A Summary of the CMS ECAL with Applications to HEP Analysis Daniel - - PowerPoint PPT Presentation

A Summary of the CMS ECAL

with Applications to HEP Analysis

Daniel Klein Thursday Pizza Lecture 11/7/2013

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Outline

I. Motivation

1. What is an ECAL? 2. Design requirements for CMS ECAL

II. Design

1. Materials 2. Crystal geometry 3. Large-scale geometry

III. Measurement

1. Superclustering 2. Triggers 3. Object reconstruction

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Motivation

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What is an ECAL? What does it do?

• Stands for Electromagnetic CALorimeter
• Used to measure the energy of

electrons/positrons and photons, and (indirectly) their parent particles

• Help with identification of EM particles (more
• n this later)
• Help determine (rough) positions of EM

particles, in conjunction with tracker

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Some searches influencing CMS ECAL design

• Higgs search

– H → γγ dominant decay mode for 114 GeV < mH < 130 GeV – H → ZZ → 4ℓ the “mode of choice” for 2mZ < mH < 600 GeV

• SUSY searches

– GMSB: LSP → G~ + γ (expect lots of hard photons) – / → γ + jets

• New vector bosons

– Z' → ee

• Lots and lots of standard model physics

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Technical Requirements

From TDR: Summary of ECAL requirements in order to meet LHC physics program goals:

• “Good” electromagnetic energy resolution
• ee and γγ mass resolution of ~1% at 100 GeV
• Coverage out to |η| = 2.5
• Measurement of γ direction, or PV localization
• Rejection of π0
• Efficient photon and lepton isolation at high luminosity

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Design

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Materials

• Primary detection material:

lead-tungstate crystals (PbWO4)

– Radiation length X0 = 0.89 cm

Recall:

– Moliere radius RM = 2.2 cm – Fast: 80% of light emitted within 25ns.

Comparable to bunch-crossing time.

– Radiation-hard – up to 10 Mrad – Emit blue-green scintillation light peaking

at ~420 nm

• Photodetectors

– Stuck onto the back of each crystal – Barrel: silicon avalanche photodiodes

(APDs)

– Endcap: vacuum phototriodes (VPTs)

• Endcap also has preshower detector

– Sits just inside endcap crystal array – Sampling calorimeter – (Lead “radiator” + silicon strip sensors)

* 2 layers

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Crystal geometry/resolution

• Reminder:

– Rad. length X0 = 8.9 mm – Moliere radius RM = 22 mm

• Crystals shaped like truncated pyramids
• Barrel section:

– Made of 61,200 crystals – Front face: 22x22mm = 1x1 RM ~ 1°x1° – Length: 230mm = 25.8 X0 – Most energy (~94%) from a single particle

will be contained in 3x3 crystals

• Endcap section:

– 2x endcaps, containing 7324 crystals each – Front face: 28.6x28.6mm = 1.3x1.3 RM – Length: 220mm = 24.7 X0 – Most energy from a particle will be

contained in 3x3 crystals

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Large-scale geometry: Barrel

• Range: 0 ≤ |η| ≤ 1.479
• Inner radius: 1.29 m
• 61,200 crystals = 360 around * 170

lengthwise

• 5x2 crystals in a “submodule”

– Each submodule matches up with a

trigger tower in η and φ

• Submodules arranged into modules
• 4 modules (85x20 crystals) in one

“supermodule”

– Each covers ½ the length in η and 20° in

φ (36 total)

• Crystal axes point 3° away from

nominal interaction point

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Large-scale geometry: Endcaps

• Range: 1.479 ≤ |η| ≤ 3.0
• Set back 3.14 m from nominal

interaction point

• Each endcap made of two “Dees,”

3662 crystals per dee

• Crystals are arranged in 5x5

“supercrystals”

– Each dee holds 138 supercrystals

and 18 partial supercrystals

• Supercrystals arranged in an x-y

grid, NOT an η-φ grid.

• Crystal axes point to a spot 1.3 m

past the nominal interaction point

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Measurement

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ECAL superclustering

• Photon conversion and electron bremsstrahlung cause shower to be

spread out in φ direction.

– Form “superclusters” - clusters of clusters, with some spread in φ

• Hybrid algorithm: start with a “bar” 3-5 crystals wide in η, then search

dynamically in φ for more deposits

– Works well for high-energy electrons in barrel

• Island algorithm: start with one crystal, then keep adding adjacent

crystals with energy deposits until you form a cluster

– Add nearby clusters (within a narrow η window, broader φ window) to form

a supercluster

– Works well when small, isolated clusters are needed

• Use log(energy)-weighted averaging to find center of a cluster

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Triggers

• Level 1 trigger: ET threshold, applied to superclusters that match in η and

φ with a trigger tower

– 50% efficiency levels: single isolated: 23 GeV, double isolated: 12 GeV, double

non-isolated: 19 GeV

– Isolation determined from HCAL and tracker

• High-level trigger (HLT) selection has three sub-levels:

– Level 2: an ET cut on ECAL superclusters – Level 2.5: Look for pixel hits in tracker consistent with an electron (positron)

hypothesis

– Level 3: If passing level 2.5, use full tracker info (including tracker isolation) to

attempt to match electron tracks to ECAL deposit

• If a deposit doesn't pass the level 2.5 trigger, it can still be used as a photon candidate
• Object-specific HLT cuts:

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Photon Reco & Selection

• Energy is a sum over 5x5 cluster, or hybrid

supercluster (EB), or island supercluster (EE)

• 3 tracker-based isolation variables used, based
• n sum pT, angle, or number of tracks within

some cone size of ECAL cluster

– Used to reject photons from π0 or k

• 4 ECAL isolation variables used, based on

energy deposited in a certain cone size around supercluster, or on R9 (E3x3 / Esupercluster)

– Used to reject photons from π0

• HCAL isolation based on simple sum of HCAL

ET in a cone around ECAL supercluster

– Used to reject photons from jets – H/E variable shows worse performance than simple

sums in HCAL

• Also use tracks to reject photons that converted

in the tracker

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Electron (positron) Reco & Selection

• Bremsstrahlung spreads out electron

energy in φ

– Brem photons can even convert in tracker – Electron energy best measured using

superclusters, not NxN windows

• Electron ID makes heavy use of tracker

information, including isolation, E/p, primary vertex reconstruction, etc.

– Another slideshow unto itself (Liam)

• Shower shape variables used in electron

ID include: σiηiη, Σ9/Σ25

• HCAL isolation used to reject electron

candidates coming from jets

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Summary

• CMS requires an efficient, high-precision electromagnetic

calorimeter

• This requirement was met by designing an ECAL made

mostly of lead-tungstate crystals, with scintillation light read

• ut by photodiodes/triodes

– Crystals have short radiation length and Moliere radius, allowing

fine resolution in eta and phi

• Energy deposits are collected into (super)clusters, the basic

blocks of energy measurement

• Measurements from other detector subsystems aid in ID and

selection of electrons and photons

For next iteration: learn more about how ECAL measurements go into

• ur more detaild analysis-level selections