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Alice Experience with Geant4

Alice Experience with Geant4. Isidro González ALICE / Houston University Geant4 Users Workshop 14 - November - 2002. Summary. ALICE Experiment AliRoot framework Virtual MC Hadronic benchmarks ALICE interest Proton thin-target benchmark Neutron transmission benchmark G4UIRoot

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Alice Experience with Geant4

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  1. Alice Experience with Geant4 Isidro González ALICE / Houston University Geant4 Users Workshop 14 - November - 2002

  2. Summary • ALICE Experiment • AliRoot framework • Virtual MC • 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. Detectors and Events ALICE Detector complexity similar to ATLAS and CMS Typical ALICE event/100

  5. AliRoot Framework • AliRoot is the ALICE software framework for simulation, reconstruction and analysis • It is based on ROOT • The user code is in C++ • There is some legacy code in FORTRAN: • Geant3 • Event generators • “microcernlib” • Integrates simulation, reconstruction and analysis ALICE software • Each detector subsystem has one single package (one directory, one library)

  6. AliRoot layout AliEn STRUCT CRT START FMD MUON TPC RALICE PMD EMCAL TRD ITS PHOS TOF ZDC RICH ISAJET STEER HIJING AliRoot EVGEN MEVSIM HBTAN HBTP PDF PYTHIA6 ROOT Virtual MC Geant3 VMC Geant4 VMC Geant4 Geant3 Fluka VMC Fluka

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

  8. 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 implementated: Geant3 Geant4 ALICE effort is now concentrated on including also Fluka Geant4 VMC limitations The geometry part is based on G3toG4 G3toG4 limitations (reflections, MANY) minimized with Geant4 4.0 Limited support for “MANY” Overlapping volumes have to be specified explicitly (via G4Gsbool function) A few more minor limitations None of them a real obstacle for using the VMC For more information see: http://root.cern.ch/root/vmc/VirtualMC.html Virtual MC and Geant4

  9. 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 11 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 0.2 Tesla Results Finished successfully Protection against looping particles Hits for 9 (from 11) 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 times slower than Geant3 Still preliminary Not a worry Geant4 VMC and Alice

  10. 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.

  11. Proton Thin-Target Benchmark • Experimental and simulation set-up • Conservation laws • Azimuthal distributions • Double differential cross sections • Conclusions Note:Revision of ALICE Note 2001-41 with Geant4.4.1 (patch 01)

  12. 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

  13. Physics Processes used: Transportation Proton Inelastic:G4ProtonInelasticProcess Models: Parameterised: G4L(H)EProtonInelastic Precompound: G4PreCompoundModel 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

  14. 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

  15. Conservation Laws in 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

  16. Conservation Laws in the Precompound Model • There were some quantities not conserved in the initial tested versions • Charge and baryon number are now conserved • Momentum is exactly conserved • Energy conservation: • Is very sensitive to initial target mass estimation • Width can be of the order of a few MeV

  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 Precompound model: Not able to produce p, they should be produced by some intranuclear model y  x z Azimuthal distributions

  18. Now Before Parameterised model:jpions: (p,Al) @ 597 MeV

  19. Now Before Parameterised model:jnucleons: (p,Al) @ 597 MeV

  20. Double differentials • Real comparison with data • We plot • Which model is better?… Difficult to say • GHEISHA is better in the low energy region (E < 10 MeV) • Precompound is better at higher energies(10 MeV < E < 100 MeV) • None of the models reproduce the high energy peak

  21. Double Differentials Precompound Parameterised

  22. Double Differential Ratio Al @ 113 Precompound Parameterised

  23. Double Differential Ratio Fe @ 256 Parameterised Precompound

  24. Double Differential Ratio Pb @ 800 Precompound Parameterised

  25. 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 the physics ALICE requires • Precompound model agreement with data improved for • Light nuclei • Low incident energies • Low angles • An intranuclear cascade model would be very welcome • May solve the double differentials disagreement • May produce correct distribution of particle flavours

  26. Neutron Transport Benchmark • Experimental and simulation set-up • Simulation physics • Flux distribution • Conclusions Note: Linux gcc 2.95 (supported compiler) was used

  27. Tiara Facility

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

  29. 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

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

  31. Preliminary Results: 68 MeVTest Shield: Iron – Thickness: 20 cm

  32. Preliminary Results: 43 MeVTest Shield: Concrete – Thickness: 25 cm

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

  34. 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 43 MeV seems better than 68 MeV • Higher statistics will come soon (hopefully)

  35. 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

  36. 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

  37. Final conclusions • ALICE has done a big effort to use GEANT4 as one of its MC • It is already integrated in the Virtual MC framework and thus in AliRoot • 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 there are still important missing parts (kinetic model) • Educated Guess Physics lists are a big step to ease the use of hadronic physics • Some more work needs to be done in ALICE: • Test EGPLs and contribute with plots/experience • Improve the results from the neutron transport benchmarks (using the recent biasing utilities?) • Test if the geometrical problems found are solved

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