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Alice Experience with Geant4 F.Carminati 1 , I.Gonz á lez 2 , I.Hrivnacova 3 , A.Morsch 1 for the ALICE Collaboration ( 1 CERN, Geneva; 2 IFCA, Cantabria; 3 IPN, Orsay). Isidro González Instituto de F ísica de Cantabria CHEP 2003 La Jolla, 24 March 2003. Outline. ALICE Experiment

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isidro gonz lez instituto de f sica de cantabria chep 2003 la jolla 24 march 2003

Alice Experience with Geant4F.Carminati1, I.González2, I.Hrivnacova3, A.Morsch1 for the ALICE Collaboration(1CERN, Geneva; 2IFCA, Cantabria; 3IPN, Orsay)

Isidro González

Instituto de Física de Cantabria

CHEP 2003

La Jolla, 24 March 2003

  • ALICE Experiment
  • Virtual MC & Geant4 VMC
  • Hadronic benchmarks
    • ALICE interest
    • Proton thin-target benchmark
    • Neutron transmission benchmark
  • G4UIRoot
  • Conclusions
alice experiment
Alice Experiment

Alice collaboration

online system

multi-level trigger

filter out background

reduce data volume

Total weight 10,000t

Overall diameter 16.00m

Overall length 25m

Magnetic Field 0.4Tesla

8 kHz (160 GB/sec)

level 0 - special hardware

200 Hz (4 GB/sec)

level 1 - embedded processors

30 Hz (2.5 GB/sec)

The ALICE collaboration includes 1223 collaborators from 85 different institutes from 27 countries.

level 2 - PCs

30 Hz

(1.25 GB/sec)

data recording &

offline analysis

virtual mc and geant4
Virtual MC advantages

Provides an interface to Monte Carlo programs

No coupling between the user code and the concrete MC

The same user application may be run with several MCs

2 MCs already implemented:



ALICE effort is now concentrated on including also Fluka

Geant4 VMC

Built as a new package external to Geant4

A big effort has been done in order to minimize the limitations

The geometry part is based on G3toG4

From Geant4 4.0 there is support for reflections

Limited support for “MANY”

Overlapping volumes have to be specified explicitly (via G4Gsbool function)

Virtual MC and Geant4

Detailed information in the presentation from I. Hrivnacova: The Virtual MonteCarlo or

geant4 vmc and alice
ALICE background event

HIJING parameterization event generator

5000 primary particles (5.8 % of full background event)

Modular physics list according to the physics list in G4 example N04 (electromagnetic and hadronic physics)

Included 12 detectors and all structures

ITS coarse geometry (due not resolved MANY)

The kinetic energy cuts equivalent to those in G3 were applied in G4 using a special process and user limits objects

Standard AliRoot magnetic field map


Finished successfully

Protection against looping particles

Hits for 10 (from 12) detectors. Missing:

ITS (coarse version does not produce hits)

RICH (requires adding own particles to the stack – not yet investigated)

Comparisons of hits x, z distribution

No detailed analysis yet

2 to 3 times slower than Geant3

Still preliminary

Geant4 VMC and ALICE
hadronic benchmarks reasons
Hadronic benchmarks: Reasons
  • Low momentum particle is of great concern for central ALICE and the forward muon spectrometer because:
    • ALICE has a rather open geometry (no calorimetry to absorb particles)
    • ALICE has a small magnetic field
    • Low momentum particles appear at the end of hadronic showers
  • Residual background which limits the performance in central Pb-Pb collisions results from particles "leaking" through the front absorbers and beam-shield.
  • In the forward direction also the high-energy hadronic collisions are of importance.
proton thin target benchmark
Proton Thin-Target Benchmark
  • Experimental and simulation set-up
  • Conservation laws
  • Azimuthal distributions
  • Comparisons with data: Double differential cross sections
  • Conclusions

Note:Revision of ALICE Note 2001-41 with Geant4.5.0 (patch 01)

proton thin target experimental set up

Beam energies: 113, 256, 597 & 800 MeV

  • Neutron detectors at: 7.5º, 30º, 60º, 120º & 150º
  • Detector angular width: 10º
  • Materials: aluminium, iron and lead
  • Thin target only one interaction
  • Data information from Los Alamos in: Nucl. Sci. Eng., Vol. 102, 110, 112 & 115
Proton Thin TargetExperimental Set-Up
proton thin target simulation set up

Processes used:


Proton Inelastic:G4ProtonInelasticProcess

2 sets of models:

Parameterised (GHEISHA): G4L(H)EProtonInelastic

Cascade and Precompound:G4CascadeInterfaceG4PreCompoundModel

The Cascade code is new and “fresh” since 5.0


Very low cross sections: Thin target is rarely “seen” CPU time expensive

One very large material block: One interaction always takes place Save CPU time

Stopevery particle after the interaction: Store its cinematic properties

Proton Thin TargetSimulation Set-Up
conservation laws
Systems in the reaction:

Target nucleus

Incident proton

Emitted particles

Residual(s): unknown in the parameterised model

Conservation Laws:

Energy (E)

Momentum (P)

Charge (Q)

Baryon Number (B)

Conservation Laws
conservation laws in the parameterised model
Conservation Laws in the Parameterised Model
  • The residual(s) is unknown It must be calculated
    • Assume only one fragment
  • Residual mass estimation:
    • Assume B-Q conservation:
      • We found negative values of Bres and Qres
    • Assume E-P conservation
      • Eres and Pres are not correlated  unphysical values for Mres
  • Aluminum is the worst case
conservation laws in the cascade precompound models
Conservation Laws in the Cascade & Precompound Models
  • There were some quantities not conserved in the initial tested versions (Precompound alone)
  • Charge and baryon number are now conserved
  • Momentum is not conserved.
    • But it was exactly conserved in previous versions (Precompound alone)
    • Can be up to 30 MeV
    • It is correlated with:
      • The target mass number: the smaller A, the bigger non-conservation
      • The incident proton energy: Non-conservation increases with proton energy
    • For Lead it shows a strange bump
  • Energy is not conserved:
    • Precompound alone had a small non-conservation width of the order of a few MeV
    • Now the width is bigger and shows spikes.
energy non conservation in the cascade model
Energy non-conservation in theCascade Model


Cascade &Precompound

azimuthal distributions
What, how, why?

Known bug in GEANT3 implementation of GHEISHA

Expected to be flat

Separated for p and nucleons


j distributions are correct! However…

Parameterised model:

At 113 & 256MeV: No p is produced

At 597 & 800MeV:

Pions are produced in Aluminium and Iron

(Almost) no p is produced for Lead

Cascade & Precompound models:

Are now able to produce p




Azimuthal distributions
double differentials
Double differentials
  • Real comparison with data
  • We plot
  • Which model is better?…
    • With Precompound alone it was difficult to say
    • Now Cascade & Precompound are much better than the parameterised models
    • Still we see big discrepancies for low incident proton energies and light targets
double differentials1
Double Differentials



double differentials2
Double Differentials



double differential ratio al @ 256
Double Differential Ratio Al @ 256



conclusions proton
Conclusions Proton
  • We found several bugs in GEANT4 during proton inelastic scattering test development
    • Most of them are currently solved.
  • The parameterised model cannot satisfy ALICE physics requirements
  • The Precompound model combined with the new Cascade model:
    • Improves a lot the agreement with data for the double differential cross sections!
    • Is able to produce pions in the reaction
    • But… introduces a new energy-momentumnon-conservation!
neutron transport benchmark
Neutron Transport Benchmark
  • Experimental and simulation set-up
  • Simulation physics
  • Flux distribution
  • Conclusions

Note: Linux gcc 2.95 (supported compiler) was used

Note2: It has not been redone with the latest Geant4 version

simulation set up



x = 0, 20 & 40 cm





401 cm

Simulation set-up
  • Incident neutrons energy spectra.
    • Peak at 43 and 68 MeV
  • Test shield material and thickness:
    • Iron (20 & 40 cm)
    • Concrete (25 & 50 cm)
simulation physics
Simulation Physics
  • Electromagnetic: for e± and g
  • Neutron decay
  • Hadronic elastic and inelastic processes for neutron, proton and alphas
    • Tabulated (G4) cross-sections for inelastic hadronic scattering
    • Precompound model is selected for inelastic hadronic scattering
  • Neutron high precision (E < 20 MeV) code with extra processes:
    • Fission
    • Capture
  • 1 million events simulated for each case
conclusions neutron
Conclusions Neutron
  • The MC peak, compared to the data, is narrower an higher
  • Low energy disagreement:
    • Attributed by H.P. to backscattering due to so simple geometry
    • Needs more investigation
  • Though the simulation does not match the data:
    • Iron simulation shows better agreement than Concrete
    • For concrete lower energies seem better
  • A GUI for Geant4:
    • Built with ROOT
  • …providing:
    • an easy way to explore G4 command tree
    • a quick inspection of standard/error output
  • A C++ Interpreter (CINT)
    • That may allow run time access to G4 classes
    • That certainly allows access to all ROOT functionallity
  • More info in:

g4uiroot features
Full Geant4 command tree displayed in a “file system” like structure

Availability clearly marked

Non available commands are identified and cannot be selected.

The availability is correctly updated with Geant4 status

Normal Geant4 command typing is also possible

Selecting a command in the tree will automatically update the command line input widget and vice-versa

Automatic command completion using the TAB key

The navigation through the successful commands executed before may be done using the arrow keys

Full and short command help

External Geant4 macros and ROOT TBrowser accessible through the menu

Customisable main window title and pictures

Different windows for error and normal output with saving capabilities

History window with saving capabilities.

History is always tracked.

Successful commands may be recalled at any point hitting the up arrow at the command line.

Root interpreter (CINT) included

It runs in the terminal.

Will give run-time access to Geant4 if it is CINTified

G4UIRoot Features
final conclusions
Final conclusions
  • ALICE has done a big effort to use GEANT4
    • It is already integrated in AliRoot through the Virtual MC framework
    • But the PPR production will be done with Geant3
    • The effort is now concentrated on bringing Fluka into the VMC.
  • Concerning the hadronic benchmarks:
    • We see and important improvement in the quality of the models
      • But it seems there is still space for more
    • Some more work needs to be done in ALICE:
      • Test EGPLs and contribute with plots/experience
      • Improve the results from the neutron transport benchmark
  • The ALICE effort has contributed:
    • To spot bugs/deficiencies in Geant4 Most of them already corrected!
    • To develop new functionality (reflections, G3toG4)
    • In providing an easy and clear way to compare Geant3 and Geant4 (and soon Fluka) in big applications via de VMC