Isidro González Instituto de F ísica de Cantabria CHEP 2003 La Jolla, 24 March 2003

1. 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

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

3. 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

4. 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) Virtual MC and Geant4 Detailed information in the presentation from I. Hrivnacova: The Virtual MonteCarlo or http://root.cern.ch/root/vmc/VirtualMC.html

5. 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 Geant4 VMC and ALICE

6. Geant3 and Geant4 VMC in ALICEHits in the TPC Geant3 Geant4

7. Geant3 and Geant4 VMC in ALICEHits in the TRD Geant3 Geant4

8. 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.

9. 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)

10. 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

11. 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 Stopevery particle after the interaction: Store its cinematic properties Proton Thin TargetSimulation Set-Up

12. 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

13. 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

14. 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.

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

16. Energy non-conservation in theCascade Model Precompoundalone Cascade &Precompound

17. What, how, why? Known bug in GEANT3 implementation of GHEISHA Expected to be flat Separated for p and nucleons Results 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 y  x z Azimuthal distributions

18. 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

19. Double Differentials Precompound Parameterised

20. Double Differentials Precompound Parameterised

21. Double Differential Ratio Al @ 256 Precompound Parameterised

22. 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!

23. 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

24. Tiara Facility

25. Experimental Simulated x = 0, 20 & 40 cm Experimental x Simulated y 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)

26. 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

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

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

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

30. 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

31. 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

32. 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

33. 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