Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel

Fany Dudziak (LAL)Fany Dudziak (LAL) Rencontres Moriond EW 2010 - YRencontres Moriond EW 2010 - Young Scientist Forumoung Scientist Forum

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Electron reconstruction and Electron reconstruction and identification in ATLAS. Implications identification in ATLAS. Implications

for the Higgs -> 4e channel. for the Higgs -> 4e channel.

Fany DudziakFany Dudziak

On behalf of the ATLAS collaborationOn behalf of the ATLAS collaboration


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- Outline- Outline

The Higgs -> ZZ* -> 4 electrons channelThe Higgs -> ZZ* -> 4 electrons channel

Electron reconstruction and identification in ATLASElectron reconstruction and identification in ATLAS

Improvements since ATLAS expected performance Improvements since ATLAS expected performance (Dec 2008) (Dec 2008) ((“Expected Performance of the ATLAS Experiment” , Dec 2008, arXiv:09010512; “Expected Performance of the ATLAS Experiment” , Dec 2008, arXiv:09010512; CERN-OPEN 2008-020 CERN-OPEN 2008-020 ))

Impact on the Higgs to 4 electrons analysisImpact on the Higgs to 4 electrons analysis


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- The H -> ZZ* -> 4 electrons analysis- The H -> ZZ* -> 4 electrons analysis

Clean electron signal => Golden channel for masses above 130 GeV

Hig

gs

Deca

ys

Bra

nch

ing R

ati

os

Hig

gs

Deca

ys

Bra

nch

ing R

ati

os

Direct search limit ~114 GeV

l+ l-

Z |(mZ)mes - (mZ)| < 15 GeV

l+ l-

Z(*)

mH

o 4 leptons with Pt > 7 GeV, at least 2 with Pt > 20 GeV et |η| < 2.5 o Lepton quality cuts : 4 e mediumo Isolation cuts : (jets,b,t)o Impact parameter cut (b)

Resonance??

Analysis skeleton


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- The Higgs to 4 electrons analysis- The Higgs to 4 electrons analysis

14TeV, 30 fb-1130 GeV

600GeV

180GeV

400GeV

300GeV

150GeV

Backgrounds :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- The electron reconstruction in ATLAS detectoro Electron = TRACKTRACK + 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 windowSliding window algorithm to reconstruct a cluster in the electromagnetic calorimeter.

o Search for a reconstructed trackSearch 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 identificationidentification and background discrimination

=> 3 levels of identification : Loose, Medium and Tight

calorimeter

trackers


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- Electron identification in the latest ATLAS publication (ATLAS - Electron identification in the latest ATLAS publication (ATLAS expected performance book dec 2008)expected performance book dec 2008)

Loose :Loose :Acceptance, hadronic leakage and constraints on 2nd sampling of the calorimeter variables

Medium :Medium :Cuts on 1st sampling of the calorimeter Quality of the track (number of hits in the different layers of the trackers)

Tight 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)


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- Improvements of the electron reconstruction- Improvements of the electron reconstruction

o Better track choice for match -> priority to Si Better track choice for match -> priority to Si trackstracks

o Ambiguities between conversions and electrons Ambiguities between conversions and electrons are now reconstructed as both photons & electronsare now reconstructed as both photons & electrons

o Identification criteria optimized to harmonize Identification criteria optimized to harmonize trigger and offline selection and to increase signal trigger and offline selection and to increase signal background separation, starting from 5 GeVbackground separation, starting from 5 GeV

o New background-discriminating variablesNew background-discriminating variables

Gain of +12% for mediumGain of +12% for medium Slightly improved jet rejection power Slightly improved jet rejection power

optimized

Pt>20GeV


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- Summary and perspectives- Summary and perspectives

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.

o 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 Waiting for first data to study electron efficiency and expected backgrounds to H->4eexpected backgrounds to H->4e


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BACK-UP SLIDESBACK-UP SLIDES


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- Lar Calorimeter- Lar Calorimeter Lead absorbers “accordion” shaped, in a Lead absorbers “accordion” shaped, in a liquid argon bath at 88.5Kliquid argon bath at 88.5K Cells are graved in the readout electrodes.Cells are graved in the readout electrodes. Detector radiation hard, fully hermetic Detector radiation hard, fully hermetic in phi, good energy resolution, “easy” and in phi, good energy resolution, “easy” and stable calibration. stable calibration. Covers the full azimuth and -3.2< Covers the full azimuth and -3.2< ηη < 3.2 < 3.2 following beams axis. 173K channelsfollowing beams axis. 173K channels

Segmented longitudinally in four layersSegmented longitudinally in four layers - PRESAMPLER:- PRESAMPLER: to measure the energy loss to measure the energy loss before the calo before the calo - FRONT:- FRONT: High granularity to disentangle High granularity to disentangle ππ0s vs single photons 0s vs single photons - MIDDLE:- MIDDLE: Contains the majority of an Contains the majority of an

electromagnetic electromagnetic shower energyshower energy- BACK :- BACK : offers rejection power against offers rejection power against hadronic showers.hadronic showers.

Longitudinal segmentation offers direction measurement for photons

Strips

Middle

Back

TEST B

EAM

2004


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- ATLAS trackers- ATLAS trackers

The Inner Detector is composed by 3 sub-systems:

Pixels :1 removable barrel layer 2 barrel layers4 end-cap disks on each side

=> 0.8x108 channels

Silicon Tracker (SCT)4 barrel layers9 end-cap wheels on each side

=> 6x108 channels

Transition Radiation Tracker (TRT)Axial barrel strawsRadial 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- 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 - Identification cuts


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- - Electron efficiencies in the latest ATLAS publication (ATLAS expected Electron efficiencies in the latest ATLAS publication (ATLAS expected performance book dec 2008)performance book dec 2008)


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- Electrons in 900 GeV data - 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

Documents

Electron reconstruction and identification in ATLAS. Implications for the Higgs -> 4e channel