calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_calib.pdf · r....

Calorimetry in particle physicsexperiments

Unit n. 8Calibration techniques

Roberta Arcidiacono

R. Arcidiacono Calorimetria a LHC 2

Lecture overview

● Introduction● Hardware Calibration● Test Beam Calibration● In-situ Calibration (EM calorimeters)


Introduction

The goal of the calibration strategy is to achieve the mostaccurate energy measurement for the particlesabsorbed by the calorimeter.

ADC counts MeV

conversion factor

Relationship between the energy deposited and theresulting calorimeter signal


Introduction

In general, one can always parametrize the energydeposited inside one em or hadronic shower as

Ep = G Fp i ci si Ai

Ai Single channel amplitude

si Single channel time dependent correction for response variations

ci Intercalibration coefficient (IC): relative single channel response

Fp Particle energy correction (geometry, clustering, etc…)

G Global scale calibration (absolute energy scale)


Introduction

The calibration strategy includes:

● Calibration of all hardware components: electronicchain, detector modules

– Specific calibration systems designed for theread-out chain

– Specific Test Beams studies carried on with allor part of the calorimeter modules

● Continuous monitoring of the calibration constantsthroughout the lifetime of the calorimeter, in theexperimental set-up (in-situ calibration)


Hardware calibration

Used to equalize and monitor the cell-to-cell responseof the detector and associated electronics, and totrack the time-variations of the response

● The electronics calibration system injects a knownpulse at the input of the readout chain (typically at pre-amplifier - PA - level). Channel-to-channel dispersionsas small as 0.2% can be achieved (precision andstability of the calibration system are essential)

● However, this system does not allow a calibration of thedetector response, for which other devices (e.g. lasers,radioactive sources) are used, that inject a well-knownlight or charge signal into the active elements of thedetector


Hardware calibration

● For Gain Switching PA: calibration system plays a crucial rolein monitoring the gain stability and the gain offset values

● Calibration pulses are typically issued regularly during datataking, in allocated time slots without physics

– ex: in SPS cycles: calibration time is right after the 3s physicsspill - extra 0.5 s every cycle (14 s);

– ex: in LHC cycles: using LHC gaps (1 of ~ 3m s every ~ 3200Bunch crossings)

● radioactive sources used to calibrate hadron calorimeters orcalorimeter designed for low energies, or to track transparencychanges in crystals (→ light transmission curve)

– Radioactive sources have very well defined decay energy [ Cobalt 60(2.8 MeV) Cesium 137 (1.2 MeV) ]


Test Beam calibration

Usually some (or ALL) calorimeter modules are exposed totest beams (like e- / pion /muon beams) before beinginstalled in the final detector

Among the reasons:

● Commission the hardware/read-out/software systemsaround a detector; study detector performance

● compute a first set of inter-calibration constants

● set the preliminary absolute energy scale [ G ] forelectrons and pions, given that the incident beam energyis well known.


In-situ calibration

In-situ calibration is performed with physics samples.

Every calorimeter needs to be calibrated (re-calibrated) afterinstallation in the experimental hall.

The experimental environment (ex. presence of material in frontof the calorimeter) is different from test beam environment,and it is not seen by the hardware calibration.

Also, calorimeter response to jets and the missing transverseenergy cannot be measured at the test beam where only singleparticles are available.

Finally, calibration stability has to be monitored.


In-situ calibration (2)

In-situ calibration allows correction of residual nonuniformities, to follow the detector responsevariations with time, and to set the final absoluteenergy scale under experimental conditions.

Well-known control physics samples (having highbranching ratios) are used, such as:

● 0/ , Ke3 (KL e)

● J/ψ,Z ee , W e (collider)● W jj (collider)

or, when possible (fixed-target), calibration electron beams


Muons for HCAL as well...

● Energy deposited by muons over a given length is awell-known quantity (MIPs)

● Muons calibrate detector response to ionization energy● Muons are also perfect to calibrate calorimeters with a

longitudinal segmentation, with EM and HAD parts→ use of muons from J/ψ events, to have a sample ofa well identified energy


Setting the absolute energy scale

Issue of setting the absolute energy scale:

– Hadron colliders - see next

– e+e- colliders● precise knowledge of the center-of-mass

energy provides useful constraints andrenders this operation easier

– fixed target experiments● use decays of well-known particles


Absolute Energy Scale: Hadron colliders

The electromagnetic absolute energy scale at hadron colliders is set

mainly by using well-known resonances such as 0/ , J/ ee, ee in the low-energy range and Z ee athigher energies.

Resonance Mass error o

0 134.9766 0.0006 MeV (8.4 ± 0.6) 10−17 s

547.853 0.024 MeV 1.30 ± 0.07 keV

J/ 3096.916 0.011 MeV 93.2 ± 2.1 keV

Z 91.1876 0.0021 GeV 2.4952 GeV

W 80.398 0.025 GeV 2.141 GeV

= ℏ/


Absolute Energy Scale: Hadron colliders

Another method: transfer the energy scale from the tracker to theelectromagnetic calorimeter by measuring the E/p ratio for isolatedelectrons (E from calorimeter, p from tracker)

The tracker momentum scale in the inner tracker is calibrated by using

isolated muons, (ex: from Z decays)

For the electron momentum scale, Monte Carlo simulation of thetracker material distribution is used to compute the electron energylosses (bremsstrahlung) and hence obtain the initial electronmomentum.

Finally, the momentum scale is transferred to the calorimeter byadjusting the E/p distribution for electrons to 1.


Ex: D0 calorimeter calibration

Energy scale is calibrated by using Z ee events:

Etrue = Emeas +

where Emeas is the electron energy measured in the

calorimeter and the parameters and are varied untilthe reconstructed Z mass peak agrees with the nominalvalue.

In RUN I , Z peak was ;5% below the nominal mass. Wronginitial scale because no module of the final D0 centralcalorimeter was calibrated with test beams ⇒ indicatesthe importance of performing test beam measurements,to keep the energy correction factors minimal.


Ex: D0 and CDF

The di-electron mass spectrum reconstructed inthe D0 central calorimeter before the finalenergy scale calibration, Zee data sample(Abbott et al.,1998).

Both D0 and CDF achieved a precision on the absolute electronenergy scale of ~0.1%.

The E/p ratio for isolated electrons from W

decays as obtained from the CDF run-IBdata (Abe et al., 1995; Kim, 1999).


Ex: D0 and CDF

Absolute Energy scale precision was limited by:

● the statistics of the physics samples used to calibrate themass peak or the E/p peak.

● systematic uncertainties: dominant sources are theincomplete knowledge of the dead material, calorimeterresponse non-linearities, the knowledge of the mass of theresonance used, and radiative Z decays ( Z → eeg with lowenergy undetected photons) .


Ex: effect of calibration in CMS ECALhttp://arxiv.org/pdf/1306.2016.pdf


On CMS ECAL calibration procedure

0

10

20

30

40

50

60

70

80

300 350 400 450 500 550 600 650 700

initial

after irradiation

wavelength (nm)

T(%

)

)(T

)(Tln

L

1)(

rad

0

xtl

Optical Transmission curve

Light loss is characterized by an‘induced absorption’

M1 M2 M3 M4

ZH4


On CMS ECAL calibration procedure


Measurement of Jets

● Jet is a collimated group ofparticles that result from thefragmentation of quarksand gluons

● measured as clusters bythe calorimeters

● Previous calorimetercalibrations are notsufficient to get calibratedjet energy


Jet Energy Scale

The setting of the energy scale of the jet, inferring the original partonenergy from the measured jet debris, is more complex than the setting ofthe electron scale: there are more numerous (and more difficult tocontrol) sources of uncertainties

Samples used at hadron colliders:

● events with associated production of a single jet with a photonor a boson, like Z → ll

If there is only one jet and one boson in the event, then theboson and the jet must have equal and opposite momenta inthe plane transverse to the beam ( ∑ET = 0 )

The transverse momentum of the photon or Z particle can bedetermined with high precision


Jet energy calibration● Response to single pion non-

linear (in test beam)● For a 50 GeV jet: calibration is

not the same whether:– One 50 GeV pion– 10 times 5 GeV pion

● Solution:

– Get the average energyscale

– Simulate an “average”particles configurationinside jet

● Jet energy measurementdepend on location indetector and relativeenergy scale

Another sample: QCD dijet events,should have equal transverse momentum


Jet energy scale

Corrections:● Out-of-cone energy

– Cone of fixed radius used to identify jets

– Need to correct for fraction of energy out-of-cone (typically15%)

● Underlying event– Spoils jet energy measurement

– Depends on the number of primary interactions per event

– Extracted from “minimum bias” events


Final JES uncertainty

● Dominated by out-of-cone (low-pT) and absoluteenergy scale (high-pT)

● Ranges from 10% to 3%

W→jj calibration● In ttbar events, invariant mass of two jets from W boson decay

should be equal to MW

● Can use W→jj decays to further constraint JES

calorimetry in particle physics experimentspersonalpages.to.infn.it/~arcidiac/calo_calib.pdf · r....

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