An Overview of the CMS ECAL with Applications to HEP Analysis - - PowerPoint PPT Presentation

▶
an overview of the cms ecal
SMART_READER_LITE
LIVE PREVIEW

An Overview of the CMS ECAL with Applications to HEP Analysis - - PowerPoint PPT Presentation

Apr 16, 2023 344 likes •557 views

An Overview of the CMS ECAL with Applications to HEP Analysis Daniel Klein Thursday Pizza Lecture 11/14/2013 Outline I. Motivation 1. What is an ECAL? 2. Design requirements for CMS ECAL II. Design 1. Materials 2. Crystal geometry

slide-1
SLIDE 1

An Overview of the CMS ECAL

with Applications to HEP Analysis

Daniel Klein Thursday Pizza Lecture 11/14/2013

slide-2
SLIDE 2

14-Nov-2013 2

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. Particle reconstruction and selection

slide-3
SLIDE 3

14-Nov-2013 3

Motivation

slide-4
SLIDE 4

14-Nov-2013 4

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

slide-5
SLIDE 5

14-Nov-2013 5

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
slide-6
SLIDE 6

14-Nov-2013 6

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
slide-7
SLIDE 7

14-Nov-2013 7

Design

slide-8
SLIDE 8

14-Nov-2013 8

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

slide-9
SLIDE 9

14-Nov-2013 9

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

slide-10
SLIDE 10

14-Nov-2013 10

Energy Resolution

(In case you're not sick of this plot yet...) → Comes from electron test-beam studies on a supermodule.

slide-11
SLIDE 11

14-Nov-2013 11

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

slide-12
SLIDE 12

14-Nov-2013 12

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

slide-13
SLIDE 13

14-Nov-2013 13

Particle Reconstruction

slide-14
SLIDE 14

14-Nov-2013 14

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
slide-15
SLIDE 15

14-Nov-2013 15

Supercluster examples

Probably hybrid algorithm Probably island algorithm

slide-16
SLIDE 16

14-Nov-2013 16

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 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:
slide-17
SLIDE 17

14-Nov-2013 17

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 (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

  • Variables from multiple subsystems are also

combined using neural networks

  • Also use tracks to reject photons that converted
slide-18
SLIDE 18

14-Nov-2013 18

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: σηη, Σ9/Σ25

  • HCAL isolation used to reject electron

candidates coming from jets

slide-19
SLIDE 19

14-Nov-2013 19

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

slide-20
SLIDE 20

14-Nov-2013 20

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