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Simulation of HEP experiments the Geant 4 toolkit

Learn about the importance of simulation in high-energy physics experiments and how the Geant 4 toolkit is used to accurately simulate particle interactions with matter. Explore the complexity of detector geometry, material definition, and physics input.

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Simulation of HEP experiments the Geant 4 toolkit

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  1. Simulation of HEP experimentsthe Geant 4 toolkit Tommaso Boccali SNS, INFN Pisa 23/5/2004

  2. Outline • Simulation – a step towards physical understanding • How do we simulate today a HEP experiment? • The Geant4 toolkit 23/5/2004

  3. Why? • First of all, why do we simulate at all? • A lot of experiments in the past were without detailed simulations … and they worked! • Short answer: today’s theories do not predict such an overwhelming hint from data in most interesting measurements • And, moreover, we want are interested not only in discovery - see what LEP has done J/Psi discovery 23/5/2004

  4. Detailed tests of theories: when the precision level required for validating/discarding a theory is high, effects due to the “bias” introduced by the detector are not negligible • Finite resolution • non-homogeneity • Particle misidentification • such that we cannot simply look at the data and say “Oh, it is clear that the answer is X” “B” events 23/5/2004

  5. simulation theory data simulation 23/5/2004

  6. So, the more subtle is the hint we want to look at, the more detailed the simulation must be • We have to thrust the background distributions to a level much greater than before • We need a tool capable to simulate (realistically) particle interactions with matter from the scale • of the TeV: primary particles generated in LHC collisions • of the keV: delta rays from secondary (tertiary…) particles leaving energy for ionization in our detectors Correct physics simulation needed for ~ 9 orders of magnitude 23/5/2004

  7. Geant 4: what is it? • Once upon a time … a Fortran code called Geant 3 was used to simulate virtually all the HEP experiments of the ‘80-’90 • Very well tested, at the heart of the LEP Simulation • Agreement in physics up or better than % level • A product at the end of lifetime … • Fortran is not suited for a 10x complexity • Fortran is not maintainable for ~20 years Visible energy in ALEPH(calo+tracker), DT vs MC 23/5/2004

  8. Geant 4 • The project started in 94 with the aim to completely rewrite Geant3 • To use new technologies (C++) • To implement all the lessons learned • To use new data and ideas for calibrations • March 25th 2004: Geant 4 6.1 Release 23/5/2004

  9. HARP Geant4 PPARC Univ. Barcelona Lebedev Collaborators also from non-member institutions, including Budker Inst. of Physics IHEP Protvino MEPHI Moscow Pittsburg University Helsinki Inst. Ph. 23/5/2004

  10. What exactly is G4? • It is not a program … there is nothing to run but examples when you download it • It a is a toolkit: a set of libraries which know how to • Handle a complex geometry • Treat decays / energy loss of all the common particles • Extend its capabilities with user code • New physics inputs • New features: visualization, data analysis 23/5/2004

  11. Construct the detector Well, usually not done by us ;) Build an accelerator Switch it on Let it run, and save data Plot histograms, and eventuallypublish physics papers Analogy with a real experiment Define a Geometry Define physics Define primary particles(the beam) Beam ON!(N events) Analyze data 23/5/2004

  12. Geometry • Define a detector • Shapes of each single component (a screw, a silicon wafer, a power cable) • Box • Cylinder • … • Hierarchical placement of each component • Materials used • Define passive and active materials, instrumented with measurement capabilities • For these, define format of read data, as close as possible to the real data taken with the apparatus Define a Geometry Define physics Define primary particles(the beam) Beam ON!(N events) Analyze data 23/5/2004

  13. Complex facilities • Not only easy volumes, but also • Polycones • Useful for HEP detectors, useful with cylindrical symmetry • Boolean solids G4SubtractionSolid G4UnionSolid G4IntersectionSolid 23/5/2004

  14. Level of complexity… • CMS Tracker Simulation • 900 different components defined • 20000 sensitive detector • 200000 global objects in the simulation • 600 materials and composite materials 23/5/2004

  15. Materials… • For some kind of “easy” physics, the definition of materials provides enough informations: • double density = 1.390*g/cm3; • double a = 39.95*g/mole; • G4Material* lAr = • new G4Material("liquidArgon",z=18.,a,density); • a = 1.01*g/mole; • G4Element* elH = • new G4Element("Hydrogen",symbol="H",z=1.,a); • a = 16.00*g/mole; • G4Element* elO = • new G4Element("Oxygen",symbol="O",z=8.,a); • density = 1.000*g/cm3; • G4Material* H2O = • new G4Material("Water",density,ncomp=2); • H2O->AddElement(elH, natoms=2); • H2O->AddElement(elO, natoms=1); 23/5/2004

  16. … • Materials also define automatically the “material budget of a detector” • “Quantity of matter” seen by a particle starting from the interaction point • Two parameters are used: • Electromagnetic interactions: X/X0 is the number of radiation lengths of the given detector • A radiation length is the distance in which an (high energy) electron loses all but 1/e of its energy • Hadronic interactions: l/l0 is the same due to hadronic effects (nuclear interaction length) • The same for nuclear interactions; used in hadronic calorimeters 23/5/2004

  17. CMS Tracker 23/5/2004

  18. CMS Hadron Calo This is not an input! Once materials are defined it is automatic 23/5/2004

  19. Physics… • It is completely unrealistic to try and develop a physics model which covers different particles and energy ranges… • Like: model for LHC physics, model for TeVatron physics • Please note it now: it is NOT the LHC physics (qqHqq); it is only the physics of particle-matter interaction Define a Geometry Define physics Define primary particles(the beam) Beam ON!(N events) Analyze data 23/5/2004

  20. A different approach… • G4 allows more than a single physics model to contribute to the simulation of the interactions • Physics processes are defined with • a given energy range • a given particle type • By combining these wisely and paying some attention at the overlaps, a model for the interesting field can be constructed Hadronic inelasticinteractions Energy range E>75 GeV model E<100 GeV model Particle typeProcess type 23/5/2004

  21. But … • This needs expertise from the user • It is easy to forget processes and so generate invalid simulations • Often more than 1 physics model is available for the same range: • You can decide to use the more detailed or the fastest… at a price! compt: Total cross sections from a parametrisation. Good description from 10 KeV to (100/Z) GeV. 23/5/2004

  22. Concept of “range” • Given a process, the user can define how accurate it has to be • The more accurate = the slower ! • Consider Brem photon emission in a material • The charged track loses energy, and a photon is emitted. • In principle, infrared divergence which would lead to infinite # of photons of E0 23/5/2004

  23. Range • We have to specify a minimum range for the process: the charged track always loses correctly energy, but we can veto the creation of photons with Range smaller than a threshold • Passive materials: if the track goes inside a block of iron, there is no point in generating particles which would not exit from that – set range of the order of the volume size. • In an active detector, no need to simulate particle which would give effects below the experimental resolution. 23/5/2004

  24. Calorimeters • Classic example, an electron enters the calorimeters • Where/when to stop the showers? • In principle, G4 could simulate electrons down to O(eV) – hours needed to simulate an event • What is usually done is to try and simulate events with increasing cuts, and define a point in which the price performance ratio is considered acceptable 23/5/2004

  25. LAr Pb LAr Pb • 500 MeV proton in a LAr-Pb (ATLAS) sampling calorimeter • Range at 1.5 mm • Corresponds to 450 keV in LAr • Corresponds to 2 MeV in Pb • Silicon detectors in CMS • Strip pitch in ~100 mm • No need to produce delta rays which would travel less than 10 mm 23/5/2004

  26. A few examples… • EM physics • “standard” processes valid from ~ 1 keV to ~ PeV • “low-energy” valid from 250 eV to ~ PeV • optical photons • Weak physics • decay of subatomic particles • radioactive decay of nuclei • Hadronic physics • pure hadronic processes valid from 0 to ~100 TeV •  and e-nuclear valid from 10 MeV to ~TeV • Parameterized or “fast simulation” physics • Tracking physics • Particle transportation in the field 23/5/2004

  27. A review… • Standard em processes: • Gamma • Photo-electric effect • Compton scattering • Electro, muon pair production • Electron • e ionization • e bremsstrahlung • e+e- annihilation • Syncrotron radiation • Muons • mu ionization • Mu bremsstrahlung • e+e- pair production • Charged hadrons • Hadron ionization • All charged particles • Multiple scattering • Transition radiation • Scintillation • Cerenkov radiation 23/5/2004

  28. A review… • Hadronic processes: • Capture • Neutron capture • Fission • Neutron-induced, de-excitation • Most of these are data driven – eagerly waiting for declassified data! • At rest • Stopped mu, p, K, anti-proton • Elastic • Same processes for all the hadrons • Inelastic • Different processes for each hadron • Photo-nuclear, electro-nuclear, muon-nuclear • Ions (for example, Fe+++) • Radioactivity! 23/5/2004

  29. Process At rest In flight Direct implementations Cross sections Models Isotope production Event biasing Directimpl. Directimpl. Directimpl. Directimpl. Theoryframework Highenergy Spallation framework Stringparton Transport utility Precompound Cascade Direct impl. Evaporationutil. Directimpl. Stringfragmenation util. Directimpl. Directimpl. Directimpl. Frag function intfc Fragfunctionimpl. Hadronic models 23/5/2004

  30. Processes • Basic distinction: • Continuous processes: ionization, multiple scattering • Energy loss is continuous and not localized • Discrete: decay, Compton scattering, annihilation • These are the ones limiting the step length 23/5/2004

  31. Startofsteppoint Endofsteppoint How does it work? • When a particle is under study, all of the possible energy losses mechanisms and decay processes must be able to “interfere” with the particle • A step is a “time” quantum for a particle traveling in a material 23/5/2004

  32. The Step • The step length is limited by the processes of the particle: for example, after a few mm, the particle can decay • Each physics process must propose a step length • Given a lifetime for a decay, a random number is thrown and the resulting distance before decaying is returned • The “physicalsteplength”is the minimum of all proposed lengths • If the minimum between that step and the distance to the next detector is taken 23/5/2004

  33. The beam • Well, not really the beam! In most HEP cases, Geant 4 is not starting from e+e- (pp), but it is interfaced via to specialized codes for the simulation of primary interactions • Pythia • CompHEP • ALPGEN • … • Geant 4 is able to read directly the output of these programs, and treat these particles as “the beam” • Exception: test beam simulations • The beam is a single particle thrown inside the experimental setup Define a Geometry Define physics Define primary particles(the beam) Beam ON!(N events) Analyze data 23/5/2004

  34. List of particlesto be tracked How an event is processed List of primary particles Push secondariesin the list Track nextparticle Interactionparticle-matter Out of the world, decayed or stopped 23/5/2004

  35. ADC counts Energy depositin the strip Simulation of the response of a detector • Some volumes are special: they are “active” in the real detector. • Data is read from these in real world • Geant 4 allows to simulate their response: • Special user code is needed • The output of this code must be as close as possible to the real response • Usually experimental parameterization used 23/5/2004

  36. Visualization • With complex geometries, often the best way to debug it to see … • Holes in the geometry • Overlapping volumes • Geant 4 provides natively (3) visualization features, most notably OpenGL • This means geometry, tracks, etc can be visualized without any user code 23/5/2004

  37. LCD in JAS3 BaBar Offline BaBar Online Geant4 Visualization BaBar Offline GLAST 23/5/2004

  38. 23/5/2004

  39. Validation • So, most of the HEP community is / will be using Geant4 • The standard of the Physics simulation must be very high Huge effort on validation 23/5/2004

  40. Compton ScatteringData, G4LowE , G4standard 23/5/2004

  41. Rayleigh ScatteringData, G4LowE 23/5/2004

  42. Stopping Power Data, G4 LowE, G4Standard 23/5/2004

  43. Pair ProductionData, G4 LowE, G4Standard 23/5/2004

  44. Multiple Scattering (1) 23/5/2004

  45. Production from 730 MeV p(LEP Model) 23/5/2004

  46.  Production from 730 MeV p(Bertini Model) 23/5/2004

  47. From the experiments point of view… • Use test beam data CMS HCAL 23/5/2004

  48. 87654321 f 1 7 17 h Crystal 25 23/5/2004

  49. Just a glimpse … • Geant 4 is aimed / supported / funded mostly by HEP experiments, but is is becoming a de-facto standard • Medical applications • Simulation of radiotherapic apparata • Simulation of dose absorption in human tissues • Space applications • Detectors • Damages from high energy cosmic rays • Effects of cosmic rays on astronauts • Industrial applications • CCD of digital cameras • Implantation of oxides in silicon wafers 23/5/2004

  50. Simulation • Simulation of dose deposit by radiotherapy in human tissues • Very useful to plan therapy cycles… 23/5/2004

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