Electron reconstruction and
identification in ATLAS. Implications
for the Higgs -> 4e channel.
Fany Dudziak
On behalf of the ATLAS collaboration
Fany Dudziak (LAL)
Rencontres Moriond EW 2010 Young Scientist Forum
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- Outline

The Higgs -> ZZ* -> 4 electrons channel

Electron reconstruction and identification in ATLAS

Improvements since ATLAS expected performance (Dec
2008)
(“Expected Performance of the ATLAS Experiment” , Dec 2008, arXiv:09010512;
CERN-OPEN 2008-020 )

Impact on the Higgs to 4 electrons analysis
Fany Dudziak (LAL)
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- The H -> ZZ* -> 4 electrons analysis
Higgs Decays Branching Ratios
Analysis skeleton
o 4 leptons with Pt > 7 GeV, at least 2
with Pt > 20 GeV et |η| < 2.5
o Lepton quality cuts : 4 e medium
o Isolation cuts : (jets,b,t)
o Impact parameter cut (b)
l+
l+
l-
Z
|(mZ)mes - (mZ)| < 15 GeV
l-
Z(*)
Direct search limit ~114 GeV
Clean electron signal => Golden channel for
masses above 130 GeV
Fany Dudziak (LAL)
Rencontres Moriond EW 2010 Young Scientist Forum
mH
Resonance??
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- The Higgs to 4 electrons analysis
130 GeV
180GeV
400GeV
150GeV
14TeV, 30 fb-1
300GeV
600GeV
Backgrounds :
irreducible : ZZ->4e,
reducible : Zbb, tt, Z+jets
=> Their importance varies with the Higgs mass
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- The electron reconstruction in ATLAS detector
o Electron = TRACK + ENERGY in CALORIMETER
=> calo = LArg Accordion calorimeter
3 longitudinal layers with different transverse
segmentation, and a presampler layer in the front .
=> tracker = pixels + SCT (silicium strips) + TRT
(transition radiation tracker)
o Sliding window algorithm to reconstruct a cluster in
the electromagnetic calorimeter.
o Search for a reconstructed track which points to the
cluster
-> If so, calibrate the cluster as an electron in an
adequate size. Otherwise, calibrate the cluster as a
photon.
o Various selection criteria are used for electron
identification and background discrimination
=> 3 levels of identification : Loose, Medium and Tight
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- Electron identification in the latest ATLAS publication
(ATLAS expected performance book dec 2008)
 Loose :
Acceptance, hadronic leakage and constraints on 2nd sampling of the calorimeter
variables
 Medium :
Cuts on 1st sampling of the calorimeter
Quality of the track (number of hits in the different layers of the trackers)
Tight
TRT hits , E/p & tighter cuts on track match
=> Optional calo and track isolation
Efficiency
ATLAS expected
Performance book Dec 2008
Impact on H->4e analysis for M=130GeV
(ATLAS expected Performance book Dec 2008)
Fany Dudziak (LAL)
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- Improvements of the electron reconstruction
o
Better track choice for match -> priority to Si
tracks
o
Ambiguities between conversions and electrons are
now reconstructed as both photons & electrons
o
Identification criteria optimized to harmonize
trigger and offline selection and to increase signal
background separation, starting from 5 GeV
o
New background-discriminating variables
Pt>20GeV
optimized
 Gain of +12% for medium
 Slightly improved jet rejection power
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- Summary and perspectives
o
o
The Higgs to 4 electron channel is a very promising channel to discover the Higgs boson
for masses above 130 GeV thanks to its clean signal of 4 electrons.
Since the Atlas expected performance book a lot of work has been done to optimize :
- the electron reconstruction
- the electron identification efficiency
 The efficiency gain is 12% (absolute) for medium criteria, with slightly improved jet
rejection.
o
The optimization on the electron identification leads to an expected improvement of the
Higgs to 4 electrons recontruction efficiency of ~(relative gain)^4
o
Waiting for first data to study electron efficiency and expected
backgrounds to H->4e
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BACK-UP SLIDES
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- Lar Calorimeter
Lead absorbers “accordion” shaped, in a
liquid argon bath at 88.5K

Cells are graved in the readout electrodes.

Detector radiation hard, fully hermetic
in phi, good energy resolution, “easy” and
stable calibration.

Covers the full azimuth and -3.2< η < 3.2
following beams axis. 173K channels
Middle
Segmented longitudinally in four layers
- PRESAMPLER: to measure the energy loss
before the calo
- FRONT: High granularity to disentangle
π0s vs single photons
- MIDDLE: Contains the majority of an electromagnetic
shower energy
- BACK : offers rejection power against
hadronic showers.
Fany Dudziak (LAL)
Strips
Back
Longitudinal segmentation offers
direction measurement for photons
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- ATLAS trackers
The Inner Detector is composed by 3 subsystems:
Pixels :
1 removable barrel layer
2 barrel layers
4 end-cap disks on each side
=> 0.8x108 channels
Silicon Tracker (SCT)
4 barrel layers
9 end-cap wheels on each side
=> 6x108 channels
Transition Radiation Tracker (TRT)
Axial barrel straws
Radial end-cap straws
~36 straws by track
=> 4x105 channels
=> electron identification capability
 Total coverage |η| < 2.5
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- Perspective for Higgs searches at 7 TeV
o
The only mass region where we have sensitivity with low luminosity is around 160 GeV
(H->WW mode favourized in this region).
o
A Higgs discovery with 1 fb-1 is unlikely for any Higgs mass at 7 TeV (~2010+2011 @ 7TeV)
=> a minimum of 3.3 fb-1 is needed for optimistic uncertainties
o
If data are well understood a Higgs can be excluded with 1 fb-1 in the mass range
140 < MH < 185 GeV at 95% CL
H -> 4 leptons channel @ 7 TeV :
o No H->4 leptons signal expected but :
o Little irreducible background, can be estimated from Drell-Yann
o Plenty of events to measure rejection of reducible background
o Study of electron’s reconstruction with data
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- Identification cuts
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-
Electron efficiencies in the latest ATLAS publication (ATLAS
expected performance book dec 2008)
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- Electrons in 900 GeV data
Monte Carlo predictions for electron
candidates:
- ~33% from conversions
- ~1% of prompt electrons from b and c
- ~3% of backgrounds electrons (Dalitz…)
- fakes from hadrons
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