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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

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Outline

ALICE Experiment Virtual MC & Geant4 VMC Hadronic benchmarks

– ALICE interest– Proton thin-target benchmark– Neutron transmission benchmark

G4UIRoot Conclusions

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The ALICE collaboration includes 1223

collaborators from 85 different institutes from

27 countries.

online systemmulti-level triggerfilter out backgroundreduce data volume

level 0 - special hardware8 kHz (160 GB/sec)

level 1 - embedded processors

level 2 - PCs

200 Hz (4 GB/sec)

30 Hz (2.5 GB/sec)

30 Hz

(1.25 GB/sec)

data recording &

offline analysis

Alice collaboration

Total weight 10,000tOverall diameter 16.00mOverall length 25mMagnetic Field 0.4Tesla

Alice Experiment

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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:– Geant3– Geant4

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)

Detailed information in the presentation from I. Hrivnacova: The Virtual MonteCarlo or http://root.cern.ch/root/vmc/VirtualMC.html

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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

Results

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

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Geant3 and Geant4 VMC in ALICEHits in the TPC

Geant3 Geant4

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Geant3 and Geant4 VMC in ALICEHits in the TRD

Geant3 Geant4

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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.

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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)

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Proton Thin TargetExperimental 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

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Proton Thin TargetSimulation Set-Up

Physics

Processes used:– Transportation– 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

Geometry 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

Stop every particle after the interaction: Store its cinematic properties

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Conservation Laws

Systems in the reaction:1. Target nucleus

2. Incident proton

3. Emitted particles

4. Residual(s): unknown in the parameterised model

Conservation Laws:1. Energy (E)

2. Momentum (P)

3. Charge (Q)

4. Baryon Number (B)

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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

Energy Q<0 B<0 Nneu < 0

113 MeV 0.00 % 0.00 % 0.00 %

256 MeV 0.38 % 0.02 % 0.44 %

597 MeV 0.77 % 0.00 % 0.90 %

800 MeV 1.20 % 0.00 % 1.50 %

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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.

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Momentum non-conservation in the Cascade Model

Lig

ht n

ucle

usH

eavy

nuc

l eus

Proton energy

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Energy non-conservation in theCascade Model

Precompoundalone

Cascade &Precompound

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Azimuthal distributions

What, how, why?

Known bug in GEANT3

implementation of

GHEISHA

Expected to be flat

Separated for and

nucleons

Results

distributions are correct! However…

Parameterised model:– At 113 & 256 MeV: No is

produced– At 597 & 800 MeV:

Pions are produced in Aluminium and Iron

(Almost) no is produced for Lead

Cascade & Precompound models:

– Are now able to produce

x

y

z

p

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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

ΩE dd

d2

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Double Differentials

Parameterised

Precompound

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Double Differentials

Parameterised

Precompound

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Double Differential Ratio Al @ 256

Parameterised

Precompound

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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-momentum

non-conservation!

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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

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Tiara Facility

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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)

x = 0, 20 & 40 cm

y

x

401 cm

ExperimentalSimulated

ExperimentalSimulated

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Simulation Physics

Electromagnetic: for e± and 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

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Preliminary Results: 43 MeVTest Shield: Iron – Thickness: 20 cm

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Preliminary Results: 43 MeVTest Shield: Concrete – Thickness: 50 cm

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Preliminary Results: 68 MeVTest Shield: Concrete – Thickness: 50 cm

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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

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G4UIRoot

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:http://home.cern.ch/iglez/alice/G4UIRoot

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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

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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