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Calorimeter Calibration and Jet Energy Scale
Jean-Francois Arguin
November 28th, 2005
Physics 252B, UC Davis
Outline
Quick remainder of calorimetry Calibration before the experiment starts: test beam Calibration when the experiment is running:
Hardware calibration Collider data
Measuring jets at high-energy colliders Example of a physics measurement: top quark
mass
Basics of Calorimetry
Incident particle creates a shower inside material Shower can be either
electromagnetic or hadronic
Energy is deposited in material through ionization/excitation
Basics of Calorimetry II
Basic principle of calorimetry: deposited energy is proportional to incident energy
Calorimeter calibration translate detector response to incident energy
Great feature of showers for detector use: length is proportional to logE
Electromagnetic showers
Created by incident photon and electron electrons emit
bremstrahlung photons undergo pair
production
Length of shower expressed in term of X
0
X0 depends on material
95% containment requires typically about 20X
0
Created by incident charged pion, kaon, proton, etc Typical composition:
50% EM (e.g. ) 25% Visible non-EM energy 25% invisible energy (nuclear break-ups)
Requires longer containment (expressed in λ)
Hadronic showers
0
Calorimeter detectors
Detector hardware must: 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
Example of electron shower with lead absorber:
Sampling calorimetry (Ex.: CDF)
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
Calorimeter is segmented into towers that are read-out independently
Lead (iron) interspersed with scintillators for EM (HAD) calorimeters
Each central tower covers
Each tower has an EM calorimeter followed by an HAD calorimeter
151.0
CDF Calorimeters
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
Go on and built the thing after it is designed! Many institutions in the world participate
First calibration: test beam
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 Set absolute scale for all
towers Relative scale for other
towers obtained later
Wedge getting ready to receive beam:
How does the test beam works (Ex.: plug calorimeter)
Performed at Fermilab meson beam facilities
Beams characteristics: Various types for EM and HAD
showers: electrons, pions, muons Various energy: 5-120 GeV (electrons),
5-220 GeV (pions)
Beams can be contaminated → bias the calibration constants E.g. use Cherenkov detector in front of
calorimeter to identify proton contamination in pion beam
WhyMuons?
Calorimeter response linearity
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:
– HAD: EEE /%80/
EEE /%14/
Final detector assembly: getting ready for physics!
The Tevatron
Proton-antiproton collisions at Most energetic collider in the
world Collisions every 0.4 μs Circumference of 6.3 km
TeVs 96.1
The CDF Detector
CDF II: general purpose CDF II: general purpose solenoidal detectorsolenoidal detector
7 layers of silicon tracking – Vertexing, B-tagging
COT: drift chamber– coverage – Resolution:
Muon chambers– Proportional chamber
interspersed with absorber– Provide muon ID up-to
Calorimeters Central, wall, plug calorimeter
1|| %1.0/ 2 Tp p
T
5.1||
Calibration when the detector is installed
Only a few towers saw test beam, how to calibrate the whole thing?? Test beam sets the absolute scale as a function energy Two solutions:
Hardware calibrations Physics calibration (using collider data)
These calibrations need to: 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
Can use radioactive sources that have very well defined decay energy 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
The lasers are connected directly to PMTs Skip scintillator/WLS steps
Used to uniformize PMTs response over towers and time
Physics calibrations
Use real collider data For calibration, you have to have some “known” and
some “unknown” (the calorimeter response) Examples of “known” information:
Mass of a well-known particles Ex.: Z→ee (Z mass measured at LEP)
Energy deposited by muons over a given length Muon sample
Energy measured in tracker (assuming tracker in calibrated) Redundant to energy measured in calorimeter for electrons
Example: Z boson mass
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
Z mass peak:
Example: E/p of electrons
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
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
Calorimeter calibration not the only issue
Electron/photon physics also rely on tracking
Removal of background E.g. remove pion
background by studying shower shape
Search for new physics: Z' candidate:
Precision measurement: W mass:
What are jets?
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
Quark/gluon produced from ppbar interaction
Fragmentation into hadrons Jets clustering algorithm:
Adds towers inside cone
Fraction of energy is out-of-cone
Underlying event contributes
Jet versus calorimeter energy scale
Jets are complicated processes Previous calorimeter calibrations are not sufficient to get
calibrated jet energy More work needs to be done!!
Jet energy scale is crucial for many important measurements: Top quark mass (used to constrain Higgs boson) Jet cross-sections (comparison to QCD predictions)
Measurements often performed by comparing real data with simulations Need to get both physics and detector simulation right
Relative energy scale
Jet energy measurement depend on location in detector
True even after all previous calibrations!
How come? Jets are wide Some regions of CDF
calorimeter are not instrumented
Relative energy scale: Use QCD dijet events Should have equal
transverse momentum
Absolute energy scale
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: One 50 GeV pion 10 times 5 GeV pions
Solution: Get the average energy scale
Simulate an “average” particles configuration inside jet
Use test beam information to get calibration factor for single particles
Out-of-cone energy
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 How well is the physics
generator representing fragmentation?
Underlying event energy
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
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
Top produced in pairs at Tevatron
Top decays to W boson and b-quark 100% of time in SM
Typical event selections: 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:
• 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
jjjjltt
Top mass reconstruction
Event-by-event kinematic fitter (assumes event is ttbar)
Attempts all jet-parton assignments Assign b-tag jets to b-
quarks
The one most consistent with ttbar hypothesis is kept
More correct combinations with b-tags!
The strategy
Construct reconstructed top mass distributions for many true top mass So-called “templates”
Compare distribution reconstructed in data with templates Using likelihood fit
Account for background contamination Dominated by W+jets
production
The measurement (spring 2005)
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
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?? No: Mjj (almost) independent
of Mtop
Remaining correlations are accounted for
The measurement(adding W→jj information)
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
Mtop, MW connected to Higgs boson mass through radiative corrections
MH< 186 GeV/c2 @ 95%C.L.
Can constrain mass of supersymmetric particles
Conclusion
Detector calibration needed to translate detector response in energy
Various techniques used for calorimetry: Test beam Radioactive sources Lasers Collider data
Calorimeter can be used to measure: Electrons, photons, jets, missing ET
Good calorimeter and jet calibration needed for measurements like top quark mass
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