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Physics in G4MICE

MICE Collaboration meetingBerkeley 11 Feb 2005

Rikard SandströmGeneva University

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Outline

• Particles• Processes• Cuts• Steps• Summary

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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.

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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

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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.

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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.

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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.

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Example: EMCal fiber hits

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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.

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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.

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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.

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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.

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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.

x1,t1 x2,t2

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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.

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Example of use

transportation

eIoni

eBrem

muIoni

scat

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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.

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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?)