An Overview of the CMS ECALwith Applications to HEP Analysis

Daniel KleinThursday Pizza Lecture

11/14/2013

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OutlineI. Motivation

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

II. Design1. Materials2. Crystal geometry3. Large-scale geometry

III. Measurement1. Superclustering2. Triggers3. Particle reconstruction and selection

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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 on this later)

● Help determine (rough) positions of EM particles, in conjunction with tracker

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Some physics goals that influenced 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 → + 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 cmRecall:

– 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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Energy Resolution(In case you're not sick of this plot yet...)

→ Comes from electron test-beam studies on a supermodule.

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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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Particle Reconstruction

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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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Supercluster examplesProbably hybrid algorithm Probably island algorithm

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Triggers● Level 1 trigger: E

T 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 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 on

sum pT, angle, or number of tracks within some cone size of ECAL cluster– Used to reject photons from charged π or k

● 4 ECAL isolation variables used, based on energy deposited in a certain cone size around supercluster, or on R9 (E

3x3 / E

supercluster)

– 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● Variables from multiple subsystems are also

combined using neural networks● Also use tracks to reject photons that converted

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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: σ

ηη, Σ

9/Σ

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● HCAL isolation used to reject electron candidates coming from jets

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Example (ECAL-based) cuts from CMS2

NtupleMacros/CORE/electronSelections.cc

● electronIsolation_ECAL_rel_v1 < 0.20

● Transition region veto (reject 1.442 < η < 1.556)

● cms2.els_hOverE < 0.15● cms2.els_eOverPIn > 0.95

NtupleMacros/CORE/photonSelections.cc

● cms2.photons_ecalIso03 < [pt-dependent threshold]

● Barrel-only (η < 1.479)● cms2.photons_hOverE < 0.05● cms2.photons_sigmaIEtaIEta <

0.013

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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 out 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