Calorimeter Calibration and
Jet Energy Scale
Jean-Francois Arguin
November 28th, 2005
Physics 252B, UC Davis
Outline
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Quick remainder of calorimetry
Calibration before the experiment starts: test beam
Calibration when the experiment is running:
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Hardware calibration
Collider data
Measuring jets at high-energy colliders
Example of a physics measurement: top quark
mass
Basics of Calorimetry
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Incident particle creates a
shower inside material
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Shower can be either
electromagnetic or
hadronic
Energy is deposited in
material through
ionization/excitation
Basics of Calorimetry II
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Basic principle of
calorimetry:
deposited energy is
proportional to incident
energy
Calorimeter calibration
translate detector
response to incident
energy
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Great feature of showers
for detector use: length is
proportional to logE
Electromagnetic showers
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Created by incident
photon and electron
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electrons emit
bremstrahlung
photons undergo pair
production
Length of shower
expressed in term of X0
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X0 depends on material
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95% containment requires
typically about 20X0
Hadronic showers
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Created by incident charged pion, kaon, proton, etc
Typical composition:
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50% EM (e.g. 0
)
25% Visible non-EM energy
25% invisible energy (nuclear break-ups)
Requires longer containment (expressed in λ)
Calorimeter detectors
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Detector hardware must:
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Favor shower development
Collect deposited energy
Can do both at the same
time (e.g. BaBar/Belle
crystal calorimeters)
Or have calorimeters with
alternating passive and
sensitive material
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Example of electron
shower with lead
absorber:
Sampling calorimetry (Ex.: CDF)
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Scintillators (sensitive
material) emit lights with
passage of ionizing
particles
Collect light deposited in
sensitive material using
wavelength shifter (WLS)
WLS → photomultipliers
that convert light into
electric signal
CDF Calorimeters Segmentation
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Calorimeter is segmented
into towers that are readout independently
Lead (iron) interspersed
with scintillators for EM
(HAD) calorimeters
Each central tower covers
    0.1  15
Each tower has an EM
calorimeter followed by an
HAD calorimeter
CDF Calorimeters
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Three regions: central,
wall and plug
Use “projective” geometry
Designed to measure
electrons, photons,
quarks, gluons, hadrons,
neutrinos
Note: design of
calorimeter performed
with a simulation of the
most important processes
you plan to measure (ex.:
Higgs at LHC)
Construction
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Go on and built the thing after it is designed!
Many institutions in the world participate
First calibration: test beam
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Take one calorimeter
“wedge”, send beam of
particles with known
energy
Obtain correspondence
detector response →
energy in GeV
A few towers only
submitted to test beam
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Set absolute scale for all
towers
Relative scale for other
towers obtained later
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Wedge getting ready to
receive beam:
How does the test beam works
(Ex.: plug calorimeter)
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Performed at Fermilab meson beam
facilities
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Beams characteristics:
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Various types for EM and HAD
showers: electrons, pions, muons
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Various energy: 5-120 GeV (electrons),
5-220 GeV (pions)
Beams can be contaminated → bias
the calibration constants
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E.g. use Cherenkov detector in front of
calorimeter to identify proton
contamination in pion beam
Why
Muons?
Calorimeter response linearity
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Extract calibration constant
for many energy point
Can test linearity of
calorimeter
Can add “artificial” material
in front of calorimeter to
simulate tracker+magnet
material
Send pions and electrons
to hadronic calorimeter
Why sending
electrons in
hadron
calorimeter?
Performance determined from test
beam
• From RMS of tower
response to same beam
energy → measure
calorimeter resolution
• Can test tower transverse
uniformity (influences
resolution)
• Stochastic term resolution:
– EM:  E / E  14% / E
– HAD:  / E  80% / E
E
Final detector assembly: getting
ready for physics!
The Tevatron
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Proton-antiproton collisions at
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Most energetic collider in the
world
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Collisions every 0.4 μs
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Circumference of 6.3 km
s  1.96TeV
The CDF Detector
CDF II: general purpose
solenoidal detector
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7 layers of silicon tracking
–
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COT: drift chamber
–
–
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coverage |  | 1
2

/
p
Resolution: pT T  0.1%
Muon chambers
–
–
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Vertexing, B-tagging
Proportional chamber
interspersed with absorber
Provide muon ID up-to |  | 1.5
Calorimeters
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Central, wall, plug calorimeter
Calibration when the detector is
installed
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Only a few towers saw test beam, how to calibrate the
whole thing??
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Test beam sets the absolute scale as a function energy
Two solutions:
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Hardware calibrations
Physics calibration (using collider data)
These calibrations need to:
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Cross-check absolute scale (e.g. test beam not 100% realistic)
Track detector response through time
 Expected degradation of scintillator and PMT
 PMT sensitive to temperature
Uniform response through all towers
Hardware calibration
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Can use radioactive sources
that have very well defined
decay energy
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Cobalt 60 (2.8 MeV)
Cesium 137 (1.2 MeV)
Source calibration can be
performed between colliders run
Sources are movable and can
expose one tower at a time
Check uniformity over all towers
and over time
Sources are sensitive to both
scintillator and PMT responses
Laser calibrations
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The lasers are connected
directly to PMTs
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Skip scintillator/WLS steps
Used to uniformize PMTs
response over towers and
time
Physics calibrations
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Use real collider data
For calibration, you have to have some “known” and
some “unknown” (the calorimeter response)
Examples of “known” information:
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Mass of a well-known particles
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Energy deposited by muons over a given length
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Ex.: Z→ee (Z mass measured at LEP)
Muon sample
Energy measured in tracker (assuming tracker in calibrated)
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Redundant to energy measured in calorimeter for electrons
Example: Z boson mass
Z mass peak:
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Z mass measured with
great accuracy at LEP
using beam energy
Background is very small
for Z→ee
Sample is relatively small,
but good enough
Example: E/p of electrons
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Used for relative scale
over towers
Cannot be used in forward
region (no tracker)
In plug: rely on sourcing
and lasers
Example: muons for HAD
calorimeters
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Muon calibrate detector
response to ionizing
energy
Use muon from J/ψ for
identification (mass not
used like Z boson)
Again, not used for PHA
(rely on sourcing, laser)
Physics with photons/electrons
Search for new physics: Z' candidate:
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Calorimeter calibration not
the only issue
Electron/photon physics
also rely on tracking
Removal of background
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E.g. remove pion
background by studying
shower shape
Precision measurement: W mass:
What are jets?
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Jets are a collimated
group of particles that
result from the
fragmentation of quarks
and gluons
They are measured as
clusters in the calorimeter
momentum of cluster of
towers is correlated with
the momentum of the
original quark and lepton
Why not
using tracker
(has better
resolution)?
Phenomenology of jets
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Quark/gluon produced from
ppbar interaction
Fragmentation into hadrons
Jets clustering algorithm:
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Adds towers inside cone
Fraction of energy is out-ofcone
Underlying event
contributes
Jet versus calorimeter
energy scale
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Jets are complicated processes
Previous calorimeter calibrations are not sufficient to get
calibrated jet energy
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Jet energy scale is crucial for many important
measurements:
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More work needs to be done!!
Top quark mass (used to constrain Higgs boson)
Jet cross-sections (comparison to QCD predictions)
Measurements often performed by comparing real data
with simulations
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Need to get both physics and detector simulation right
Relative energy scale
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Jet energy measurement
depend on location in
detector
True even after all
previous calibrations!
How come?
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Jets are wide
Some regions of CDF
calorimeter are not
instrumented
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Relative energy scale:
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Use QCD dijet events
Should have equal
transverse momentum
Absolute energy scale
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Response to single pion
non-linear (in test beam)
However, jets are identified
as one single objects
For a 50 GeV jet:
calibration is not the same
whether:
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One 50 GeV pion
10 times 5 GeV pions
Solution:
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Get the average energy scale
Simulate an “average” particles
configuration inside jet
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Use test beam information to get
calibration factor for single
particles
Out-of-cone energy
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Cone of fixed radius used
to identify jets
Need to correct for fraction
of energy out-of-cone
(typically 15%)
This is mostly physics
related
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How well is the physics
generator representing
fragmentation?
Underlying event energy
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Proton/antiproton
remnants splash energy in
calorimeter
Spoils jet energy
measurement
Depends on the number of
ppbar interaction per event
Extracted from “minimum
bias” events
Small effect: ~0.4 GeV per
jet
Final jet energy scale uncertainty
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Estimate of jet energy scale
uncertainty is important to
estimate systematic
uncertainties of
measurements
Dominated by out-of-cone
(low-pT) and absolute
energy scale (high-pT)
Ranges from 10% to 3%
energy uncertainties
Example physics measurement:
top quark mass
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Top produced in pairs at
Tevatron
Top decays to W boson
and b-quark 100% of time
in SM
Typical event selections:
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Well-identified electron(s) or
muon(s)
Large missing ET
Several reconstructed jets
identified in calorimeters
Note: 4 jets in final state!
Identification of b-quark jets
• Complicated final state:
tt  ljjjj
• Which jets come from
which parton?
• Can identify b-quark jets
using one characteristic:
– Long b-quark lifetime
• Note: lots of semileptonc
B-hadrons decay
(involving neutrino)
– Require special b-jets
calibration
Top mass reconstruction
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Event-by-event kinematic
fitter (assumes event is
ttbar)
Attempts all jet-parton
assignments
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Assign b-tag jets to bquarks
The one most consistent
with ttbar hypothesis is
kept
More correct
combinations
with b-tags!
The strategy
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Construct reconstructed top
mass distributions for many
true top mass
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Compare distribution
reconstructed in data with
templates
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So-called “templates”
Using likelihood fit
Account for background
contamination
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Dominated by W+jets
production
The measurement (spring 2005)
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Using 138 candidate ttbar events, fit yields:
Mtop= 173.2 +2.9/-2.8 (stat.) +/- 3.4 (syst.) GeV/c2
By shifting by JES
uncertainty defined
before: Mtop
changes by 3.1
GeV/c !
JES uncertainty
limiting factor for
Mtop measurement
Improvement: W→jj calibration
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Inside ttbar events,
invariant mass of two jets
from W boson decay
should equal MW
Can use W→jj decays to
further constraint JES
Use same data for
measurement and
calibration… cheating??
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No: Mjj (almost) independent
of Mtop
Remaining correlations are
accounted for
The measurement
(adding W→jj information)
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Using same dataset as previously:
Mtop= 173.5 +2.7/-2.6 (stat.) +/- 2.8 (syst.) GeV/c2
Total Mtop
uncertainty
improved by 10%
JES uncertainty
decreased by 20%
Good prospect for
future
Impact of Mtop measurement
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Mtop, MW connected to
Higgs boson mass through
radiative corrections
MH< 186 GeV/c2 @ 95%C.L.
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Can constrain mass of
supersymmetric particles
Conclusion
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Detector calibration needed to translate detector
response in energy
Various techniques used for calorimetry:
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Calorimeter can be used to measure:
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Test beam
Radioactive sources
Lasers
Collider data
Electrons, photons, jets, missing ET
Good calorimeter and jet calibration needed for
measurements like top quark mass