coseners house forum on lhc startup 13th april 2007 david futyan imperial college 1 david futyan...

David FutyanImperial College

1Coseners House Forum on LHC Startup13th April 2007


Calibration of the CMS ECAL

Using Vector Bosons

Calibration of the CMS ECAL

Using Vector Bosons



Overview

Introduction:The CMS ECALCalibration requirements and strategy at LHC startup

Precalibration:Laboratory measurementsTestbeamCosmic Rays

In-situ calibration with physics events:Phi independenceVector bosons:

• Single electrons: We• Double electrons: Zee• Single photons: Z

Low mass resonances: 0



The CMS Detector

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

ECAL



The CMS ECAL

75848 Lead Tungstate crystals.

Barrel geometry:Front face dimensions: 0.01740.0174 in (2222mm = Molliere radius)

Crystal depth: 25.8X0 (230mm)Crystal axes tilted by 3o w.r.t. line from nominal vertex

3X0 preshower in front of most of endcap

1.29m



The CMS ECAL

Barrel “supermodule” (1700 crystals) composed of 4 modules. Each Half barrel contains 18 supermodules.

Readout:Barrel: avalanche photo-diodes (APDs)Endcap: vacuum phototriodes (VPTs)

• Photomultipliers with single gain stage• Able to operate in 4T magnetic field and high neutron flux



ECAL Energy Resolution

Physics reach of the ECAL, in particular the H discovery potential, depends on its excellent energy resolution.

Achievement of deign performance requires high precision calibration.

Intrinsic ECAL energy resolution of the CMS ECAL:

€

σE

=2.7%

E⊕0.5%⊕

150MeV

E

€

σE

=2.7%

E⊕0.5%⊕

150MeV

E

Constant term dominated by intercalibration precision (most of the energy of an electron or photon goes into a single crystal)



Calibration Requirements

Challenge is the relative channel-to-channel intercalibration of the ~80K crystals.

Intercalibration precision feeds directly into the constant termGlobal absolute energy scale can be obtained using a relatively small no. of Ze+e- or Z events.

Main source of channel-to-channel response variation:Barrel: crystal scintillation light yield, RMS ≈ 8%Endcaps: VPT signal yield, RMS ≈ 25%

Aim is to achieve an intercalibration precision of better than 0.5%.



Calibration Requirements

Ultimate goal is to achieve the most accurate energy measurement for electrons and photons:

G x ci x Ai are calibrated RecHits

• G = global absolute scale

• ci = calibration coefficients

• Ai = signal amplitudes in ADC counts

F = cluster level energy corrections to correct for energy loss due to bremsstrahlung and containment variations

• dependent on type of particle, position, momentum, clustering algorithm…

€

Eγ ,emeasured = F × G × c i

cluster

∑ × Ai

€


cluster

∑ × Ai



ECAL Intercalibration Strategy at LHC Startup

Preliminary estimates of intercalibration coefficients:Laboratory measurements of crystal light yield

Test beam precalibration of some supermodules

Commissioning of further supermodules with cosmic rays

Target precision can only be achieved in-situ using physics events:Impose -independence of energy deposited from minimum bias or jet triggers to rapidly intercalibrate to a precision of around 2%. Intercalibrate between regions using Ze+e-.

Finally intercalibrate to design goal of <0.5% using the momentum measured in the tracker for electrons from We (requires tracker alignment to be complete)

Complimentary method, not relying on tracker: invariant mass reconstruction from and resonances.



Precalibration



Laboratory Measurements

Excite crystal with a 60Co source. Measure light yield (LY) with PM tube.

Prediction of calibration coefficient ci:

Q = photo-detector quantum efficiency

cele = electronics chain calibration

M = photo-detector gain

Precision of lab measurement of ci can be determined by comparing to ci

determined from testbeam: €

1

c i

∝ LY ⋅εQ ⋅cele ⋅M

€

1

c i

∝ LY ⋅εQ ⋅cele ⋅M

σ 4%



Testbeam Precalibration

8 supermodules (~1/4 or ECAL barrel) precalibrated in summer 2006Remaining 28 barrel supermodules will be intercalibrated in-situ

One of the four endcap “Dees” will be precalibrated in the summer 2007 testbeam

Full scan of supermodule using high energy electron beam

Crystal response depends on electron impact position.4th order polynomial, separately in the 2 lateral coordinates used to correct for this dependence.

Only electrons incident on central 7mm 7mm of a given crystal are used.

ci defined as ratio of mean value of corrected response w.r.t. reference value

Statistical uncertainty with 1000 events/crystal < 0.1%

Intercalibration precision limited by response variations during time between testbeam measurement and LHC data taking.

Repeat precalibration of one module to quantify reproducibility



Calibration Using Cosmic Ray Muons

Calibrate using cosmic muons well aligned to crystal axes.

Select events with nearly all energy in one crystal

Supermodule exposed to cosmic muons for 41 hours in Nov 2004

Precision of 3% achievable in 1 week of data taking for barrel modules 1-3, and 3.5% for module 4.

data simulation

Agreement with test beam calibration:

σ 3%



In-Situ Intercalibration Using Physics Events

1: Phi-Independence



Phi Independence

-symmetry of event activity Total ET deposited by a large number of events

should be the same for all crystals at a given .

Can perform intercalibration by comparing ET deposited in a crystal with the

mean ET for all crystals at the same .

Aim: reduce the number of intercalibration constants from ~76000 (number of ECAL crystals) to 248 (number of fixed rings). Rings can then be intercalibrated using Ze+e- events.

y

x

Invariant quantity isET/A for endcap rings

170 barrel rings

39 endcap ring pairs



Phi Independence: Event trigger and Selection

Event trigger: Level-1 Jet trigger

Alternatives considered:Random bunch-crossings (minimum-bias):

• No trigger bias from the event trigger, but sensitivity to noise due to low energies, and large extrapolation from calibration energies (few hundred MeV) to physics energies

Electromagnetic triggers:• Trigger bias a severe issue

Assume 1kHz of Level-1 bandwidth allocated to single jet triggers

Event selection consists only of an ET threshold: ET> 120 GeV, chosen such

that the 1kHz bandwidth is approximately saturated at LHC startup luminosity.



Only crystals with ET in the following ranges contribute to the ET sums:

1 < ET < 6 GeV (barrel)

1 < ET < 4 GeV (endcap)

~10 crystal hits per event pass this selection.

Write out highly compacted data stream to be processed offline:Only information stored for each event is energy and crystal ID for each selected hit

Lower threshold excludes noise; upper threshold removes low statistics tail (improve stability of ET sum)

Eliminate trigger bias by excluding crystals associated with the triggering (highest ET) jet:

Require that crystals are separated from the position of the triggering jet by:

R = (2 + 2) > 1rad

Selection of Crystal Energy Deposits



Calculate ET (ET/A for endcap) for each crystal.

Obtain ET (mean value of ET for all crystals in pair of rings).

For each crystal:Calculate fractional deviation of ET from ET:

Since ET is obtained from a truncated ET distribution, is proportional rather than equal to the miscalibration. Constant of proportionality k determined empirically for each pair of rings (value is typically ~1.5).

Estimate of miscalibration:

Calibration coefficient:

To test calibration procedure:Gaussian miscalibration applied with spread 4.5%Determine residual miscalibration after correction

Determination of Calibration Coefficients

€

= ET∑ ET∑( ) −1

€

= ET∑ ET∑( ) −1

€

M = ε k

€

M = ε k

€

c i =1/(1+ εM )

€

c i =1/(1+ εM )11 million

events



Intercalibration Precision

Assumes no knowledge of tracker material distribution

Limit of the technique reached when inhomogeneity of tracker material and crystal geometry breaks -symmetry of energy deposition.



Limit is approached with a few tens of millions of events - equivalent to around 10 hours of data taking assuming 1kHz of Level-1 bandwidth allocated to single jet triggers.

With increasing knowledge of tracker material distribution, potential for rapid repeated calibration of the ECAL to high precision.

Limit on Precision



2: Single Electrons from W Bosons



Calibration with We: Introduction

Intercalibrate using the tracker momentum measurement for isolated electronsBenchmark technique to achieve target precision (0.5%)

Requires tracker is fully operational and well aligned

We: source of single electrons with a high HLT rate of ~10 Hz at 2x1033cm-2s-1

Main difficulty: Bremssrahlung radiated in tracker material degrades electron energy and momentum measurements

<10% radiated energy

No cut on radiated energy

E(5x5)/p(track)



ECAL Energy Measurement

ECAL energy measured by summing 5x5 array of crystals around crystal with maximum signal.

In the endcaps, energy deposited in the preshower is added

Choice of 5x5 rather than clustering algorithms designed to recover bremsstrahlung in standard electron reconstruction:

Cleanly separate intercalibration from complex algorithmic corrections required for bremsstrahlung recovery

Energy in 5x5 gives best measurement of energy for unconverted photons, for which calibration accuracy is most important (H)



Regional Calibration

Amount of bremsstrahlung depends on amount of material

varies significantly as a function of .

Average value of E/p distribution therefore also varies with .

Divide intercalibration task into 2 steps:1) Intercalibrate within small regions for which <E/p> is rather constant

• Can be achieved rapidly for each region due to the reduced no. of constants to be determined

2) Intercalibrate between regions.• Use very tight electron selection requiring

minimal bremsstrahlung energy loss



Event Selection

Event selection based on variables correlated with amount of bremsstrahlung emission. Electrons with little or no radiated bremsstrahlung have:

More accurate reconstruction of energy and momentum

Most of their energy deposited in 5x5 crystal array

Barrel selection variables: E(5x5)/p(track), plus:

E(3x3)/E(5x5)No. of hits in

the track2/n.d.f.

of the track



Endcap Event Selection

Selection variables used in the endcaps:E(5x5)/p(track)

E(3x3)/E(5x5)

Ratio of track momentum at outermost and innermost points: pout/pin

Fraction of energy radiated beforeradius = 80cm



Selection Cut Optimization

Selection cuts chosen by scanning 4 dimensional phase space and identifying point (global minimum) giving best precision in determination of calibration constants.

Global minimum found to be broadi.e. stable - insensitive to potential differences between simulation, used to derive the cuts, and real data used to perform calibraition

Sensitivity of minimum on no. of events per crystal also found to be small

Selection cuts optimized separately for the different regions

Selection efficiency varies with Mean efficiency in barel: 30%

Efficiency in endcap ranges from 10% to 30%

E(5x5)/p(track)(Endcap)

Selectedevents



Calibration Algorithms

Each energy measurement contains contribution from 25 crystals. Two techniques have been studied to extract calibration constants for individual crystals:

1) L3 iterative algorithm:• Used for in-situ calibration of the

BGO crystals in the L3 experiment at LEP

2) Matrix inversion algorithm:• Householder decomposition

or 2 minimization

Based on minimization of difference between EECAL and ptrack

Both techniques perform similarly, both in terms of precision and speed.

L3 iterative algorithm




To test calibration procedure:Gaussian miscalibration applied with spread 4%.

Many (>50) MC experiments performed each with different randomly chosen miscalibration constants

Intercalibration precision vs :

barrelendcaps




Dependency on no. of electrons per crystal:

1.31<<1.481.31<<1.48

0.78<<0.960.78<<0.96

0<<0.260<<0.26



HLT Rates and Background Contamination

Most of the background rate comes from b/ce decays.

Since these are real electrons, can still be useful for calibration

Otherwise Can be strongly suppressed with negligible loss of signal efficiency using isolation cuts

We

background

No isolation cut



3: Electrons from Z Bosons



Calibration with Zee: Introduction

Z mass constraint in Zee events is a powerful tool for ECAL calibration

Phi independence and We concerned only with intercalibration. Zee can also be used to determine the global energy scale.

Independent of the tracker measurements: can be used from the beginning of the data taking.

Several uses are envisaged:

Intercalibration between rings previously intercalibrated using phi-independence.

Determination of global energy scale.

Tuning of algorithmic (clustering) energy scale corrections for reconstructed electrons and photons, currently determined using Monte Carlo.

Measurement of electron trigger, reconstruction and identification efficiencies using “tag and probe” approach.

€


cluster

∑ × Ai

€


cluster

∑ × Ai



Extraction of Calibration Coefficients

Measured invariant mass:

Weighted mean of miscalibration factors in event i:

For each ring, plot distribution of <>i, with each entry weighted by the fraction of reconstructed electron energy contained in the ring in that event

Estimate of miscalibration for the ring given by position of peak obtained from Gaussian fit

€

< >i=1

2⋅

M invi

MZ

⎛

⎝ ⎜

⎞

⎠ ⎟

2

−1 ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

€

< >i=1

2⋅

M invi

MZ

⎛

⎝ ⎜

⎞

⎠ ⎟

2

−1 ⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

€

M inv2 = 4E1 ⋅E2 ⋅sin2 ϑ 12

2

€

M inv2 = 4E1 ⋅E2 ⋅sin2 ϑ 12

2



Extraction of Calibration Coefficients

Procedure is iterated until the calibration coefficients converge:

Calibration coefficient for an individual crystal is the product of the ring coefficient C() and the relative coefficient for the crystal within the ring obtained from Phi Independence:€

C j = c jk

iteration,k=1

n

∏ =1

1+ ε jk

iteration,k=1

n

∏

€

C j = c jk

iteration,k=1

n

∏ =1

1+ ε jk

iteration,k=1

n

∏

€

C(η ,ϕ ) = C(η ) ⋅CηPhiInv (ϕ )

€

C(η ,ϕ ) = C(η ) ⋅CηPhiInv (ϕ )



Intercalibration of Crystal Rings

Crystals within rings already intercalibrated using phi independence

Event selection: require two “golden” electrons (i.e. little bremsstrahlung radiation), in order to minimize dependence on tracker material and hence

Set up miscalibrations:2% miscalibration between crystals within a ring (precision obtained after -symmetry intercalibration)

5% miscalibration between rings

Use simulated event sample corresponding to an integrated luminosity of 2.0fb-1

Intercalibration precision is RMS

spread of residual miscalibration

after correction: σ = 0.6%σ = 0.6%



Intercalibration Precision Using Zee

Intercalibration precision with 2 fb-1 for different ECAL barrel modules (increasing in ):

Intercalibration precision as function of event statistics:

2 fb-1



4: Inner Bremsstrahlung Photons in Z



Z: Introduction

Radiative decays of Z bosons to muons:Clean source of high pT isolated photons with significant rate and very little background

Energy scale determination independent of the ECAL

Z is an important tool for several commissioning tasks:Calibration between regions previously intercalibrated using We.

Determination of overall energy scale, using Z mass constraint

Probe for measuring photon trigger, reconstruction and identification efficiencies

Tuning of algorithmic (clustering) energy scale corrections for photons and electrons (from photon conversions).



Z: Preliminary Studies

Investigated using events generated with full matrix element calculation of radiative decays

Separate generation using ALPGEN and CompHEP

Background: Z bosons produced with additional jet(s)

Event selection:Muon pair invariant mass:

40 < M < 80 GeV

Reconstructed photon with pT > 15 GeV,within R < 0.8 of either muon3 body invariant mass:

87.2 < M < 95.2 GeV

Signal to background ratio:~80 for 15 < ET

< 30 GeV

~1 photon per crystal for 1fb-1 of data



5. Low Mass Resonances



Calibration using 0

0and currently being studied as additional calibration tools

Unconverted photons insensitive to tracker material distribution:No dependence on

Minimum separation for photons from 0 in the barrel with ET=5GeV is ~65mm

3 times crystal granularity

For QCD events accepted by the Level-1 triggers, perform ECAL cluster reconstruction in small region identified by the trigger, using an online filter farm.

Event selection:Require 2 ECAL clusters with energy in range 1.5 < E < 5 GeV and separation at ECAL front face between 60 and 90mm.

Tight requirements on ECAL shower shape to select unconverted photons

Assuming a Level-1 trigger rate of 10 kHz:Rate after selection >1000 0s/crystal per day of data taking at 2x1033cm-2s-1

signal-to-background ratio ~2



Summary

Accurate intercalibration of the CMS ECAL to the design goal of 0.5% essential for physics discovery reach, in particular for the H channel.

Can only be achieved in-situ using physics events.

High pT isolated electrons produced from decay of W and Z bosons are key to

achieving this goal.

Baseline strategy:Phi independence of energy from jet trigger events to intercalibrate rapidly within rings at startup to 2% precision (< 1 day)

Electrons from Z to intercalibrate between rings

Once tracker is fully operational and aligned, design goal precision of 0.5% can be achieved with 5 fb-1 of data using single electrons from We

coseners house forum on lhc startup 13th april 2007 david futyan imperial college 1 david futyan...

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