

Generated events

ESD/AOD/TAG

Real life versus ATLAS full simulation

Simulation of the material effect

Simulated hits

Simulation of the read-out electronics

Digits/RDOs

Full Simulation

Pile-up

LHC

Interaction with the detector materials

Trigger + DAQ

Real events

Signal in the subdetectors

Readout output

Generated events are plugged in a full ATLAS Simulation chain

Generated events

Simulated ATLAS

detector


Physics Analysis

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Analysis of the H→ZZ(*)→4l channel in ATLAS

• Purpose of this H→ZZ(*)→4l study is to prepare the analysis of ATLAS data as they will be at day-1 of the data taking

• Uses and develops the physics analysis tools in athena

• Re-evaluates the expected performance estimated at the times of the ATLAS Physics TDR

• Focus on detector performance

• Only studies on full-simulation samples

• Include the full trigger chain in the analysis

• This work is included in the ATLAS software framework and integrated within the Higgs working group activities

• Results will appear in an ATLAS-CSC (Computing System Commissioning) document

• The analysis performance are going to be revaluated with the “as-built” detector, including misalignments, calibrations, deformations, realistic effects, full pileup, etc.


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• Selection similar to the Physics TDR, very loose preselection:

- all reconstructed leptons with pT>6 GeV and ||<2.5

• First selection applied to reconstructed leptons quality

• Kinematic cuts (for mH=130 GeV)

- two leptons with pT>20 GeV and ||<2.5

- one pair of opposite-charge leptons (same flavor) with invariant mass in a window mZ15 GeV

- the other pair of leptons with an invariant mass >20 GeV

- Higgs mass in a 2 window

• Impact parameter d0 cut (against Zbbbar background)

- z0 resolution will be very important with pileup: identify primary vertex out of minimum bias pileup

• Lepton isolation energy (against Zbbbar and ttbar backgrounds)

Selecting Higgs events


Cut on leptons: knowledge of the exact NLO distributions is fundamental

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Selecting Higgs events (cont’d)


Effect of the aforementioned selections on signal and the main backgrounds

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Expected events for 30 fb-1 - Results LO – no K factors

Expected signal and background (preliminary)

missing qqbar→ZZ background in the TDRTwo on-shell Z production becomes critical at 180 GeV


mH (GeV)

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• So far, kinematic cuts are applied to offline-reconstructed objects (electrons and muons), which should satisfy typical trigger configurations → the trigger system coverage is not fully accounted

Trigger Aware Analysis

Trigger Aware Analysis = physics analysis including the results of the full trigger slices (for both and e) → see A. Nisati’s talk tomorrow

Possible trigger menu for H→ZZ(*)→4l including only LVL1

efficiencies (in %) for:

- all events (filter: 4-leptons with pT>5 GeV and ||<2.7)

- events after all H4l offline selection cuts (kinematic cuts + isolation + impact parameter cuts)

Errors are ~0.3% (~0.5%)


Ongoing also the LVL2 and EF integration

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Summary and Plans: H→ZZ(*)→4l analysis in ATLAS

• We set up a tool for a detailed H→ZZ(*)→4l analysis

- The analysis algorithm runs within the ATLAS software framework and uses the ATLAS physics analysis tools

- Performance studies and comparisons with the Physics TDR

- The same code can be applied to the first real data: ready to analyze the first data of the experiment

• Complete the studies on all relevant backgrounds, re-evaluate the significances, taking into account the NLO calculations

• Final results should take into account the “as-built” simulation, the “real detector” will affect the physics observables


→ without a complete simulation of the detector we cannot understand the physics processes

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Backup Slides

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The H→ZZ(*)→4l channel

Most promising signature for the discovery of the SM Higgs for 130 GeV < mH < 2mZ and dominant channel for mH > 200 GeV

• Branching ratio H→ZZ(*) - from 3.8% @ mH=130GeV to

30.7%@ mH=300GeV


• Signature:

- 4 high pT isolated leptons from the primary vertex

• Production channels at LHC -Gluon-gluon fusion (ggF)-Vector boson fusion (VBF)

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HIGLU predictions

• HIGLU configured with PY parameters with S(MZ) =0.1158

[M. Spira, hep-ph/9510347]

The agreement with HW is very good..

Overview Generator Studies H→ZZ(*)→4 Analysis Summary

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• Under investigation the contributions of the different part from the Higgs decays- From BR(H →ZZ(*)) and tot at different masses, evaluation of (H →ZZ(*))

• Larger differences for (H →W+W-) but especially for (H→bbbar) and (H→ccbar)

To be understood: partial widths

w.r.t HW default, differences are within the statistical errors

* Tuned with PY parameters and allowing also Z*Z* events


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ATLAS Higgs discovery potential in H→ZZ(*)→4l

Overview Generator Studies H→ZZ(*)→4 Analysis Summary

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ATLAS software full offline chain

• Generation – use generators to produce events

• Simulation – run products through detector

• Digitization (with/without pile-up with minimum bias and cavern background events) – digitize hits

• Reconstruction - reconstructs particles, jets, tracks etc.

• Preparation of the reconstructed data, stored in structures called AODs, Analysis Objects Data

• Analysis

For all of these steps, we use Athena, the common framework for the ATLAS offline software

Overview Generator Studies ATLAS Simulation H→ZZ(*)→4 AnalysisSummary

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Real life versus simulation

LHC


Trigger + DAQ

Reconstruction

Real events


ADC/TDC output

Event Generator

Generated events

ESD/AOD/TAG

Real life

Simulated ATLAS

detector


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Simulating the ATLAS detector

Fast Simulation: Atlfast

• No particle propagation, nor interaction with the detector material • Only the basic information on the detector acceptance • Gaussian smearing (on the MC truth information) with resolutions measured in

full simulation studies

Full Simulation G4Atlas (geant4 simulation of the ATLAS detector)

• High level of details and precision• Electronic noise, dead channels, dead time, misalignment and deformations• Event overlapping due to pile-up with mininum bias and cavern background• Detector responses validated and tuned with:

-Test beam data-Today: in situ calibration data with cosmics-In the next future: halo muons, calibration data from LHC collisions (Z→μ+μ-,e+e-, π0→γγ; …)

• We want to be able to analyze data of the day-0

Timing: one complete G4Atlas physics event takes ~800 s Four to five orders of magnitude difference full/fast simulation


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Real life versus simulationEvent

Generator

ESD/AOD/TAG

LHC


Trigger + DAQ

Real events


ADC/TDC output

Reconstruction

Fast Simulation

AOD

Fast simulation

Atlfast

•Jet reconstruction in the calorimeter, momentum/energy smearing for leptons and photons, magnetic fields effects and missing transverse energy

•List of reconstructed jets, isolated leptons, photons and muons and expected missing transverse energy

• (optionally) the list of reconstructed charged tracks

Generated events

Simulated ATLAS

detector


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Full simulation of the ATLAS experiment

•The ATLAS full simulation uses geant4 for particle tracking into the detector, evaluating all possible interactions the particle could be submitted to

•The geometry description is provided by an external service which builds modularly the ATLAS subdetectors reading from external databases

•The geometry is under constant development, and there exists several versions of the detector descriptions

- amongst them also misaligned (and deformed) geometries, to check the effect of the “real detector” on the physics observables

•the detector description and the simulation detector response are extremely accurate

Without a complete and deep simulation of the detector we cannot understand the physics processes

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ATLAS fast and full simulation events

parameterized resolutions andparticle identification efficiencyFocus on simplicity and velocity

detailed showering and clustering models, simulation of detector electronics, events pileup, etc.

Atlfast G4Atlas


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• AOD objects contain all the information necessary for the analysis:– Quality flags, 4-vectors, isolation energies, impact parameter…– Calo clusters and reconstructed tracks parameters and covariance

matrices

• Electrons:– Access to the results of the electron Id cuts– Momentum from the Inner Detector, energy from the calo

measurement

• Muons:– Access to both algorithms available– Combined or “LowPt” muons (hits or segments associated)– Access to the single tracks from inner detector and muon

spectrometer for combined muons

Lepton AOD


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H Mass Resolution (GeV), MH=130 GeV, Z mass constraint

2e2 4

Mass Resolution


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3 points 3 points

angle-point

1TeV 5GeV

B

B

B~0

3 points 3 points

angle-point

1TeV 5GeV

B

B

B~0

Muon Reconstruction in ATLAS

• Tracks are back-extrapolated to the IP

• Parameters corrected for energy losses and multiple scattering

• Energy loss ~3 GeV at =0

• Look for match with tracks reconstructed in the ID

• Inner Detector in a Solenoidal Field of 2 T

Muon resolution vs pT

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Cuts applied at Level-1 selection:

• e15i EM cluster ET>11 GeV

• e25i EM cluster ET>21 GeV

• e60 EM cluster ET>51 GeV• Isolation cuts (for e15i and e25i):

– isolation in ring around 2x2 trigger tower cluster core in EM < 3 GeV – leakage into 4x4 and 2x2 trigger towers behind the EM cluster <2 GeV

• Accessing Level-1 results for electrons and muons

Trigger Aware Analysis: electrons

• Single electron efficiencies vs pT

• Using CSC samples (ZZ*/*4l or HZZ*4l)

• True electron from theAOD truth container, select onlye within ||<2.5

15 GeV Threshold

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• Dedicated detector systembased on RPC (barrel) andTGC (endcap)

• 3-dimensional coincidencesbetween hits in both space and time

• Accessing Level-1 result from the LVL1_ROI container in the AOD

Trigger Aware Analysis: muons

10 GeV Threshold

• Single Muon efficiencies• Barrel and endcap together• Plateau:

86% for LowPt82% for High Pt (20 GeV) – also external station in the coincidence

simulation of the h →zz (*) →4l channel in atlas

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