Simulation of HEP experiments the Geant 4 toolkit

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Simulation of HEP experiments the Geant 4 toolkit

Tommaso Boccali

SNS, INFN Pisa

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Outline

Simulation – a step towards physical understanding

How do we simulate today a HEP experiment?

The Geant4 toolkit

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Why?

First of all, why do we simulate at all?

A lot of experiments in the past were without detailed simulations … and they worked!

Short answer: today’s theories do not predict such an overwhelming hint from data in most interesting measurements

And, moreover, we want are interested not only in discovery - see what LEP has

done J/Psi discovery

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Detailed tests of theories: when the precision level required for validating/discarding a theory is high, effects due to the “bias”

introduced by the detector are not negligible

Finite resolution

non-homogeneity

Particle misidentification

such that we cannot simply look at the data and say “Oh, it is

clear that the answer is X”

“B” events

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theory data simulation

simulation

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Hadronic physics

Even the theorists are not able to present analytical results for x-sections

Parton showers, string models …

Sometimes the best thing they provide are code to generate primary interactions

In some sense, Monte Carlo code has become the

“equation”

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So, the more subtle is the hint we want to look at, the more detailed the simulation must be

We have to thrust the background distributions to a level much greater than before

We need a tool capable to simulate (realistically) particle interactions with matter from the scale

of the TeV: primary particles generated in LHC collisions

of the keV: delta rays from secondary (tertiary…) particles leaving energy for ionization in our detectors

10

9

keV 1

TeV

1 =

Correct physics simulation needed for ~ 9 orders of magnitude

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Geant 4: what is it?

Once upon a time … a Fortran code called Geant 3 was used to simulate virtually all the HEP

experiments of the ‘80-’90

Very well tested, at the heart of the LEP Simulation

Agreement in physics up or better than % level

A product at the end of lifetime …

Fortran is not suited for a 10x complexity

Fortran is not maintainable for ~20 years

Visible energy in ALEPH (calo+tracker), DT vs MC

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Geant 4

The project started in 94 with the aim to completely rewrite Geant3

To use new technologies (C++)

To implement all the lessons learned

To use new data and ideas for calibrations

March 25

^th

2004: Geant 4 6.1 Release

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Geant4

Collaborators also from non- member institutions, including

Budker Inst. of Physics IHEP Protvino MEPHI Moscow Pittsburg University

Helsinki Inst. Ph.

PPARC Univ. Barcelona

HARP

Lebedev

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What exactly is G4?

It is not a program … there is nothing to run but examples when you download it

It a is a toolkit: a set of libraries which know how to

Handle a complex geometry

Treat decays / energy loss of all the common particles

Extend its capabilities with user code

New physics inputs

New features: visualization, data analysis

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Analogy with a real experiment

Define a Geometry Define physics

Define primary particles (the beam)

Beam ON!

(N events)

Analyze data Construct the detector

Well, usually not done by us ;)

Build an accelerator

Switch it on

Let it run, and save data

Plot histograms, and eventually

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Geometry

Beam ON!

(N events)

Analyze data

1. Define a detector

1. Shapes of each single component (a screw, a silicon wafer, a power

cable)

1. Box

2. Cylinder

3. …

2. Hierarchical placement of each component

3. Materials used

4. Define passive and active materials, instrumented with measurement capabilities

1. For these, define format of read

data, as close as possible to the real data taken with the apparatus

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Complex facilities

Not only easy volumes, but also

Polycones

Useful for HEP detectors,

useful with cylindrical symmetry

Boolean solids

G4UnionSolid G4SubtractionSolid G4IntersectionSolid

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Level of complexity…

CMS Tracker Simulation

900 different components defined

20000 sensitive detector

200000 global objects in the simulation

600 materials and composite materials

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Materials…

For some kind of “easy” physics, the definition of materials provides enough informations:

double density = 1.390*g/cm3;

double a = 39.95*g/mole;

G4Material*

G4Material* lArlAr ==

new G4Materialnew G4Material("liquidArgon",z=18.,a,density);("liquidArgon",z=18.,a,density);

a = 1.01*g/mole;

G4Element*

G4Element* elHelH ==

new G4Elementnew G4Element("Hydrogen",symbol="("Hydrogen",symbol="H",zH",z=1.,a);=1.,a);

a = 16.00*g/mole;

G4Element*

G4Element* elOelO ==

new G4Elementnew G4Element("Oxygen",symbol="("Oxygen",symbol="O",zO",z=8.,a);=8.,a);

density = 1.000*g/cm3;

G4Material* H2O = G4Material* H2O =

new G4Materialnew G4Material("Water",density,ncomp=2);("Water",density,ncomp=2);

H2OH2O->->AddElementAddElement(elH(elH, , natomsnatoms==22););

H2OH2O->->AddElementAddElement(elO(elO, , natomsnatoms==11););

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…

Materials also define automatically the “material budget of a detector”

“Quantity of matter” seen by a particle starting from the interaction point

Two parameters are used:

Electromagnetic interactions: X/X₀ is the number of radiation lengths of the given detector

A radiation length is the distance in which an (high energy) electron loses all but 1/e of its energy

Hadronic interactions: λ/λ₀ is the same due to hadronic effects (nuclear interaction length)

The same for nuclear interactions; used in hadronic calorimeters

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CMS Tracker

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CMS Hadron Calo

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Physics…

It is completely unrealistic to try and develop a physics

model which covers different particles and energy ranges…

Like: model for LHC physics, model for TeVatron physics

Please note it now: it is NOT the LHC physics (qq → Hqq);

it is only the physics of particle-matter interaction

Beam ON!

(N events)

Analyze data

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E>75 GeV model E<100 GeV

model

Hadronic inelastic interactions

G4 allows more than a single

physics model to contribute to the simulation of the interactions

Physics processes are defined with

a given energy range

a given particle type

By combining these wisely and paying some attention at the overlaps, a model for the

interesting field can be constructed

A different approach…

Particle type Process type

Energy range

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But …

This needs expertise from the user

It is easy to forget processes and so generate invalid simulations

Often more than 1 physics model is available for the same range:

You can decide to use the more detailed or the fastest…

at a price!

compt: Total cross sections from a parametrisation.

Good description from 10 KeV to (100/Z) GeV.

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Concept of “range”

Given a process, the user can define how accurate it has to be

The more accurate = the slower !

Consider Brem photon emission in a material

The charged track loses energy, and a photon is emitted.

In principle, infrared divergence which would lead to infinite # of photons of E→0

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Range

We have to specify a minimum range for the

process: the charged track always loses correctly

energy, but we can veto the creation of photons with Range smaller than a threshold

Passive materials: if the track goes inside a block of iron, there is no point in generating particles which would not exit from that – set range of the order of the volume size.

In an active detector, no need to simulate particle

which would give effects below the experimental

resolution.

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Calorimeters

Classic example, an electron enters the calorimeters

Where/when to stop the showers?

In principle, G4 could simulate electrons down to O(eV) – hours needed to simulate an event

What is usually done is to try and simulate events with increasing cuts, and define a point in which the price performance ratio is considered

acceptable

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500 MeV proton in a LAr-Pb (ATLAS) sampling calorimeter

Range at 1.5 mm

Corresponds to 450 keV in LAr

Corresponds to 2 MeV in Pb

Silicon detectors in CMS

Strip pitch in ~100 µm

No need to produce delta rays

which would travel less than 10 µm

Pb

LAr LAr Pb

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A few examples…

● EM physics

 “standard” processes valid from ~ 1 keV to ~ PeV

 “low-energy” valid from 250 eV to ~ PeV

 optical photons

● Weak physics

 decay of subatomic particles

 radioactive decay of nuclei

● Hadronic physics

 pure hadronic processes valid from 0 to ~100 TeV

 γ−, µ− and e-nuclear valid from 10 MeV to ~TeV

● Parameterized or “fast simulation” physics

● Tracking physics

● Particle transportation in the field

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A review…

Standard em processes:

Gamma

Photo-electric effect

Compton scattering

Electro, muon pair production

Electron

e ionization

e bremsstrahlung

e+e- annihilation

Syncrotron radiation

Muons

mu ionization

Mu bremsstrahlung

e+e- pair production

Charged hadrons

Hadron ionization

All charged particles

Multiple scattering

Transition radiation

Scintillation

Cerenkov radiation

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A review…

Hadronic processes:

At rest

Stopped mu, p, K, anti- proton

Elastic

Same processes for all the hadrons

Inelastic

Different processes for each hadron

Photo-nuclear, electro- nuclear, muon-nuclear

Ions (for example, Fe⁺⁺⁺)

Radioactivity!

Capture

Neutron capture

Fission

Neutron-induced, de- excitation

Most of these are data driven – eagerly waiting for

declassified declassified

data!

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Hadronic models

At rest In flight Direct implementations Cross sections Models Isotope production Event biasing

Direct impl. Direct impl. Theory framework

High energy Spallation framework Cascade Precompound

Direct impl.

Frag function impl. Process

Direct impl. Direct impl.

Direct impl.

Transport utility String parton

String fragmenation util. Evaporation util. Direct impl.

Frag function intfc Direct impl.

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Processes

Basic distinction:

Continuous processes: ionization, multiple scattering

Energy loss is continuous and not localized

Discrete: decay, Compton scattering, annihilation

These are the ones limiting the step length

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How does it work?

When a particle is under study, all of the possible energy losses mechanisms and decay processes must be able to “interfere” with the particle

A step is a “time” quantum for a particle traveling in a material

Start of step point End of step point

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The Step

The step length is limited by the processes of the particle: for example, after a few mm, the particle can decay

Each physics process must propose a step length

Given a lifetime for a decay, a random number is thrown and the resulting distance before decaying is returned

 The “physical step length” is the minimum of all proposed lengths

If the minimum between that step and the distance to the next detector is taken

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The beam

Well, not really the beam! In most HEP cases, Geant 4 is not starting from e⁺e^- (pp), but it is interfaced via to specialized codes for the simulation of primary

interactions

Pythia

CompHEP

ALPGEN

…

Geant 4 is able to read directly the output of these programs, and treat these particles as “the beam”

Exception: test beam simulations

The beam is a single particle thrown inside the experimental setup

Beam ON!

(N events)

Analyze data

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How an event is processed

List of primary particles

Track next particle

Push secondaries in the list

Interaction particle-matter

Out of the world, decayed or stopped

List of particles to be tracked

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Simulation of the response of a detector

Some volumes are special:

they are “active” in the real detector.

Data is read from these in real world

Geant 4 allows to simulate their response:

Special user code is needed

The output of this code must be as close as possible to the real response

Usually experimental parameterization used

Energy deposit in the strip

ADC counts

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Visualization

With complex geometries, often the best way to debug it to see …

Holes in the geometry

Overlapping volumes

Geant 4 provides natively (3) visualization features, most notably OpenGL

This means geometry, tracks, etc can be

visualized without any user code

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Visualization

LCD in JAS3

GLAST

BaBar Offline

Geant4 BaBar Offline

BaBar Online

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Validation

So, most of the HEP community is / will be using Geant4

The standard of the Physics simulation must be very high

Huge effort on validation

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Compton Scattering

Data , G4 LowE , G4 standard

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Rayleigh Scattering

Data , G4 LowE

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Stopping Power

Data , G4 LowE , G4 Standard

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Pair Production

Data, G4 LowE, G4 Standard

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Multiple Scattering (1)

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π Production from 730 MeV p

(LEP Model)

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π Production from 730 MeV p

(Bertini Model)

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From the experiments point of view…

Use test beam data

^{CMS HCAL}

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Crystal 25

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Just a glimpse …

Geant 4 is aimed / supported / funded mostly by HEP experiments, but is is becoming a de-facto standard

Medical applications

Simulation of radiotherapic apparata

Simulation of dose absorption in human tissues

Space applications

Detectors

Damages from high energy cosmic rays

Effects of cosmic rays on astronauts

Industrial applications

CCD of digital cameras

Implantation of oxides in silicon wafers

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Simulation

Simulation of dose deposit by radiotherapy in human tissues

Very useful to plan therapy cycles…

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γ astrophysics

γγγγγγγγ-ray bursts-ray burstsray burstsray burstsray burstsray burstsray burstsray bursts

AGILE

GLAST

Typical telescope:

Tracker

Calorimeter Anticoincidence

γ conversion

electron interactions

multiple scattering

δ-ray production

charged particle tracking

GLAST

NASA mission

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