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

2 Outline ALICE Experiment Virtual MC & Geant4 VMC Hadronic benchmarks – ALICE interest – Proton thin-target benchmark – Neutron transmission benchmark G4UIRoot Conclusions

3 The ALICE collaboration includes 1223 collaborators from 85 different institutes from 27 countries. online system multi-level trigger filter out background reduce data volume level 0 - special hardware 8 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 weight10,000t Overall diameter 16.00m Overall length25m Magnetic Field0.4Tesla Alice Experiment

4 The Virtual MC User Code VMC Virtual Geometrical Modeller G3 G3 transport G4 transport G4 FLUKA transport FLUKA Geometrical Modeller Reconstruction Visualisation Geant3.tar.gz includes an upgraded Geant3 with a C++ interface Geant4_vmc.tar.gz includes the TVirtualMC Geant4 interface classes Generators

5 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

6 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

7 Geant3 and Geant4 VMC in ALICE Hits in the TPC Geant3 Geant4

8 Geant3 and Geant4 VMC in ALICE Hits in the TRD Geant3 Geant4

9 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.

10 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 with Geant4.5.0 (patch 01)

11 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

12 Proton Thin Target Simulation Set-Up Physics Processes used: – Transportation – Proton Inelastic: G4ProtonInelasticProcess 2 sets of models : – Parameterised (GHEISHA): G4L(H)EProtonInelastic – Cascade and Precompound: G4CascadeInterface G4PreCompoundModel 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

13 Parameterised Physics List theParticleIterator->reset(); while ((*theParticleIterator)()) { G4String particleName = theParticleIterator->value()->GetParticleName(); G4ProcessManager* pmanager = particle- >GetProcessManager(); if (particleName == "proton") { G4ProtonInelasticProcess* theInelasticProcess = new G4ProtonInelasticProcess("Had_Inelastic_proton"); G4LEProtonInelastic* theLEInelasticModel = new G4LEProtonInelastic; G4HEProtonInelastic* theHEInelasticModel = new G4HEProtonInelastic; theInelasticProcess->RegisterMe(theLEInelasticModel); theInelasticProcess->RegisterMe(theHEInelasticModel); pmanager->AddDiscreteProcess(theInelasticProcess); }

14 Precompound Physics List G4CascadeInterface* theCascade = new G4CascadeInterface(); G4PreCompoundModel* thePreEquilib = new G4PreCompoundModel(new G4ExcitationHandler); theCascade->SetDeExcitation(thePreEquilib); theParticleIterator->reset(); while ((*theParticleIterator)()) { G4String particleName = theParticleIterator->value()->GetParticleName(); G4ProcessManager* pmanager = particle- >GetProcessManager(); if (particleName == "proton") { G4ProtonInelasticProcess* theInelasticProcess = new G4ProtonInelasticProcess("Had_Inelastic_proton"); theInelasticProcess->RegisterMe(theCascade); pmanager->AddDiscreteProcess(theInelasticProcess); }

15 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)

16 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 B res and Q res – Assume E-P conservation E res and P res are not correlated  unphysical values for M res Aluminum is the worst case EnergyQ<0B<0N neu < MeV0.00 % 256 MeV0.38 %0.02 %0.44 % 597 MeV0.77 %0.00 %0.90 % 800 MeV1.20 %0.00 %1.50 %

17 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.

18 Momentum non-conservation in the Cascade Model Light nucleus Heavy nucleus Proton energy

19 Energy non-conservation in the Cascade Model Precompound alone Cascade & Precompound

20 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 

21 Parameterised model:  pions : 597 MeV BeforeNow

22 Parameterised model:  nucleons : 597 MeV NowBefore

23 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

24 Double Differentials Parameterised Precompound

25 Double Differentials Parameterised Precompound

26 Double Differential Ratio 113 Parameterised Precompound

27 Double Differential Ratio 256 Parameterised Precompound

28 Double Differential Ratio 256 Parameterised Precompound

29 Double Differential Ratio 597 Parameterised Precompound

30 Double Differential Ratio 597 Parameterised Precompound

31 Double Differential Ratio 800 Parameterised Precompound

32 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!

33 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

34 Tiara Facility

35 Target Views Top ViewSide View

36 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 Experimental Simulated Experimental Simulated

37 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

38 Preliminary Results: 43 MeV Test Shield: Iron – Thickness: 20 cm

39 Preliminary Results: 68 MeV Test Shield: Iron – Thickness: 20 cm

40 Preliminary Results: 43 MeV Test Shield: Iron – Thickness: 40 cm

41 Preliminary Results: 68 MeV Test Shield: Iron – Thickness: 40 cm

42 Preliminary Results: 43 MeV Test Shield: Concrete – Thickness: 25 cm

43 Preliminary Results: 68 MeV Test Shield: Concrete – Thickness: 25 cm

44 Preliminary Results: 43 MeV Test Shield: Concrete – Thickness: 50 cm

45 Preliminary Results: 68 MeV Test Shield: Concrete – Thickness: 50 cm

46 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

47 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:

48 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

49 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