Expected Performance forElectron and Photon in ATLAS
東城 順治（KEK）
LHCが切り拓く新しい物理東京大学
2009年4月1日

Contents• Electron/photon performance の重要性
• Reconstruction• Correction and calibration• Identification• Expected performance• Performance in early data and What’s going on
now and next step2

Electron/Photon Performance の重要性 (1/3)• 多くの bench mark physics channel で、high pT electronと photon は重要な signal である– Higgs search
H→γγ, H→Z(*)Z→4e, 2e2μ
– BSM searche/γ signatures in SUSY, extra gauge bosons (Z’→ee etc.)– SM physicsZ →ee, W →eν, γ+jet

3Electron/Photon Performance の重要性 (2/3)• 検出器（subsystemでは主に内部飛跡検出器と電磁カロリメータ）の性能を data-driven で理解する in-situ tool として、electron/photon を final state に持つ SM process
は重要（特にresonance）– Electron
W →eν, Z →ee, J/ψ→ee, Υ→ee
– Photonγ+jet, γγ+jet
Note : high pT photon を重い resonance はない4

内部飛跡検出器
SemiConductor Tracker (SCT)- Silicon strip detector- Barrel : 4 cylindrical layers- End-cap : 9 disks per sidePixel Detector- Hybrid silicon pixel detector- Barrel : innermost cylindrical layer (B-layer)and 2 outer cylindrical layers- End-cap : 3 disks per side

- Surrounded by 2 T
super-conducting
solenoid magnet- |η| < 2.5

Transition Radiation Tracker (TRT)- Straw-tube tracking chamber w/ transition radiation capability.
- Straws run in axial direction in barrel and radial direction in end-caps.

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LAr 電磁カロリメータ

3rd sampling (back)

EM Barrel Calorimeter : |η| < 1.475

2nd sampling(middle)

EM End-cap Calorimeter : 1.375 < |η| < 3.2
1st sampling (strip)- Pb/LAr sampling calorimeter w/ accordion-shaped electrodes- Three longitudinal segmentation- Cell size in Δη×Δφ
1st (strip) : 0.003×0.1, 2nd (middel) : 0.025×0.025, 3rd (back) : 0.05×0.025- Presampler in front of calorimeter in |η| < 1.8 : Δη×Δφ ∼ 0.025×0.1

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Electron/Photon Performance の重要性 (3/3)• LHC 環境下での Severe な検出性能の要求– Energy resolution ： σE/E ∼ 10%/√E ⊕ 0.7%
– Energy linearlity : < 0.5% up to 300 GeV– EM cluster pointing resolution : σRφ ∼ 9mm/√E, σz ∼ 3mm/√E– Electron identification
Rejection factor ~105 for pT > 20 GeV/c( Note : electron rate / jet rate ~ 10-5)– Photon identification
Rejection factor ~5×103 for pT > 20 GeV/c7

Reconstruction of Electron and Photon (1/2)
System-level reconstructione/γ combined reconstruction

LAr 電磁カロリメータ

cluster(seed)electron/photon candidate
cluster-track
matching

correction &calibrationelectron/photon candidate
内部飛跡検出器
identification

track
Identified electron/photon
8Reconstruction of Electron and Photon (2/2)• e/γ combined reconstruction の algorithm flow
1. Cluster seed
Cluster (fixed-size window Δη×Δφ = 5×5 in cell) を seed とする

2. Cluster-Track matching (in Δη×Δφ = 0.05×0.1 atcalorimeter position)••

Match する track あり → electron or converted photon candidate
Match する track なし → un-converted photon candidate

3. Correction
Electron, converted photon, un-converted photon candidate 別に、cluster の re-build と calibration

4. Identification••

Cut-based identification (default)
Multivariate discriminants : Likelihood etc.
9Correction and Calibration (1/3)• Cluster re-build と position correction
Electron, converted photon, un-converted photon ごとに別々のsizeのwindow– BarrelΔη×Δφ in cellelectronconverted photon

un-converted photon

3×7

3×7

3×5

– Endcap : Δη×Δφ = 5×5• η/φ position correction
Electron 100 GeV

Electron 100 GeV

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Correction and Calibration (2/3)• Energy calibration と correction の実験前初期値として、singleparticle (electron と photon) MC の Geant4 の情報（CalibrationHit）を用いて、energy を再構成
– Active layer (pre-sampler, layer1(strip), layer2(middle), layer3(back)) へのenergy deposit– Inactive layer (material in front of pre-sampler and LAr ) へのenergy
deposit– Out-of cluster energy– Shower leakage
物質量分布11Correction and Calibration (3/3)• MC-base correction & calibration 後の energy resolution とlinearity
ElectronPhoton

12Identification of Electron and Photon (1/3)• Cut-based method (現在ならびに実験初期のdefault)
A brief list of cut variables– Leakage into Hadronic calorimeter– Calorimeter shower shapes in 2nd sampling• Shower shape in η and φ• Energy-weighted lateral width– Calorimeter shower shapes in 1st sampling• Details of energy deposition structure in cells (2nd maximum in energy etc)
• Shower width– Track quality• Number of hits in pixel, SCT, TRT• Transverse impact parameter– Track-cluster matching

Red : Calorimeter-relatedBlue : ID-related
Green : track-cluster• Δη×Δφ position matching at calorimeter, E/p13

Identification of Electron and Photon (2/3)
Z→eedijet

ET/(ET+ET(had1))Δη(track-cluster)E(3×3)/E(3×7)

E(3×7)/E(7×7)

14Identification of Electron and Photon (3/3)
H→γγdijet

15Expected Performance (1/6)• Efficiency and jet rejection for electronreconstruction/identification– Medium cut : efficiency = 77%, rejection = 2.1×103– Tight cut : efficiency = 62%, rejection = 8.9×104“Tight” = “medium” & (cut in TRT hits)

16Expected Performance (2/6)• Efficiency and jet rejection for photon reconstruction/identification– Efficiency = 84%, rejection = 5.1×103

Fake rate = 1 / rejection

17Expected Performance (3/6)• Efficiency and jet rejection for photon
reconstruction/identification– Efficiency = 84%, rejection = 5.1×103

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Expected Performance (4/6)• Photon conversion reconstruction– Photon conversion vertex in MC truth for Minimum bias events

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Expected Performance (5/6)• Photon conversion reconstruction– Track reconstruction• Algorithm seeded in silicon detector (smaller radius) or TRT (lagerradius)• Combine tracks from both categories– Conversion vertex fitting assuming electron mass
– 現在も発展途上

H→γγ20Expected Performance (6/6)• Brems recovery– 現在も発展途上Electron 25 GeV
３つのalgorithmを検討中- GSF (Gaussian Sum Filter)- DNA (Dynamic NoiseAdjustment)- CaloBremFit21

Performance in Early Data (1/4)• 現在の LHC プラン– 2009年9月末：1st beam, 2009年10末：1st collision– 11 months (44 weeks) running at 5 TeV + 5 TeV– 期待される delivered luminosity : ~200 pb-1
• 1st 100 days : ~100 pb-1
• 2nd 100 days : ~200 pb-1
• Last 5 weeks : Heavy ion run– 200 pb-1でのプランを策定中

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Performance in Early Data (2/4)• 1st beam – 1st collision at 10 TeV• 900 GeV (injection energy) か 10 TeV より low energy の runがあると予想• Start to look at Minimum bias
• π0→γγ, energy flow, photon conversion を使って、カロリメータの response の（非）一様性、内部飛跡検出器の物質量の評価を行う• 物質量評価は特に重要– 初期は 5% X0 の精度、ゴールは 1-2% X0 精度

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Performance in Early Data (3/4)• 1 - 10 pb-1
• Start to use J/ψ→ee, Υ→ee, Z→ee, W→eν, γ+jet• 実データに対して、 e/γ combined reconstruction の tuning・実用化を目指す– Algorithm flow
– Correction and calibration– Cut-based identification• Efficiency と fake のstudy– 5% 精度• 10 - 200 pb-1
• In-situ calibration using Z→ee
– 0.1% 精度• Efficiency と fake のstudy– 2% 精度(統計＋系統)
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Performance in Early Data (4/4)• その他議論中の重要な item•••••

MC-based calibration
MC-based calibration と in-situ calibration の比較・理解
内部飛跡検出器のalignment
内部飛跡検出器 – カロリメータ間のaligmnent
Data quality monitoring25