「LHCが切り拓く新しい素粒⼦物理学」 March 28-29, 2008 東京大学

Lepton Identification and Trigger Decisionat Atlas Experiment
Shimpei YamamotoICEPP, Univ. of Tokyo

Outline◊◊◊◊

Introduction
Leptons (e/mu/tau)– Leptonic signatures at LHC– Reconstruction/Identification– In-situ calibration using early dataTrigger– Trigger system overview– Menus at low luminosity run
Summary

March 28-29, 20082

Leptons

March 28-29, 20083

Introduction◊

Leptons:– Leptonic signatures are clean.
• Easy to trigger and select.
• Suppression of physics (QCD) and non-physics backgrounds.
• Excellent for calibration and alignment of the detector.

– Thus primary channels of LHC discoveries are leptonic.
◊We summarize;– lepton reconstruction, identification and predictedperformances at the ATLAS detector,
and also give our perspectives of;– In-situ calibration using early data and expected performances– They are sufficient for early discoveries??

March 28-29, 20084

Leptonic signatures
Physics shopping list w/ leptonic signatures•••••

Standard Model Higgs– H → ZZ → llll
– H → WW → lνlν
– qqH → qqττ (one or both τ → lνν)
MSSM Higgs– gg → bbH(A), H(A) → ττ,μμDoubly Charged Higgs– H±± → W±W± → l±νl±νMassive Vector Bosons (KK, Gravitons, etc.)– Z’,G → ll
– Z’,G → WW → lνlν
– W’ → lν or WZ
SUSY– g̃ → qq̃Lq̃L → qχ̃20 → ql l̃R → qllχ̃10March 28-29, 2008

◊◊

◊

◊

GMSB (χ̃10 → G̃γ, l̃R → G̃l)— χ̃20 → l l̃R → llχ̃10 → llG̃γ
Right-Handed W— WR → l + N → l + ljj
Excited & Heavy Leptons— pp → ll’ → llZ → l + ljj (resonances)— gg → Z,Z’ → LL → lZ + lZ → ljj + ljj
Etc.
— SM precision measurements, e.g.

5Detector Overview
ATLAS (A Toiroidal Lhc ApparatuS)“Small” 2T Solenoid for Tracking
3 Large Toroids for Muon Spectroscopy
 High BL2 for Standalone Measurements
Muon Spectrometer Precision: MDT, CSC
 Trigger: RPC, TGC
 Excellent acceptance at poles
Calorimetry
 Lateral & Longitudinal Segmentation
 FCAL only 4.9m from IP
Inner Tracking (Pixel, SCT, TRT)
 Pixels: 50μm (r-φ) x 400μm (z) Δp/p (1 GeV)= 0.013,0.02 (η≈0,2.5)
 Δp/p (100 GeV) = 0.038,0.11
 TRT for e/π identification

ATLAS
March 28-29, 2008

6

ATLAS calorimetry
Calorimeter Parameters

March 28-29, 2008

Outer Dimensions (r /z)

2.25m / ±6.65m (ECAL)
4.25m / ±6.10m (HCAL)

Coverage (η)±3.2 (ECAL, HCAL)±4.9 (FCAL)

ECAL Technology

Presampler
Pb / LAr (Liquid Argon)
Accordion (190,000)

HCAL Technology

Fe / Scintillator (10,000)
Cu / LAr (HEC)
Cu / W / LAr (FCAL)

Samplings

1 + 3 + 3 (Barrel)(1) + 3 + 4 (Endcap)
3 + 3 (Forward)

Material

24 X0 - 26 X0 (ECAL)
11 λ (HCAL)

Resolution (η / φ)0.025 / 0.025 mrad (ECAL)
0.100 / 0.100 mrad (HCAL)

7

Basic detector performance
Trackingη≈0η ≈ 2.5

δp/p at pT = 1 GeV

1.3%

2.0%

δp/p at pT = 100 GeV

3.8%

11.0%

ε(pions) at pT = 1 GeV

84.0%

ε(electrons) at pT = 5 GeV

90.0%

Calorimeter
ECAL δE/E (100 GeV Photons)

1 - 1.5%

ECAL δE/E (50 GeV Electrons)

1.3 - 2.3%

ECAL+HCAL Stochastic Term

55% / √E

ECAL+HCAL Constant Term

2.3%

Muon SpectrometerStandalone

March 28-29, 2008

Combinedη≈0η≈2η≈0η≈2

δp/p at p = 10 GeV

3.9%

6.4%

1.4%

2.4%

δp/p at p = 100 GeV

3.1%

3.1%

2.6%

2.1%

δp/p at p = 1000 GeV

10.5%

4.6%

10.4%

4.4%
8

Electron: reconstruction
1. Build clusters from ECAL
cells
2. Correct for geometryeffects
3. Correct for cell saturation
4. Match clusters to tracks
5. Correction forbremsstrahlung6. Require isolation
7. (kinematic cuts foranalysis)

March 28-29, 2008

ATLAS

9

Electron: performance◊

Predicted performances from MC simulation

The drop in efficiency is visible at:• gap region between the barrel and endcap calorimeters• low pt, mainly due to the loss of discrimination power of the shower shapecuts.
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In-situ calibration using ZÆee (1)◊◊Robust analysis at early stage– No track info. needed– Key tool for commissioningof electron reconstruction
and ID.
Correct the residual nonuniformity under Z massconstraint.
– ~30,000 events of ZÆeeenough to achieve aresponse uniformity of 0.7%

March 28-29, 2008

11In-situ calibration using ZÆee (2)Tag and probe method to determine efficiencies:• “tag” electron: well identified electron on one side
• “probe” electron: track/EM cluster on the other side
Single electron trigger provides unbiased probes.
~2% accuracy with 100pb-1
Agrees with the predicted performances.

March 28-29, 2008

15.00−25.00

25.00−40.00

40.00−70.00

0.00−0.80

83.60±2.35±3.76

89.75±0.68±0.69

92.63±0.53±0.23

0.80−1.37

75.60±2.76±6.1387.64±0.91±0.58

90.86±0.77±0.33

1.52−1.80

71.93±4.38±4.04

76.90±1.85±1.77

83.58±1.89±0.59

1.80−2.40

78.05±2.73±4.75

79.24±1.38±1.3682.50±1.37±0.7912Muon: reconstruction
1. Build segments in muon stations
2. Build tracks from hits or segments
3. Correct for Eloss & multiple scattering
4. Match inner tracks ( or calorimeter hits)
5. Combine statistically or re-fit

March 28-29, 200813

Muon: performance◊◊◊

Predicted performance from ttbar
MC.
Efficiency is fairly flat for pt between
10 and 100 GeV/c.
Efficiency loss at eta=0.0, 1.2 due tothe poor detector coverage.

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First peaks of di-muon resonances◊◊

Statistics for 1pb-1 (@1031)– Assuming 30% accelerator + detector efficiency
Possible studies are– First sanity check– ID(tracker) alignment• <100micron– Low pt muon scale◊ZÆmumu for 100pb-1– Detector efficiencies– Scales for higher-pt muonsMarch 28-29, 200815

Tau: reconstruction
1. Search for localized energy
deposits in calorimeters
2. Require hadronic energy
3. Match with 1 or 3 tracks incone
4. Remove photon conversion
tracks
5. Require isolation incalorimeters
6. Require small jet mass
Hadronic tau decay (tau jet)– 1prong (49.5%):⎯ τ −> ντ + π+/− + n(πο)– 3prong (15.2%):– τ −> ντ + 3π+/− + n(πο)
March 28-29, 200816

In-situ calibration of tau◊ZÆtautau(lh)– Visible mass of tau pairs with an integrated L of 100pb-1 after
subtracting the SS events from the OS.
– Visible mass can be reconstructed with an error of <1%, tau scale cantherefore be determined with an accuracy of a few %.

March 28-29, 200817

Summary of start-up and ultimate performances for
leptons
Start-up (~100pb-1)

ultimate

EM energy uniformity

<2%

0.7%

Electron energy scale

~2%

0.02%

ID alignment

50-100 micron<10 micron

Muon system alignment<200 micron

30 micronMuon momentum scale

~1%

0.02%

Tau momentum scale

~2%

---

1pb-1

100pb-1

1fb-1

Integrated lumi.

Early discovery with leptonic signature? (SUSY, massive bosson…)
Detector efficiencies, in-situ calibration for leptonsCommissioning, alignment, low-pt lepton

Cannot achieve ultimate performances, but do notneed for early discoveries (<1fb-1)
March 28-29, 200818

Trigger

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Trigger: introduction◊◊◊◊

At nominal high (low) luminosity, on average ~23 (2.3) minimum
bias events superimposed on any rare discovery signal– 25 ns bunch crossing– Minimum bias rate ~7x108 (7x107) Hz~1000 (100) low-pt tracks per event !
Moreover, due to finite detector response time, out-of-time pileup from different bunch crossings– need “time stamp” to distinguish events
Trigger system should be very fast and extremely selective:– 40 MHz input and O(100 Hz) “on tape” (100 events/s )– selection at the <10-4/109 = 10-13 level, with virtually zero deadtime !

March 28-29, 200820

Trigger System
Overview

TriggerDAQ40 MHz••

Level 1:
– Hardware based : Calo +Muon– Latency 2.5 μs– Output rate ~37kHz(75k)
Level 2: ~500 farm nodes– “Regions of Interest” (RoI) toguide reconstruction– Custom algorithms– Average execution time ~40ms– Output rate up to ~1kHz (2k)•

Event Builder: ~100 farm nodes•

Event Filter (EF):~1600 farm nodes– Seeded by level 2– Access to full built event– Offline algorithms– Average execution time ~4 s
– Output rate up to ~200 Hz

Pipelines

LVL1
Calorimeter
Trigger

2.5 μs
2.5 msMuon
LVL1 Acc.
Trigger

75 kHz

CTP

ROD

ROD

RoI’s (Region of Interest)

LVL2
ROIB

HL
T

~40ms
L2SV

L2P
L2P
L2P

L2N

Event Filter
EFP
EFP
EFP

RoI
requests
RoI data
LVL2 Acc.

2 kHz

ROD

120 GB/s
ROB

ROB

ROB

ROS
3 GB/sEvent BuilderEB~4sec
EF Acc.

EFN

200 HzEvent Size ~1.5 MB

March 28-29, 2008

CaloMuTrCh Other detectors

300 MB/s
21

Trigger menu at start-up (L~1031cm-2s-1)
Unprescaled items

March 28-29, 2008

(rate in Hz)

22

Trigger menu at L~1032cm-2s-1
Unprescaled items (very preliminary)(rate in Hz)

Not yet for 1033
March 28-29, 200823

In-situ measurement of trigger efficiency(1)◊At design low luminosity (1033cm-2s-1), L1 rates do not allow to go
below ~20GeV for the single and ~12GeV for the double electronsignatures. Physics/Detector studies dominated by W and Z.

Direct J/ψ
Direct Υb,c→e
DY→ee
W→eν
Z→eeZ’(1TeV)x103

March 28-29, 200824

In-situ measurement of trigger efficiency(2)ZÆee example:• Select events accepted bysingle e trigger (e15, e15i, e25,e25i) ÆTAG• Events with 2 electrons. Build
inv. mass & keep only thosewith inv. mass close to Z peak.
• For this selection, check how
many times the 2nd electron
also triggered (2e15, 2e15i,2e25, 2e25i) ÆprobeMarch 28-29, 2008

Tag and probe method still works for trigger
efficiency measurements.

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Triggers for inclusive SUSY searches◊◊◊◊

Early SUSY searches will focus on inclusive signatures of:– Multiple jets
– Large missing ET– Leptons
To minimize biases and systematic effects, we should keep trigger
selection criteria as simple as possible.
– Due to its rich phenomenology, triggering SUSY events is easy inprinciple.
We estimate Etmiss trigger takes time to get established and Etmiss isoften crucial for signal selection (instrumental effects);– de-emphasize Etmiss trigger and loosen lepton selection are better
choice in early days??

Lepton triggers could be robust and urgent issue is– Strategy/method for evaluation of multi-jet/XE trigger.

March 28-29, 200826

Summary◊

Data is coming this year and leptons play an important role forearly physics at LHC.
– Detector alignment will be performed with e/mu using very
early data(~1pb-1).
– pt scales of leptons are determined with an accuracy of ~2%in the first 100pb-1
ÆSatisfactory performances for early discoveries.

◊Trigger menu at start-up (1031) is well tuned and close to final.
– Menus for 1032 and1033 are presently under study.
Tag and probe method works for determination of lepton trigger
efficiencies.
Also need in-situ measurements of jet/XE trigger efficiencies◊◊

March 28-29, 200827