lectures on calorimetry - otranto 2016

The Art of Calorimetry

Michele Livan Pavia University and INFN

lectures given at the XXVIII Seminario Nazionale di Fisica Nucleare e Subnucleare

Otranto 4-9 June 2016

M. LivanThe Art of Calorimetry

Lecture I

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M. Livan Pavia University & INFN

✦ Very compact introduction to and overview of calorimetric particle detection

✦ Main focus on the physics of calorimetric measurements, very little on present and past techniques

✦ Topics:

✦ The development of electromagnetic and hadron showers

✦ Energy response and compensation

✦ Fluctuations and energy resolution

✦ The state of the art (towards future colliders)2


✦ References

✦ Wigmans, R. (2000). Calorimetry: Energy measurement in particle physics. International Series of Monographs on Physics Vol. 107, Oxford University Press. New edition coming in 2017 !

✦ G. Gaudio, M. Livan, R.Wigmans The Art of Calorimetry. Proceeding of the International School of Physics “ Enrico Fermi”. Varenna, July 2010. IOS Press - Volume 175 (ask for the pdf)

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✦ Acknowledgments

✦ Thanks to Richard Wigmans for many useful discussions and for allowing me to use his tutorial given at the 2014 CALOR Conference as inspiration for part of these lectures.

✦ Thanks to Richard also for allowing me to use some unpublished material from the forthcoming second edition of his book.

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Why Calorimeters ?✦ Sensitive to both charged and neutral particles

✦ Differences in the shower patterns ⇒ some particle identification is possible (h/e/µ/ν(missing ET) separation)

✦ Calorimetry based on statistical processes

✦ ⇒ σ(E)/E ∝ 1/√E

✦ Magnetic spectrometers ⇒ Δp/p ∝ p

✦ Increasing energy ⇒ calorimeter dimensions ∝ logE to contain showers

✦ Fast: response times < 100 ns feasible

✦ No magnetic field needed to measure E

✦ High segmentation possible ⇒ precise measurement of the direction of incoming particles

✦ In modern colliders, calorimeters form the heart and the soul of the experiments5

The physics of shower development✦ Although calorimeters are intended to measure energy deposits at

the level of 109 eV and up, their performance is in practice determined by what happens at the MeV, keV and sometimes eV levels.

✦ Since showers initiated by hadrons, such as protons and pions are distinctly different (and in particular more complicated) than the electromagnetic (em) ones initiated by electrons or photons, we will start from the latter

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Electromagnetic showers development

7


Summary of em processes

Z(Z + 1)

Z(Z + 1)

Z

Z Z4÷5

Critical energy εc

✏c =610Mev

Z + 1.24

dE

dx

��Brems

=dE

dx

��ion

8


Photon absorption

✦ Energy domains in which photoelectric effect, Compton scattering and pair production are the most likely processes to occur as a function of the Z value of the absorber material 9


Z dependence

10


Cross sections

Gammas

Electrons

11

Simple shower model✦ Consider only Bremsstrahlung and (symmetric) pair

production

✦ Assume X0 ∼ λpair

✦ After t X0:

✦ N(t) = 2t

✦ E(t)/particle = E0/2t

✦ Process continues until E(t)<Ec

✦ E(tmax) = E0/2tmax = Ec

✦ tmax = ln(E0/Ec)/ln2

✦ Nmax ≈ E0/Ec

12

Electromagnetic shower profiles (longitudinal)

Depth of shower max increases logarithmically

with energy

13

X0 = A

4↵NAZ2A z2r2

eE ln 183

Z13

The radiation length X0 is the

scale for the longitudinal

development of an em shower.

X0 (in g/cm2)scales as A/Z2

Electromagnetic Showers✦ Longitudinal development governed by radiation length (X0)

✦ Defined only for GeV regime

✦ there are important differences between showers induced by e, γ:

✦ e.g. Leakage fluctuations, effects of material upstream, ....

✦ Mean free path of γs = 9/7 X0

Distribution of energy fraction deposited in the first 5 X0 by 10 GeV electrons and γs showering in Pb.Results of EGS4 simulations

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Lateral profileRadial distributions of the energy deposited by 10 GeV electron showers in Cu.

Results of EGS4 simulations

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The Molière radius ρM is the scale for the lateral development of an em

shower.

ρM = X0/εcρM (in g/cm2)scales as A/Z

Electromagnetic Showers✦ Differences between high-Z/low-Z materials

✦ Energy at which radiation becomes dominant

✦ Energy at which photoelectric effect becomes dominant

✦ Energy at which pair production becomes dominant

✦ Showers ⇒ Particle multiplication ⇒ little material needed to contain shower

✦ 100 GeV electrons: 90% of shower energy contained in 4 kg of lead

✦ Shower particle multiplicity reaches maximum at shower maximum

✦ Depth of shower maximum shifts logarithmically with energy

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Electromagnetic Showers✦ Scaling with X0 is not perfect

✦ In high-Z materials, particle multiplication continues longer and decreases more slowly than in low-Z materials

✦ EC ∝ Z-1

✦ The number of positrons strongly increases with the Z value of the absorber material

✦ Example: number of e+/GeV in Pb is 3 times larger than in Al

✦ Need more X0 of Pb to contain shower at 90% level

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Scaling is NOT perfect

18

Pb Z = 82Fe Z = 26Al Z = 13

Electromagnetic shower leakage (longitudinal)

• The absorber thickness needed to contain a shower increases logarithmically with energy

• The number of extra X0 needed to fully contain the shower energy can be as much as 10 going from high Z to low Z absorbers

• More X0 needed to contain γ initiated showers

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Importance of SOFT particles

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• The composition of em showers. Shown are the percentages of the energy of 10 GeV electromagnetic showers deposited through shower particles with energies below 1 MeV, below 4 MeV or above 20 MeV as function of the Z of the absorber material.

• Results of EGS4 simulations

Electromagnetic Showers

✦ Phenomena at E < Ec determine important calorimeter properties✦ In lead > 40% of energy deposited by e± with E < 1

MeV✦ Only 1/4 deposited by e+, 3/4 by e- (Compton,

photoelectrons!)✦ The e+ are closer to the shower axis, Compton and

photoelectrons in halo

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Angular distribution

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Lessons for calorimetry✦ In absorption process, most of the energy is

deposited by very soft particles✦ Electromagnetic showers:

✦ 3/4 of the energy deposited by e-, 2/4 of it by Compton, photoelectrons

✦ These are isotropic, have forgotten direction of incoming particle ⇒ No need for sandwich geometry

✦ The typical shower particle is a 1 MeV electron, range < 1mm ⇒ important consequences for sampling calorimetry

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Hadronic showers development

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Hadron showers✦ Extra complication: the strong interaction✦ Much larger variety may occur both at the particle

level and at the level of the stuck nucleus✦ Production of other particles, mainly pions✦ Some of these particles (π0, η) develop electromagnetic

showers✦ Nuclear reactions: protons, neutrons released from

nuclei✦ Invisible energy (nuclear binding energy, target recoil)

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Hadron vs em showers✦ Hadron showers ⇒ much more complex than em

showers✦ Invisible energy

✦ em showers: all energy carried by incoming e or γ goes to ionization

✦ had showers: certain fraction of energy is fundamentally undetectable

✦ em showers✦ e± ⇒ continuous stream of events ( ionization + bremsstrahlung)✦ γ ⇒ can penetrate sizable amounts of material before losing

energy✦ had showers

✦ ionization (as a µ) then interaction with nuclei✦ development similar to em shower but different scale ( λ vs. X0)✦ Particle sector✦ Nuclear sector 26

The electromagnetic fraction, fem✦ em decaying particles : π0, η0 ⇒ γ γ

✦ % of hadronic energy going to em fluctuates heavily✦ On average 1/3 of particles in first generation are π0s✦ π0s production by strongly interacting particles is an

irreversible process (a ”one-way street”)✦ Simple model

✦ after first generation fem = 1/3✦ after second generation fem = 1/3 + 1/3 of 2/3 = 5/9✦ after third generation fem = 1/3 + 1/3 of 2/3 + 1/3 of 4/9 = 19/27✦ after n generations fem = 1 - (1- 1/3)n

✦ the process stops when the available energy drops below the pion production threshold and n depends on the average multiplicity of mesons produced per interaction <m> ⇒ n increases by one unit every time E increases by a factor <m>

✦ fem increases with increasing incoming hadron energy

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The electromagnetic fraction, fem✦ But

✦ other particles than pions are produced (factor 1/3 wrong)✦ <m> is energy dependent✦ Barion number conservation neglected → lower fem in

proton induced showers than in pion induced ones✦ Using a more realistic model

✦ < fem > = 1- (E/E0)(k-1)

✦ E0= average energy needed to produce a π0

✦ (k-1) related to the average multiplicity✦ < fem > slightly Z dependent

✦ Consequences:✦ Signal of pion < signal of electron (non-compensation)✦ e/π signal ratio energy dependent (non-linearity)

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Energy dependence em component

✦ SPACAL : Pb - scintillating fibers✦ QFCAL: Cu - quartz fibers

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Signal non-linearity

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Non em component✦ Breakdown of the non-em component in Lead

✦ Ionizing particles 56% (2/3 from spallation protons)

✦ Neutrons 10% (37 neutrons/GeV)

✦ Invisible 34%

✦ Spallation protons carry typically 100 MeV

✦ Evaporation neutrons 3 MeV31

Where does the energy go ?

✦ Energy deposit and composition of the non-em component of hadronic showers in lead and iron.

✦ The listed numbers of particles are per GeV of non-em energy

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A typical process✦ Nuclear interaction (nuclear star) induced by a proton

of 30 GeV in a photographic emulsion

Protons(isotropic)

Fast pions and fast spallation protons( non-isotropic)

Spallation neutrons( non-visible)

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Hadronic shower profiles

✦ Shower profiles are governed by the

✦ Nuclear interaction lenght, λint

✦ average distance a high-energy hadron has to travel inside a medium before a nuclear interaction occurs

✦ λint (g cm-2) ∝ A1/3

✦ Fe 16.8 cm, Cu 15.1 cm, Pb 17.0 cm, U 10.0 cm

For comparison X0 : Fe 1.76 cm, Cu 1.43 cm, Pb 0.56 cm, U 0.32 cm

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Longitudinal profile

35

Fluctuations (em showers)

✦ Hanging file calorimeter

✦ 170 GeV electrons

36

Fluctuations (hadronic showers)

✦ Hanging file calorimeter

✦ 270 GeV pions

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Shower containment

✦ Shower containment:

✦ Depth to contain showers increases with log E

✦ Lateral leakage decreases as the energy goes up !✦ <fem> increases with energy ✦ Electromagnetic component concentrated in a narrow cone

around shower axis✦ ⇒ Energy fraction contained in a cylinder with a given

radius increases with energy

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Hadronic shower leakage (longitudinal)

39

Hadronic shower leakage (lateral)

✦ This energy dependence is a direct consequence of the energy of <fem>. The average energy fraction carried by the em shower component increases with energy and since this component is concentrated in a narrow cone around the shower axis, the energy fraction contained in a cylinder with a given radius increases with energy as well.

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Lateral distribution shower particles

41

✦ Np produced by thermal neutron capture

✦ Mo fission product of U produced by non-thermal neutrons (MeV)

✦ U produced by γ (10 GeV) induced reactions present in the em core

Lessons for calorimetry✦ In absorption process, most of the energy is deposited by

very soft particles✦ Electromagnetic showers:

✦ 3/4 of the energy deposited by e-, 2/4 of it by Compton, photoelectrons

✦ These are isotropic, have forgotten direction of incoming particle ⇒ No need for sandwich geometry

✦ The typical shower particle is a 1 MeV electron, range < 1mm ⇒ important consequences for sampling calorimetry

✦ Hadron showers:✦ Typical shower particles are a 50-100 MeV proton and a 3 MeV

evaporation neutron✦ Range of 100 MeV proton is 1 - 2 cm✦ Neutrons travel typically several cm✦ What they do depends critically on detail of the absorber

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Backup Slides

43

Particle Identification

44


Energy loss by charged particles

✦ Main energy loss mechanism for charged particles traversing matter:

✦ Inelastic interaction with atomic electrons

✦ If the energy is sufficient to release atomic electrons from nuclear Coulomb field ⇒ ionization

✦ Other processes:

✦ Atomic excitation

✦ Production of Čerenkov light

✦ At high energy: production of δ-rays

✦ At high energy: bremsstrahlung

✦ At very high energy: nuclear reactions45


The Bethe-Block Formula

✦ dE/dx in [Mev g-1cm2]

✦ Valid for “heavy particles

✦ First approximation: medium simply

dE

dx= �4⌅NAr2

ec2z2 Z

A

1�2

�12

ln2mec2⇥2�2

I2Tmax � �2 � ⇤

2

⇥

46


Energy loss by charged particles

47


✦ Critical energy Ec

✦ For electrons:

✦Ec(e-) in Cu (Z=29) = 20 MeV

✦For muons

✦Ec(µ) in Cu = 1 TeV

✦Unlike electrons, muons in multi-GeV range can traverse thick layers of dense matter.

Interaction of charged particles

dE

dx

��Brems

=dE

dx

��ion

Esolid+liq

c

=610MeV

Z + 1.24Egas

c

=710MeV

Z + 1.24

Emuc = Eelec

c

⇣mµ

me

⌘2 Energy loss (ion+rad)of e and p in Cu

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✦ Energy loss by Bremsstrahlung✦ Radiation of real photons in the Coulomb

field of the nuclei of the absorber medium

✦ For electrons:

radiation length [g/cm2]

Interaction of charged particles

�dE

dx

= 4↵NAZ

2

A

z

2r

2eE ln

183Z

13

�dE

dx

=E

X0=) E = E

� x

X00

X0 = A

4↵NAZ2A z2r2

eE ln 183

Z13

�dE

dx

= 4↵NAZ

2

A

z

2(1

4⇡✏0

e

2

mc

2)2E ln

183Z

13/ E

m

2

49


Interaction of photons✦ In order to be detected a photon has to create charged

particles and/or transfer energy to charged particles

✦ Photoelectric effect

✦ Most probable process at low energy

✦ P.e. effect releases mainly electrons from the K-shell

✦ Cross section shows strong modulation if Eγ≈ Eshell

✦ At high energies (ε>>1)

�K

photo

= 4⇡r2e

↵4Z5 1✏ �

photo

_ Z5 �photo

_ E�3

⌅Kphoto =

�32⇥7

⇥ 12

�4Z5⌅eTh ⇥ =

E�

mec2⌅e

Th =83⇤r2

e (Thomson)

50


Interaction of photons✦ Compton scattering

✦ Assume electrons quasi-free

✦ Klein-Nishina

✦ At high energy approximately

✦ Atomic Compton cross-section

Compton Cross-section (Klein-

Nishina)

d�

d⌦(✓, ✏)

�ec _ ln ✏

✏

�atomic

c

= Z�e

c

γ + e ⇒ γ’ + e’

�Compton � E�1

51


Scattering Compton ✦ For all but the high Z materials: most probable process for

γs in the range between few hundred keV and 5 MeV

✦ Typically at least 50% of the total energy is deposited by such γs in the absorption process of multi GeV e+, e- and γs

✦ Compton scattering is a very important process to understand the fine details of calorimetry

✦ Angular distribution of recoil electrons shows a substantial isotropic component. Many γs in the MeV range are absorbed by a sequence of Compton scatterings ⇒ most of the Compton electrons produced in this process are isotropically distributed

52


Interaction of photons✦ Pair production

✦ Only possible in the field of a nucleus (or an electron) if✦ Eγ > 2mec2

✦ Cross-section (High energy approximation)

53

✦ Lateral shower width scales with Molière radius ρM

✦ On average 90% of the shower energy is deposited in a cylinder with radius ρM around the shower axis

✦ ρM much less material dependent than X0

✦ Lateral shower width determined by:

✦ Multiple scattering of e± (early, 0.2 ρM)

✦ Compton γs traveling away from axis (1 - 1.5 ρM)

Electromagnetic Showers

⇢M = EsX0

EcEs = mec

2p

4⇡/↵

X0 _ A/Z2, Ec _ 1/Z ) ⇢M _ A/Z

54

Cu Pb

Z 29 82

density 8,96 11,35

X0 14,3 5,6

ρM 15,2 16

Lateral profile

✦ Material dependence✦ Radial energy deposit

profiles for 10 GeV electrons showering in Al, Cu and Pb

✦ Results of EGS4 calculations

55

Tails made out of Compton electrons. Mean free path of MeV gammas becomes smaller as Z increases

Lateral shower leakage

• No energy dependence

• A (sufficiently long) cylinder will contain the same fraction of energy of a 1 GeV or 1 TeV em shower

• Results of EGS4 simulations

56

Contributions to signal

57

Electrons and positrons

✦ The number of positrons increases by more than a factor 3 going from Al to U

✦ Aluminum (∼18 e+/GeV)

✦ Uranium (∼60 e+/GeV)

✦ Increase due to the fact that particle multiplication in showers developing in high-Z absorber materials continues down to much lower energies than in low-Z materials 58

Muons in calorimeters

✦ Muons are not minimum ionizing particles

✦ The effects of radiation are clearly visible in calorimeters, especially for high-energy muons in high-Z absorber material

Ec(µ) =hmµ

me

i2⇥ Ec(e)

=) Ec(µ) ⇡ 200 GeV in Pb

59

Muon signals in a calorimeter

✦ Signal distributions for muons of 10, 20, 80 and 225 GeV traversing the SPACAL

60

Comparison em/hadronic calorimeter properties

✦ Ratio of the nuclear interaction lenght and the radiation lenght as a function of Z

61

Particle ID with a simple preshower detector

62

Energy response and Compensation1


Lecture II

✦ Response = Average signal per unit of deposited energy, e.g. # photoelectrons/GeV, picoCoulombs/MeV, etc

✦ ➞ A linear calorimeter has a constant response

The Calorimeter Response Function

• Electromagnetic calorimeters are in general linear

• All energy deposited through ionization/excitation of absorber

• If not linear ⇒ instrumental effects (saturation, leakage,....)2

Saturation in “digital” calorimeters✦ Gaseous detector operated in “digital” mode

✦ Geiger counters or streamer chambers✦ Intrinsically non linear:

✦ Each charged particle creates an insensitive region along the stuck wire preventing nearby particles to be registered

✦ Density of particles increases with increasing energy✦ ⇒ calorimeter response decreases with increasing

energy✦ Example:

✦ Calorimeter read out using wire chambers in “limited streamer” mode

✦ Energy varied by depositing n (n = 1-10) positrons of 17.5 GeV simultaneously in the calorimeter

✦ Energy deposit profile not energy dependent✦ Calorimeter longitudinally subdivided in 5 sections 3

Saturation in “digital” calorimeters

✦ At n=6✦ Non linearity:

✦ 1 - 14.5 %✦ 2 - 14.8 %✦ 3 - 9.3 %✦ 4 - 2.4 %✦ 5 - 0.5 %

✦ Non-linearity in sect. 1 more than 6 times the one in sect. 4✦ Energy deposit in sect. 1 less than half of the one in sect. 4✦ Particle density in sect. 1 larger than in sect. 4 4

Homogeneous calorimeters I✦ Homogeneous: absorber and active media are the same✦ Response to muons

✦ because of similarity between the energy deposit mechanism response to muons and em showers are equal

✦ ⇒ same calibration constant ⇒ e/mip=1✦ Response to hadrons

✦ Due to the invisible energy π/e < 1✦ e/mip =1 ⇒ π/mip < 1✦ Response to hadron showers smaller than the electromagnetic

one✦ Electromagnetic fraction (fem) energy dependent✦ ⇒ response to electromagnetic component increases with

energy ⇒ π/e increases with energy

5

Homogeneous calorimeters II✦ Calorimeter response to non-em component (h)

energy independent ⇒ e/h > 1 (non compensating calorimeter)✦ e/π not a measure of the degree of non-compensation

✦ part of a pion induced shower is of em nature✦ fem increases with energy ⇒ e/π ⇒ tends to 1

✦ e/h cannot be measured directly (unless…..)

fem function of energy

� = fem · e + (1� fem) · h

�/e = fem + (1� fem) · h/e

e/� =e/h

1� fem[1� e/h]

6

Hadron Showers Energy dependence EM component

7

Hadron showers: e/h and the e/π signal ratio

8

e/� =e/h

1� fem[1� e/h]< fem > = 1- (E/E0)(k-1)

E=1GeV; k=0.82

Homogeneous calorimeters III✦ Response to jets✦ Jet = collection of particles resulting from the fragmentation of

a quark, a diquark or a hard gluon✦ From the calorimetric point of view absorption of jets proceeds

in a way that is similar to absorption of single hadrons✦ (Minor) difference:

✦ em component for single hadrons are π0 produced in the calorimeter

✦ jets contain a number of π0 (γ from their decays) upon entering the calorimeter (“intrinsic em component”)

✦ <fem> for jets and single hadrons different and depending on the fragmentation process

✦ No general statement can be made about differences between response to single hadrons and jets but:

✦ response to jets smaller than to electrons or gammas✦ response to jets is energy dependent 9

Sampling calorimeters✦ Sampling calorimeter: only part of shower energy

deposited in active medium✦ Sampling fraction fsamp

✦ fsamp is usually determined with a mip (dE/dx at minimum)✦ N.B. mip’s do not exist !✦ e.g. D0 (em section):

✦ 3 mm 238U (dE/dx = 61.5 MeV/layer)✦ 2 x 2.3 mm LAr (dE/dx = 9.8 MeV/layer)✦ fsamp = 13.7%

fsamp =energy deposited in active medium

total energy deposited in calorimeter

10

The e/mip ratio✦ D0: fsamp = 13.7%

✦ However, for em showers, sampling fraction is only 8.2%✦ ⇒ e/mip ≈ 0.6

✦ e/mip is a function of shower depth, in U/LAr it decreases✦ e/mip increases when the sampling frequency becomes very

high✦ What is going on ?✦ Photoelectric effect: σ ∝ Z5, (18 / 92)5 = 3 · 10-4

✦ ⇒ Soft γs are very inefficiently sampled✦ Effects strongest at high Z, and late in the shower development✦ The range of the photoelectrons is typically < 1 mm✦ Only photoelectrons produced near the boundary between

active and passive material produce a signal✦ ⇒ if absorber layers are thin, they may contribute to the signals

11

Gammas

✦ At high energy γ/mip ≈ e/mip✦ Below 1 MeV the efficiency for γ detection drops

spectacularly due to the onset of the photoelectric effect 12

The e/mip ratio: dependence on sampling frequency

✦ Only photoelectrons produced in a very thin absorber layer near the boundary between active and passive materials are sampled

✦ Increasing the sampling frequency (thinner absorber plates) increases the total boundary surface 13

Sampling calorimeters: the e/mip signal ratio

✦ e/mip larger for LAr (Z=18) than for scintillator✦ e/mip ratio determined by the difference in Z values

between active and passive media 14

✦ The hadronic response is not constant✦ fem, and therefore e/π signal ratio is a function of energy✦ ➙ If calorimeter is linear for electrons, it is non-linear for

hadrons✦ Energy-independent way to characterize hadron calorimeters: e/h

✦ e = response to the em shower component✦ h = response to the non-em shower component✦ → Response to showers initiated by pions:

✦ e/h is inferred from e/π measured at several energies (fem values)✦ Calorimeters can be

✦ Undercompensating (e/h > 1)✦ Overcompensating (e/h < 1)✦ Compensating (e/h =1)

Hadronic shower response and the e/h ratio

R� = fem e + [1� fem] h ⇥ e/� =e/h

1� fem[1� e/h]

15

Response function of a non compensating calorimeter

16

Signal non-linearity

17

Compensation✦ In order to understand how compensation could be achieved, one

should understand in detail the response to the various types of particles that contribute to the calorimeter signals

✦ Energy deposition mechanisms that play a role in the absorption of the non-em shower energy:

✦ Ionization by charged pions (Relativistic shower component). The fraction of energy carried by these particles is called frel

✦ Ionization by spallation protons (non-relativistic shower component). The fraction of energy carried by these particles is called fp

✦ Kinetic energy carried by evaporation neutrons may be deposited in a variety of ways. The fraction of energy carried by these particles is called fn

✦ The energy used to release protons and neutrons from calorimeter nuclei, and the kinetic energy carried by recoil nuclei do not lead to a calorimeter signal. This energy represent the invisible fraction finv of the non-em shower energy

18

Non-em calorimeter response✦ h can be written as follows:

✦ rel, p, n and inv denote the calorimeter responses✦ Normalizing to mips and eliminating the last term

✦ The e/h value can be determined once we know its response to the three components of the non-em shower components

✦ For compensation the response to neutron is crucial✦ Despite the fact that n carry typically not more than

~10% of the non-em energy, their contribution to the signal can be much larger than that

h = frel · rel + fp · p + fn · n + finv · inv

frel + fp + fn + finv = 1

e

h=

e/mip

frel · rel/mip + fp · p/mip + fn · n/mip

19

Compensation - The role of neutrons✦ Neutrons only loose their energy through the products of the

nuclear reactions they undergo✦ Most prominent at the low energies typical for hadronic shower

neutrons is the elastic scattering. ✦ In this process the transferred energy fraction is on average:

felastic = 2A/(A+1)2

✦ Hydrogen felastic = 0.5 Lead felastic = 0.005✦ Pb/H2 calorimeter structure (50/50)

✦ 1 MeV n deposits 98% in H2

✦ mip deposits 2.2% in H2

✦ Pb/H2 calorimeter structure (90/10)✦ Recoil protons can be measured!

✦ ⇒ Neutrons have an enormous potential to amplify hadronic shower signals, and thus compensate for losses in invisible energy

✦ Tune the e/h value through the sampling fraction!20

⇒ n/mip = 45⇒ n/mip = 350

Compensation in a Uranium/gas calorimeter

21

Compensation: the crucial role of the sampling fraction

22

Compensation: slow neutrons and the signal’s time structure

✦ Average time between elastic n-p collisions: 17 ns in polystyrene

✦ Measured value lower (10 ns) due to elastic or inelastic neutron scattering off other nuclei present in the calorimeter structure (Pb, C and O)

23

Compensation✦ All compensating calorimeters rely on the contribution of

neutrons to the signals✦ Ingredients for compensating calorimeters

✦ Sampling calorimeter✦ Hydrogenous active medium (recoil p!)✦ Precisely tuned sampling fraction

✦ e.g. 10% for U/scintillator, 3% for Pb/scintillator,…….✦ No way to get compensation in homogeneous calorimeters ✦ No way to get compensation in sampling calorimeters with

non hydrogenous active medium, e.g. LAr or Si

24

Fluctuations25

Calorimetric measurement✦ Discussing calorimeter response we examined the average signals

produced during absorption

✦ To make a statement about the energy of a particle:

✦ relationship between measured signal and deposited energy (calibration)

✦ energy resolution (precision with which the unknown energy can be measured)

✦ Resolution is limited by:

✦ fluctuations in the processes through which the energy is degraded (unavoidable)

✦ ultimate limit to the energy resolution in em showers (worsened by detection techniques)

✦ not a limit for hadronic showers ? (clever readout techniques can allow to obtain resolutions better than the limits set by internal fluctuations

✦ technique chosen to measure the final products of the cascade process26

Fluctuations (1)✦ Calorimeter’s energy resolution is determined by fluctuations✦ Many sources of fluctuations may play a role, for example:

✦ Signal quantum fluctuations (e.g. photoelectron statistics)

✦ Sampling fluctuations

✦ Shower leakage

✦ Instrumental effects (e.g. electronic noise, light attenuation, structural non-uniformity)

✦ but usually one source dominates

✦ Improve performance ⇒ work on that source

✦ Poissonian fluctuations (many, but not all):✦ Energy E gives N signal quanta, with σ = √N

✦ ⇒ σ√E ∝ √N√N = cE ⇒ σ/E=c/√E

27

Fluctuations (2)

✦ Signal quantum fluctuations

✦ Ge detectors for nuclear γ ray spectroscopy: 1 eV/quantum

✦ ⇒ If E= 1 MeV: 106 quanta, therefore σ/E = 0.1%

✦ Usually E expressed in GeV ⇒ σ/E = 0.003%/√E

✦ Quartz fiber calorimeters: typical light yield ∼ 1 photoelectron/GeV

✦ Small fraction of energy lost in Čerenkov radiation, small fraction of the light trapped in the fiber, low quantum efficiency for UV light

✦ ⇒ σ/E = 100%/√E. If E = 100 GeV, σ/E = 10%

28

Signal quantum fluctuations dominate

✦ Quartz window transmit a larger fraction of the Čerenkov light (UV component) 29

Fluctuations (3)

✦ Sampling fluctuations✦ Determined by fluctuations in the number of

different shower particles contributing to signals

✦ Both sampling fraction and the sampling frequency are important

✦ Poissonian contribution : σsamp/E = asamp/√E

✦ ZEUS: No correlation between particles contributing to signals in neighboring sampling layers ⇒ range of shower particles is very small

30

Sampling fluctuations in em calorimeters Determined by sampling fraction and sampling frequency

31

Fluctuations (4)

✦ Shower leakage fluctuations

✦ These fluctuations are non-Poissonian

✦ For a given average containment, longitudinal fluctuations are larger that lateral ones

✦ Difference comes from # of particles responsible for leakage

✦ e.g. Differences between e, γ induced showers

32

Contribution of leakage fluctuations to energy resolution

✦ Longitudinal shower fluctuations and therefore leakage are essentially driven by fluctuations in the starting point of the shower, i.e. by the behavior of one single shower particle.

✦ Lateral shower fluctuations generated by many particles33

Fluctuations (5)

✦ Instrumental effects

✦ Structural differences in sampling fraction

✦ “Channelling” effects

✦ Electronic noise, light attenuation,.....

34

Fluctuations (6)

✦ Different effects have different energy dependence

✦ quantum, sampling fluctuations σ/E ∼ E-1/2

✦ shower leakage σ/E ∼ E-1/4

✦ electronic noise σ/E ∼ E-1

✦ structural non-uniformities σ/E = constant

✦ Add in quadrature σ2tot = σ21 + σ22 + σ23 + σ24+......

35

The em resolution of the ATLAS em calorimeter

36

Fluctuations in hadron showers (I)

✦ Some types of fluctuations as in em showers, plus

✦ Fluctuations in visible energy

✦ (ultimate limit of hadronic energy resolution)

✦ Fluctuations in the em shower fraction, fem

✦ Dominating effect in most hadron calorimeters (e/h≠1)

✦ Fluctuations are asymmetric in pion showers (one-way street)

✦ Differences between p, π induced showers

✦ No leading π0 in proton showers (barion # conservation)

37

Hadron showers: fluctuations in nuclear binding energy losses

✦ Distribution of nuclear binding energy loss that may occur in spallation reaction induced by protons with a kinetic energy of 1 GeV on 238U (more of 300 different reactions contributing)

✦ Note the strong correlation between the distribution of the binding energy loss and the distribution of the number of neutrons produced in the spallation reactions

✦ There may be also a strong correlation between the kinetic energy carried by these neutrons and the nuclear binding energy loss

38

Hadron showers: Fluctuations in em shower fraction (fem)

Pion showersDue to the irreversibility of the production of π0s and because of the leading particle effect, there is an asymmetry between the probability that an anomalously large fraction of the energy goes into the em shower component

39

✦ Hadronic energy resolution of non-compensating calorimeters does not scale with E-1/2 and is often described by:

✦ Effects of non-compensation on σ/E is are better described by an energy dependent term:

✦ In practice a good approximation is:

Fluctuations in hadron showers

40

�

E=

a1⇥E� a2

�

E=

a1�E

+ a2

�

E=

a1⇥E� a2

⇤�E

E0

⇥l�1⌅ �

E=

a1pE

� a2E�0.28

Hadronic resolution of non-compensating calorimeters

41

ATLAS Fe-scintillator

prototype

Backup Slides

Signal linearity for electromagnetic showers

✦ Example:✦ Saturation

effects in PMTs✦ Voltage

between last dynodes lowered by high current

43

Fluctuations due to instrumental effects (readout)

✦ Electrons from shower leakage traversing SPDs generate signals order of magnitude larger than the ones generated by scintillation photons

44

Hadronic signal (non)-linearity at low energy

✦ At low energy response to hadrons resembles the one for mips at low energy, and if e/mip ≠1 ⇒ the calorimeter is by definition non linear, regardless the degree of compensation

45

Compensation (I)✦ Need to understand response to typical shower

particles (relative to mip)

✦ Relativistic charged hadrons✦ Even if relativistic, these particles resemble mip in

their ionization losses ⇒ rel/mip = 1

✦ Spallation protons and Neutrons✦ Spallation protons

✦ More efficient sampling✦ Signal saturation

46

Aspects of compensation: sampling of soft shower protons

✦ Results from Monte Carlo simulation✦ Quite complicated picture, many effects contribute:

✦ ratio of specific ionization in the active and passive materials ↑,

✦ multiple scattering effects ↓, ✦ sampling inefficiencies due to limited range ↓ ✦ saturation and recombination effects in the active medium ↓

47

Aspects of compensation: sampling of soft shower protons

48

Aspects of compensation: saturation effects

dL

dx= S

dE/dx

1 + kb · dE/dxBirk’s Law

49

Compensation: the spallation proton/mip signal ratio

50

Compensation: the spallation proton/mip signal ratio

✦ Response to spallation protons depends on:✦ spallation protons energy spectrum ✦ Z value of the absorber ✦ fsamp and ffreq ✦ saturation properties of the active medium 51

Compensation (II)✦ Need to understand response to typical shower

particles (relative to mip)

✦ Spallation protons and Neutrons

✦ Neutrons✦ (n, n’γ) inelastic scattering: not very important✦ (n, n’) elastic scattering: most interesting✦ (n, γ) capture (thermal): lots of energy, but process is slow

(µs)

52

Compensation (III) - The role of neutrons✦ Elastic scattering felastic = 2A/(A+1)2

✦ Hydrogen felastic = 0.5 Lead felastic = 0.005✦ Pb/H2 calorimeter structure (50/50)

✦ 1 MeV n deposits 98% in H2


✦ Recoil protons can be measured!✦ ⇒ Neutrons have an enormous potential to amplify

hadronic shower signals, and thus compensate for losses in invisible energy

✦ Tune the e/h value through the sampling fraction!✦ e.g. 90% Pb/10% H2 calorimeter structure

✦ 1 MeV n deposits 86.6% in H2


}⇒ n/mip = 45

}⇒ n/mip = 35053

average fraction of neutron’s kinetic energy transferred in elastic collisions with a nucleus containing A nucleons

Signal quantum fluctuations dominate also here (2)

✦ Asymmetric distribution typical of Poisson distribution

✦ At high energy Gaussian shape (central limit theorem)54

Sampling fraction and sampling frequency

For fiber calorimeters for equal sampling fraction, better resolution for smaller diameter fibers (higher sampling frequency)

55

How to measure signal quantum fluctuations

✦ Reduce the # of photoelectrons using a neutral density filter by a factor f (f>1)

✦ x = light yield of the calorimeter

✦ # of p.e. for an energy deposit E is xE, σ=√(xE), (σ/E)p.e.=1/√(xE)

✦ Contribution of sampling fluctuations: (σ/E)samp=asamp/√(E)

✦ If f is known ⇒ determine x

✦ Better at low energy where non stochastic contributions are

(�/E)nofilter =�

a2samp/E + 1/xE

(�/E)filter =�

a2samp/E + f/xE =

�(�/E)2nofilter + (f � 1)/xE

56

How to measure signal quantum fluctuations

✦ f = 3 and 10

✦ 3, 8,10,15 GeV

✦ 1300 ± 90 p.e./GeV

✦ 2.8%/√E contribution to resolution

✦ 5.7%/√E: total em resolution

NIM A368 (1996), 64057

How to measure the effects of sampling fluctuations (I)✦ To measure sampling fluctuations:

✦ Choose conditions in which sampling fluctuations contribute substantially to the total energy resolution

✦ Change the sampling fraction keeping everything else the same

✦ Derive the contribution of sampling fluctuations from a comparison of the total energy resolution before and after

58

How to measure the effects of sampling fluctuations (IV)

✦ Energy resolution for em showers strongly dominated by sampling fluctuations

✦ Sampling fluctuations in good agreement with the formula:✦ Sampling fluctuations strongly dominate the hadron resolution in

these compensating calorimeters✦ Hadronic sampling fluctuations are about a factor 2 larger than the

em ones✦ “Intrinsic” fluctuation contribution scale as E-1/2

asamp = 2.7%�

d/fsamp

59

Sampling fluctuations in em and hadronic showers

em resolution dominated by sampling fluctuations

sampling fluctuations only minor contribution to hadronic resolution in this non compensating

calorimeter

60

Instrumental effects: channelling in fiber calorimeters

- Sampling fraction for em showers: 2%

- Electrons entering the calorimeter at 0º exactly at the position of a fiber loose very little energy in the early stages of the shower development and can cause longitudinal leakage

- Shower particles escaping from the back traverse a region where there is no more Pb, the fibers are bundled and the sampling fraction is almost 100%

61

Fluctuations in fem: differences p/π induced showers

<fem> is smaller in proton-induced showers than in pion-induced ones: barion number conservation prohibits the production of leading π0s and thus reduces the em component respect to pion-induced showers

62

The state of art Towards the next Collider

1


Lecture III

The next Collider✦ Discussions about the post-LHC era have mainly

concentrated on a high-energy electron-positron collider with a center-of-mass energy that would allow this machine to become a factory for t-tbar and Higgs boson production.

✦ Both linear colliders (ILC, CLIC) and circular ones (FCC) are being considered in this context.

✦ A sufficiently large circular collider could additionally be used to further push the energy frontier for hadron collisions beyond the LHC limit

✦ Calorimetry R&D in the past 15 years has evolved looking forward to experiments at these machines.

2

Calorimetry for the next Collider✦ An often mentioned design criterion for calorimeters at a

future e+e- collider is the need to distinguish between hadronically decaying W and Z bosons.

✦ The requirement that the di-jet masses of events are separable by at least one Rayleigh criterion implies that one should be able to detect 80 - 90 GeV jets with a resolution of 3 -3.5 GeV.

✦ This goal can be, and has been achieved for single hadrons but not for jets.

3

W ) qq̄ and Z ) qq̄

The importance of energy resolution

4

Calorimetry for the next Collider

5

✦ Different approaches are followed to develop calorimeter systems that are up to this task✦ Compensating calorimeters

✦ Proven technology, current holders of all performance records

✦ Dual-readout calorimeters

✦ Try to improve on the performance of compensating calorimeters by eliminating the weak points of the latter

✦ Systems based on Particle Flow Analysis (PFA)

✦ Combine the information from a tracking system and a fine-grained calorimeter

Compensating calorimeters

6

✦ Reasons for poor hadronic performance of non-compensating calorimeters understood

✦ Experimentally demonstrated with Pb/scintillator calorimeters (ZEUS, SPACAL)

Hadronic signal distributions in a compensating calorimeter

SPACAL NIM A308 (1991) 481

Pros and Cons of Compensating Calorimeters

7

✦ Pros✦ Same energy scale for electrons, hadrons and jets. No ifs, ands or buts.

✦ Calibrate with electrons and you are done

✦ Excellent hadronic energy resolution (SPACAL 30%/√E)

✦ Linearity, Gaussian response function

✦ Compensation fully understood. (We know how to built these things, even though GEANT doesn’t)

✦ Cons✦ Small sampling fraction (2.4%in Pb/plastic) ⇒ em resolution limited (SPACAL:

13%/√E, ZEUS: 18%/√E)

✦ Compensation relies on detecting neutrons

✦ ⇒ Large integration volume

✦ ⇒ Long integration time (> 50 ns)

✦ Jet energy resolution not as good as for single hadrons in Pb, U calorimeters

What is the problem with the jet energy resolution ?

8

Signal non-linearities at low energy (< 5 GeV) due to non-showering hadrons.Many jet fragments fall in this categoryA copper or iron calorimeter would be much better in that respect

How to improve the excellent ZEUS/SPACAL performance ?

9

✦ Reduce the contributions of the sampling fluctuations to energy resolution

✦ (THE limiting factor in SPACAL/ZEUS)

✦ Use lower-Z absorber material

✦ to eliminate / reduce the jet problem

✦ Maintain advantages of compensation

✦ (eliminate / reduce effects of fluctuations in fem and invisible energy)

➔ Dual -Readout Calorimetry

The DREAM principle✦ Quartz fibers are only sensitive to em shower component !

✦ Production of Čerenkov light ⇒ Signal dominated by electromagnetic component

✦ Non-electromagnetic component suppressed by a factor 5 ⇒ e/h=5 (CMS)

✦ Hadronic component mainly spallation protons Ek ∼ few hundred MeV ⇒non relativistic ⇒ no Čerenkov light

✦ Electron and positrons emit Čerenkov light up to a portion of MeV

✦ Use dual-readout system:

✦ Regular readout (scintillator, LAr, ...) measures visible energy

✦ Quartz fibers measure em shower component Eem

✦ Combining both results makes it possible to determine fem and the energy E of the showering hadron

✦ Eliminates dominant source of fluctuations10

Quartz fibers calorimetry

Radial shower profiles in: SPACAL (scintillating fibers)QCAL (quartz fibers)

11

Radial hadron shower profiles (DREAM)

12

DREAM structure

Some characteristics of the DREAM detector:Depth 200 cm (10 λint)16.2 cm effective radius (0.81 λint, 8.0 ρM)Mass instrumented volume:1030 KgNumber of fibers 35910, diameter 0.8 mmX0 = 20.10 mm, ρM =20.35 mm19 hexagonal towers, 270 rods eachEach tower read-out by 2 PMs (1 for Q and 1 for S fibers) 13

The (energy independent) Q/S method

14

S = E

�fem +

1(e/h)S

(1� fem)⇥

Q = E

�fem +

1(e/h)Q

(1� fem)⇥

If e/h = 1.3(S), 4.7(Q)

Q

S=

fem + 0.21(1� fem)

fem + 0.77(1� fem)

E =S �XQ

1�X

with X =1� (h/e)S1� (h/e)Q

' 0.3

Effect of selection based on fem

15

DREAM: Effect of corrections (200 GeV “jets”)

16

Uncorrected

Q/S Method

Hadronic response:Effect Q/S correction

17

CONCLUSIONS from tests✦ DREAM offers a powerful technique to

improve hadronic calorimeter performance:✦ Correct hadronic energy reconstruction, in an

instrument calibrated with electrons !

✦ Linearity for hadrons and jets

✦ Gaussian response functions

✦ Energy resolution scales with √E

✦ σ/E < 5% for high-energy “jets”, in a detector with a mass of only 1 ton ! (dominated by fluctuations in shower leakage)

18

How to improve DREAM performance✦ Build a larger detector → reduce effects side leakage

✦ Increase Čerenkov light yield

✦ DREAM: 8 p.e./GeV → fluctuations contribute 35%/√E

✦ No reason why DREAM principle is limited to fiber calorimeters

✦ Homogeneous detector ?!

✦ ⇒ Need to separate the light into its Č, S components

✦ Increase sampling fraction

✦ For ultimate hadron calorimetry (15%/√E) Measure Ekin (neutrons).

✦ Is correlated to nuclear binding energy loss (invisible energy)

19

Expected effect of full shower containment

20

Time structure of the DREAM signals: the neutron tail (anticorrelated with fem)

✦ High resolution hadron calorimetry also requires efficient detection of the “nuclear” shower component

21

Probing the total signal distribution with the neutron fraction

22

Homogeneous calorimeters (crystals)✦ No reason why DREAM principle should be limited to fiber

calorimeters

✦ Crystals have the potential to solve light yield + sampling fluctuations problem

✦ HOWEVER: Need to separate the light into its Č, S components

✦ OPTIONS:

✦ Directionality: S light is isotropic, Č light directional

✦ Time structure: Č light is prompt, S light has decay constant(s)

✦ Spectral characteristics: Č light λ-2, S depend on scintillator

✦ Polarization: Č light polarized, S light not23

Čerenkov and Scintillation information from one signal !

24

BGO crystalUG 11 (UV) filterfrom NIM A595 (2008) 359

Conclusions on DREAM✦ The DREAM approach combines the advantages of compensating calorimetry with

a reasonable amount of design flexibility

✦ The dominating factors that limited the hadronic resolution of compensating calorimeters (ZEUS; SPACAL) to 30 - 35%/√E can be eliminated

✦ The theoretical resolution limit for hadron calorimeters (15%/√E) seems within reach

✦ The DREAM project holds the promise of high-quality calorimetry for all types of particles, with an instrument that can be calibrated with electrons

✦ Open issues:

✦ Projective geometry

✦ Read-out

✦ Copper fabrication

✦ Better fibre quality (attenuation length)

25

Particle Flow Analysis (PFA)✦ The basic idea: combine the information of the tracker and the

calorimeter system to determine the jet energy. Momenta of charged jet fragments are determined with the tracker, energies of the neutral jet fragments come from the calorimeter

✦ This principle has been used successfully to improve the performance of experiments with modest hadronic calorimetry. However, the improvements are fundamentally limited. In particular, no one ha sever come close to separating W/Z this way

✦ The problem: the calorimeters do not know that the charged fragments have already measured by the tracker: These fragments are also absorbed in the calorimeter. Confusion: Which part of the calorimeter signals comes from the neutral jet fragments ?

✦ Advocates of this method claim that a fine detector granularity will help solve this problem. Other believe it would only create more confusion. Like with all other issues in calorimetry, this issue has to settled by means of experiments, NOT by Monte Carlo simulations !

26

PFA in CDF

27

Improvement of energy resolution expected with PFA

28from NIM A495 (2002) 107

PFA at CMS

29

PFA method: “π” jet

✦ The circles indicate the characteristic size of the showers initiated by the jet fragments, i.e. ρM for em showers and λint for the hadronic ones

30

from NIM A495 (2002) 107

CALICE: the zoo of PFA calorimeters

31

PFA: High Density + Fine Granularity✦ In order to reduce problems of shower overlap, PFA R&D focuses on

reducing the shower dimensions and decreasing the calorimeter cell size

32

X0 = 1.8 cm, λI=17 cm X0 = 0.35 cm, λI=9.6 cm

Iron Tungsten

PFA: High Density + Fine Granularity✦ Unfortunately the calorimeters do not see colors !

✦ How about calibration ?

33

Iron Tungsten

On calibration problems and its non-solutions✦ Proposed PFA systems consist of millions of readout channels

(fine granularity)

✦ Question: how does one want to calibrate these calorimeters ?

✦ Answer (CALICE): DIGITAL calorimetry (energy ∝ # of channels that fired)

34

This was tried and abandoned in 1983, for good reasons: Particle density in the core of em showers is very high

⇒ Non linearity & Signal saturation

The extremely narrow electromagnetic shower profile

35

from NIM A735 (2014) 130

CALICE hadronic digital prototypes

36from Rev. Mod. Phys, 2016 vol.88, 015300

Fe or W absorber and RPC readout

CALICE W/Si + Fe/plastic Hadronic prototype

37

rms90 is referred to as “the resolution”

from Rev. Mod. Phys, 2016 vol.88, 015300

CALICE W/Si ECAL

38

data from Rev. Mod. Phys, 2016 vol.88, 015300 and NIM A608, 372

Courtesy of R. Wigmans, to be published

Emeas = Emean + α

The CALICE Philosophy

39

from Rev. Mod. Phys, 2016 vol.88, 015300

“Detailed simulation studies in the framework ofpreparing detector concepts for future electron positron linear colliders such as ILC and CLIC have shown that resolutions around 3.5%–4% can be realized for jet energies from 50 up to1500 GeV. Such a resolution was shown to provide a separation of W and Z bosons decaying into pairs of jets on the basis of the dijet invariant mass, as shown in Fig. 83.”

“It was suggested to directly test the jet energy performance in test beams by creating bundles of particles from a primary beam hadron impinging on a thin target. Leaving aside the differences in particle momenta and multiplicity, or energy density, between these “jets” and those generated in quark fragmentation at the same primary energy, such an experiment would have prohibitive cost, as simulation studies have shown (Morgunov, 2009). For particle flow reconstruction magnetic momentum spectroscopy and large acceptance are indispensable.Consequently the experimental strategy must be to validate the critical ingredients of particle flow calorimetry individually.”

Conclusions I✦ In the past 25 years calorimetry has become a mature art

✦ High quality energy measurements will be an important tool for collider experiments at the TeV scale

✦ There are no fundamental reasons why the four vectors of all elementary could not be measured with a precision of 1% or better at these energies

✦ However this goal is far from trivial, especially for the hadronic constituents of matter and unfortunately little guidance is provided by hadronic Montecarlo shower simulations in this respect.

✦ In the past 30 years, all progress in this domani has been achieved trough dedicated R&D projects

40

Conclusions II✦ Two major R&D efforts are underway to improve the quality of

hadronic energy measurements.

✦ R&D in the PFA framework is more concentrated on the technicalities of detector design, despite the fact that some fundamental questions about applicability of this concept ( e.g. calibration) are still not fully clarified.

✦ The potential of the PFA concept may be real, but will need to be experimentally demonstrated

✦ R&D on dual-readout (DREAM) project concentrates strongly on experimental tests of the validity of the principles on which improvements of hadronic calorimetry with little attention to a 4π detector design and simulations in general.

✦ Until now DREAM has been remarkably successful. It combines the advantages of compensating calorimetry with a reasonable amount of design flexibility, with an instrument that can be calibrated with electrons 41

Backup Slides

Finally a way to measure e/h

Cu/scintillator e/h = 1.3 Cu/quartz e/h = 4.7

R(fem) = p0 + p1fem withp1

p0= e/h� 1

43

lectures on calorimetry - otranto 2016

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