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About Calorimetry

Henri Videau Ecole Polytechnique

Monschau, September 6-7 2000

This presentation can be found at http://polywww.in2p3.fr/~videau/Ecole_de_Monschau

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 2

Outline

Calorimetry. What does that mean ? What is it for?

Principles, the physical bases of calorimetry

Real calorimeters, simple or composite

Designing a calorimeter: the lessons of today ALEPH the prospects of tomorrow TESLA.

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 3 The purpose

Measuring neutrals

electromagnetic :  hadronic : neutrons, K0 l

Measuring Identifying leptons, electrons and The of a is known only from the tracker UA2 D0 Measuring charged

Measuring jets energy flow Global method / analytic

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 4

Reminder on getting the charged particles

Charged particles through electromagnetic interaction leave a lot of small energy deposits (small transfers) when passing through matter.

This defines a trajectory along which the particle properties are about untouched; measuring this trajectory provides the direction of the particle, its in presence of a magnetic field, through its curvature its nature through the spectrum of the energy deposits, dE/dx

p= 0.3 B R GeV ,T , m

The error on the radius R comes from the error on the sagitta L2  p R= ∝  s 8 s p2

The uncertainty on the momentum increases like p2

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 5 The principle

To measure a neutral convert it in charged particles

 : pair conversion, electromagnetic interaction Neutral hadrons : Neutrinos : weak interaction (charged current) Then you measure the charged tracks

One needs to provide matter to interact with (radiator) a medium sensitive to charged tracks (detector)

Radiator and detector can be the same medium, homegeneous calorimeter Radiator and detector can be interleaved, sampling calorimeter

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 6 The simplest case: a wall followed by a charged in a magnetic field

ALEPH A radiated by a muon is converted in the vertex detector and appears as a pair in ITC and TPC where the an can be measured,  energy, position, direction.

Thin wall = low conversion probability Thick wall = reinteractions

If the detector is denser it can be used also as radiator but the momentum measurement is degraded. Compromise. And we rely on a magnetic field If we go for a "thick" material the electrons are degraded by which create pairs which radiate ..... it is a shower

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 7 Development of a shower

Above a given energy (critical) an electron looses energy mostly by emitting Bremsstrahlung with a spectrum in 1/k2 Above a given energy the photons convert into electron pairs which in turn ......

This develops in the depth of the radiator in what is called a shower

A shower contains electrons becoming less and less energetic, until they loose their energy purely by dE/dx, and photons down to the pair threshold, below they escape.

And the !

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 8 How do we see the charged particles in a calorimeter? By one of the ways it looses energy

Radiation Bremsstrahlung, rare but for low mass particles Synchrotron radiation, rare Transition radiation, but no wall and rare 1 Cerenkov, visible light   n Excitation Electrons of atoms are ejected to higher levels and go back radiatively

Ionisation Electrons in gases are freed from their atoms and follow the fields  rays Electrons in semiconductors (Si, Ge) go the conduction band And in liquid noble gases ??

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 9

With strong similarities it's quite different

1) Measuring the electromagnetic showers

2) Measuring the hadronic showers

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 10 Characterizing the radiator

By the cross sections of the incoming particles For electromagnetic processes (pair conversions, Bremsstrahlung) the cross sections are essentially flat above 100 MeV concept of (X ), length of material after which 0 electrons have lost 1/e of their energy. It is expressed in units of length (cm) but often in g/cm2 by dividing by the density.

A 2 Approximately X =180 g cm− 0 Z 2

Critical energy, comparing the radiation loss with the dE/dx

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 11 Main processes in an electromagnetic shower

N, e

Bremsstrahlung Pair creation But also At low (< 2 GeV )

Compton At high energy at the level of 10-4 Muon pair creation

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 12 From "Nuclei and Particles " by Segré

Interaction of a photon with the nuclear field, pair, and with the field of an electron, triplet.

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 Courtesy of NIST 13

Contribution to the cross section  Carbon in barns/atom

1 barn = 10 -24 cm 2

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 14

ALEPH

Creation of a  pair.

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 15 Shape

The shape of a photon shower is like the shower of two electrons.

As soon as the electron energy is large compared to the pair mass, ( in X and R ) the shape of the shower is rather standard 0 M . Longitudinal For a given incident energy the length (maximum) scales with X 0 It scales logarithmically with the energy The loss of energy on a given depth is proportionnal to the energy Lateral The lateral spread is due to the electron multiple scattering and the angle at which photons are emitted It is characterised by the Molière radius

Typically 90 % of the energy is within 1.7 R M 10 −3 R M cm≈ g cm  

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 16 From Rossi

Electron showering on lead plates in a Wilson chamber (1949)

Next gamma and showering in the atmosphere simulation

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 17

V

V

eV

0 Ge

s

50 G

300 Ge

p

auroi

N

de

thieu

Ma

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esy

rt

u

By co

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 18

ALEPH

A 45 GEV electron

The electromagnetic calorimeter has 45 samples in depth. This is the profile of the energy collected in each sample for a 45 GeV electron

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 19

Celeste

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 20

Celeste

Using he wave front

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 21

Hadronic showers

They develop through strong interactions. Transverse spread related to typical transverse momentum Above few GeV the cross sections are flat and not too different, concept of interaction length Below, huge differences due to resonances.

About one third of the products of a strong interaction are ’s The non interacting charged hadrons loose energy by dE/dx and nuclear collisions with nuclei (40%) The amount of energy lost by a hadron, nuclei breaking up, is a priori different from what you get from an electromagnetic shower e/ ratio

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 22

+ p and - p cross sections

From PDG

10 mbarn = 1 fm-2

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 23 From the particle to the Measuring the energy number

How do we get an estimate of the energy released in a shower? We see only the charged particles No use of magnetic field in a shower

Except for the low energy photons which escape, all the energy is released in the medium by the charged.

Even in homogeneous calorimeters only one part of the released energy is measured

We can collect visible light (crystals, glass, ), free electrons (gas), electron-hole pairs (semiconductors)

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 24 Different types of calorimeters

Homogeneous measurement by collecting light collecting electrons (holes) Scintillation Ionisation Cerenkov e-h pair creation

Crystals Na I, Cs I, BGO, PbWO4 Gases Noble gases (liq) Noble gases (liq) Ge Lead glass Water Air

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 25 Sampling calorimeters

Radiators for electromagnetic (high Z): Lead, Uranium, Tungsten beware of physical but also mechanical properties!

For hadronic (cheap, mechanically sound): iron (return yoke), stainless steel (in field), copper Detectors are similar for both: scintillators, Cerenkov light (H1 lumi), ionisation chambers, liquid argon, warm liquids, gas chambers in different modes (prop, streamer, Geiger), silicon detectors.

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 26

The energy lost in breaking nuclei Hadrons loose energy by breaking nuclei which can emit neutrons, the kinetic energy of these low energy neutrons is dissipated by scattering on nuclei.

That is invisible if the nuclei are heavy, but detected if the scatter occurs on protons.

Principle of the hardware compensation, a radiator like uranium (or even lead) a detecting medium rich enough with free protons (scintillators) will exhibit the energy of the neutrons through the energy loss of the protons. Very strong constraints Software compensation use weights in function of energy density, nature of the shower

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 27 Radiator physical properties

Material Z dE/dx density X I 0 cm MeV/cm g/cm3 cm Iron Fe 26 16.8 11.4 7.87 1.76 Tungsten W 74 9.6 22.1 19.3 0.35 Lead Pb 82 17.1 12.7 11.35 0.56 Uranium U 92 10.5 20.5 19. 0.32

Argon (liq) A 18 83.7 2.13 1.4 14. Air 74380 2.2 10-3 1.21 10-3 30300

Remarks: strong dependence on density (obvious) energy loss in one interaction length almost constant large variations of dE/dx in one radiation length large variations of  / X with Z  0

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 28 Radiator physical properties

Material Z  /X dE/dx X X I 0   MeV cm Iron Fe 26 9.5 20.1 1.76 Tungsten W 74 27.4 7.7 0.35 Lead Pb 82 30.5 7.1 0.56 Uranium U 92 32.8 6.7 0.32

Argon (liq) A 18 6.0 29.8 14. Air 2.5 66.7 30300

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 29

Collecting the signals

From light: photomultipliers, diodes (APD, HPD) Quantum efficiency Beware of the magnetic field Clear fibers From electrons: what amount of signal? Number of electrons extracted by the passage of a particle Dense media: liq. A, Si diodes. Do not need amplification in situ, but noise Gases: few electrons, 30/cm, needs local amplification Beware of the magnetic field Electric cables Pick up noise

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 30 Measuring the position

To measure the position transverse to the particle flight necessitates to read out independently transverse cells

Two regimes: The cell size is large compared to the shower size it dominates the precision The cell is of the size of the Molière radius or below. The shower properties dominate Solutions: the complete depth can be segmented (fluctuation limited) the segmentation is done only at a depth where the shower has started is still narrow

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 31 Measuring the angle  is the shower position precision To measure the mass of a two photon system Knowing the origin:  = d is the distance to the origin d

But it may be of use to know if a photon comes from the interaction

Rejection GMSB

Needs a position measurement at two depths  =2  is the distance between the two samples 

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 32 Performances

Linearity Origin of fluctuations Energy resolution

Position precision Electron/ identification

Angular precision Muon/pion identification

Separation between showers

For photons, hadrons, jets isolated, in jets Radiation damage

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 33

Linearity

Most of the calorimeters are linear, but non linearities may be introduced at different levels

Deliberate non linearity introduced to reduce the dynamical range

Saturation in the read out Inhomogeneity in depth, in particular for hadrons

Saturation at the level of the detector, high gain, cell counting,..

E = E ( 1 - E ) with 10 -3 ALEPH m t  t   Induces non additivity!

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 34

Energy resolution On isolated particles

Sources of fluctuations For electromagnetic calorimeters We measure always a fraction of the deposited energy

Detector limits leakage, Weighting insensitive material in front, .. Intrinsic limitations They have always a stochastic character We measure numbers of photons, electrons, e-hole pairs,..

Larger number of objects  better resolution But correlations and global constraint Fano factor  E  = E  E

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 35

Noise

Electronic noise  E  = incoherent, coherent E E Important only at low energies

In a multiple channel system, intercalibration error  E = Deadly at high energy E Difficult to get it smaller than 1-2 %

For an electron, do you prefer to measure the energy or the momentum? 2  p=k p = E=  E For k = 10 -4 and  = 0.1 p = 100 GeV

 E   = ⊕ ⊕ E  E E

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 36 The typical resolution for different types of calorimeters

A Cerenkov calorimeter measures the length of electrons with  > 0.9 adding the fluctuation on the number of photons + the conversion in the light detector, quantum efficiency Typical stochastic resolution: 5 % A crystal calorimeter, scintillation, is around 1-2 %

A sampling calorimeter will be worse since a large fraction of energy is lost in the radiator. The resolution scales like the (radiator thickness)1/2 For "thin" sensitive media, the best estimator is the number of tracks crossing the detector, this is degraded by the length dispersion due to the angles and by Landau fluctuations. Use of saturation. Typical resolutions for half radiation length thickness are 20 % for gas detectors, 12 % for liq. A, 10 % for scintillators

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 37

For hadron calorimeters

The main fluctuation comes from the fraction of photons

if e/  1 Depends on energy

In view of the volume of the shower, saturation never hurts

The scaling laws are, more or less, similar to electromagnetic

Typical resolutions 80%

40% with compensation or counting

At low energies problem of the nature of the particle

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 38 For particles in jets Separation between showers

Separate the particle shower from the environment Like separate the two photons of a 0 (by moments) or a photon from a charged pion shower.   Granularity lateral and longitudinal at the lowest level (Molière radius) Is that separation needed? YES  For jets A global calorimetric approach does not know if there is a  is lost with a  has the resolution of the worst part An analytic approach tries to make use of the tracker and extract gammas and neutral hadrons (10% of the energy) from the charged Signs the presence of  by charged leptons including taus

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 39

When looking at e/ or / separation

We need to consider

Probability for an electron (resp. pion) The efficiency to be recognised as electron (resp. pion)

Probability for an electron (resp. pion) The contamination to be recognised as pion (resp electron)

as a function of momentum

Efficiencies can be at the level of 99 % or more for contamination at the % level ALEPH tau physics

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 40 Electron/pion separation

An electron starts showering quickly and is stanched in 20 - 30 X 0 1/e pions do not interact in 1 . I Longitudinal separation: Choose a material which maximise / X . Z (factor 30 for lead) I 0  Longitudinal shape In first few X0 : large deposit for electrons, small for pions, (compared to momentum) In 30 X0 : Electron: energy  momentum, pion : energy ≪ momentum Lateral shape, position accuracy Limits intrinsic: charge exchange, production of muons or hadrons in e-m showers 10-4 Detector imperfections or defects Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 41

ALEPH

An electron (45 GeV) showering in the ECAL

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 42

ALEPH

A pion sailing through the ECAL to shower in the HCAL

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 43

ALEPH

A Bhabhadron

From the left shower emerge two pions seen in the HCAL

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 44 Muon/pion separation

At current energies muons and electrons can not be confused! But for muons produced in the showers

Muons almost do not radiate, they sail through the calorimeters or stop by dE/dx

Pions shower in HCAL (or stop)

Longitudinal and lateral shapes, position accuracy after lot of matter

Limits :  sail through (e-6), punch through, decay in flight

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 45

ALEPH

A muon sailing through ECAL and HCAL to the muon chambers

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 46

ALEPH

A so called llV

Z  e+ e- V

V  + - Monschau, September 6-7 2000 Low energy muon stopping in the HCAL. Identification cut-off

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 47 Examples of large calorimeters

Low energy machines CLEO, BaBar crystal Cs I,  no HCAL

OPAL lead glass, Fe-w.ch. L3 BGO LEP ALEPH, DELPHI Pb/Fe-wire chambers sandwich

SLC SLD Pb/Fe-liq.A

HERA H1 Pb/Fe-liq.A ZEUS U-scint.

Tevatron D0 U-liq. A CDF CMS PbWO4, Fe-scintillator LHC ATLAS Pb-liq. A, Fe-scintillator

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000 48

What drives your choice of a calorimeter?

Electromagnetic

Excellent (stochastic) resolution. Crystals A good resolution and stability. Liquid Argon Good resolution, fast and cheap. Scintillators High granularity. Gas chambers, silicon

Hadronic Good hardware resolution. U(Pb)-scintillator Good software resolution. Liq. Argon Good pattern identification. Gas chambers

Henri Videau, Ecole Polytechnique Monschau, September 6-7 2000