Physics in G4MICE

1. Physics in G4MICE MICE Collaboration meeting Berkeley 11 Feb 2005 Rikard Sandström Geneva University

2. Outline • Particles • Processes • Cuts • Steps • Summary

3. Introduction • G4MICE uses Geant4.6.2.p02 • Released Oct -04. • Will move to Geant4.7.0 soon. (Released Dec -04.) • Geant4 is really made for high energy physics, such as LHC. • We encounter and uncover G4 bugs no one else sees. • They fix them. • Hadron physics users gives G4 momentum we can ride on. G4 has improved greatly.

4. What we care about • A cross section of our interests: • Tracking in EM field • LH2: • dE/dx • MSC • RFBG: • bremsstrahlung • Compton • photoelectric • EMCal: • dE/dx • Conversion • annihilation • Showering • decay • Ckov: • Ckov • optical processes • Fibers: • scintillation • TPG: • delta electrons • dE/dx • drift

5. Particles • Main interest is mu+. • Other particles mostly background. • mu- ? • Photons and e- very important. • Low energy particles. • RF background. • PMT, SciFi etc. • Other particles of interest • e+ (mu+ decay) • pi+ (contamination) • p (contamination) • Less interesting • Ions (secondaries) • Kaons (too high mass) • All PDG particles can be simulated. • Exotic particles very rare downstream.

6. Processes, MSC • MICE is low energy wrt typical G4 application. • We use both normal G4 physics description, and low E. • Low E G4 makes use of special parameterizations. • Multiple scattering. • Lambda tables from 100 eV. • Based on Lewis theory, not Moliere (= only angular). • Path length correction. • Lateral displacement. • Lewis is a “condensed” msc theory. (Soft) • Effects calculated after each step. • mu+, e- and p treated using individual models. • Has improved in G4, and still is with each release.

7. Ionisation & bremsstrahlung • Muon ionisation. • Separate models for mu+ & mu -. • dE/dx tables from 100 eV. • E<0.2 MeV, Bragg peak. • E>0.2 MeV, Bethe-Block. • E>1 GeV, radiative corr. • Muon bremsstrahlung. • Too low energy. • Electron ionisation. • Separate models for e- & e+. • dE/dx tables from 100 eV. • E>0.1 MeV, Möller-Bhabha good for delta cross sections. • Electron bremsstrahlung. • dE/dx from 100 eV. • E>1 keV, good description from EEDL data.

8. Example: EMCal fiber hits

9. Photonic • Photoelectric effect • Photo absorption & e- emission. • Uses Sandia parameterization. • Compton • Uses empirical formula, E>10 keV to E<(100 GeV)/Z. • Can go down to 1 keV. • Conversion • gamma -> e+ e- • Uses Bethe-Heitler, but with Coloumb waves instead of plane waves. • E > 2 me, Good from E>1.5 MeV • Optical processes • Cherenkov is used, can add scintillation, transition radiation, etc • Was added/improved in G4 during last year.

10. Other processes • Decay • Muon decay: V-A theory, electron mass neglected. • Invoked at AtRestDoIt if set, when Ekin->0. • Good for getting rid of exotic particles (pi0 = Dalitz). • Annihilation • e+ e- -> 2 gamma • E>10 keV. • Hadron ionisation • E>2MeV*(m/mp), Bethe-Block • Low E, Bragg peak model. • Effective way to kill boring hadrons & ions. • Elastic scattering + Low E elScat • Low E extension to 250 eV.

11. Principle ideas • Simulate muons, photons and electrons as carefully as possible. • Simulate other particles, but kill them! • Let G4 do it with decay and ionisation. • Don’t do it ourselves, no need to get into trouble. • More details for these particles is overkill.

12. Cuts • Production threshold is specified as range. • Calculated as kinetic energy for each material. • Can set different cut for particles. • Possibility to make a volume a G4Region. • Allows regional difference in production threshold. • TPG is using a G4Region to ensure delta rays are simulated correctly. • Every volume can set its own max allowed step length. • Tracking precision. • Example: TPG active volume 1 mm.

13. Steps • G4MICE is a step based Monte Carlo tool. • Particles are tracked in step, change can happen along step, post step or when the particle is at rest. • I found earlier that G4 was very sensitive to step sizes. • G4 has solved our problems by giving us G4.6. • Continue to reduce step size sensitivities in G4.7. • In order to avoid confusion, I propose to team Yagmur we always take hits from G4Track instead of pre/post step point. • Small, if any difference, but neat. x2,t2 x1,t1

14. Steps in Geant4.7 This all applies to Geant4.7 (not yet official G4MICE). • New class G4StepLimiter in addition to G4UserSpecialCuts and G4SteppingManager. • G4StepLimiter limits the step, but does not kill the track. • Previously a user limited step returned NULL pointer when requesting process, now a valid pointer. • No need for my workaround in MICEStepStatistics. • G4StepLimiter must be included in physics list. • User can select what particles to invoke step limits. • AlongStepDoIt no longer kills particles but sets kinetic energy to 0. • Allows AtRestDoIt to perform decay. (Effects mu+ in EMCal.) • Still no support for reaccelerating particles which has stopped. • Cannot use G4 to simulate RF background from surface emission.

15. Example of use transportation eBrem scat muIoni eIoni

16. Flexibility • G4 supports customization • We choose what processes to switch on/off. • Possible to add/modify process models, dE/dx tables. • We could make our own MSC if we want (or charged Higgs radiative correction etc). • Thresholds, cuts, step limits can be set by user. • G4MICE preserves this freedom • Interactive mode -> access to standard G4 knobs • Datacards -> can switch on off processess for whole groups of particles. • Ex: HadronicDecayOption = Meson, -> mesons can decay.

17. Summary • We can simulate anything we want at MeV scale. • Geant4 has matured into a reliable tool. • We will move to G4.7 soon. • Many changes to stepping and physics requires testing of G4MICE physics. • With the correct background the user of G4MICE can/will be able to set up environment to fulfill her exact requirements. • G4MICE has physics defaults which makes sense. • Will need optimization. • Depending on what you do with G4MICE the idea of ideal defaults differ. (Zum Beimspiel should muon decay be on by default?)