Geant4 neutrino interaction processes

Geant4 neutrino processes include three flavour oscillation, neutrino-electron and neutrino-nucleus interactions. The interactions are available in both charged and neutral current modes.

The user neutrino process management is located in the G4NeutrinoPhysicsMessenger class (in the domain /physics_lists/nu/). The messenger is in connection with the G4NeutrinoPhysics constructor that is included in the majority of Geant4 physics lists. Neutrino activation and tuning have to be chosen before the /run/initialize command.

Neutrino oscillation process

In the user macro file:

/physics_lists/factory/addNeutrino  # adds G4NeutrinoPhysics on the top of PL
/physics_lists/nu/NuOscDistanceName   rBaseLine  # oscillation L name
/physics_lists/nu/NuOscDistanceBias   5.0e4      # L is biased
...
/run/initialize
...

The first command activates the neutrino physics. It is mandatory for all neutrino processes. Then the user defines the oscillation distance G4Region name and the reduction length bias (\(10^3\) km is not suitable for the Geant4 geometry, 1000 km/50000=20 m).

Neutrino-electron process

The G4NeutrinoElectronProcess class with biased charge and neutral current cross-sections in a detector G4Region:

/physics_lists/factory/addNeutrino
/physics_lists/nu/NuEleCcBias          1.0     # charged current is not biased
/physics_lists/nu/NuEleNcBias          5.0e14  # neutral current is biased
/physics_lists/nu/NuDetectorName       lTarget # detector region name
...
/run/initialize
...

The process gets the cross-sections (G4NeutrinoElectronNc(Cc)Xsc) as well as charge and neutral current final state models (G4NeutrinoElectronNc(Cc)Model). Here the neutral current cross-section is biased only.

Neutrino-nucleus process

User commands:

/physics_lists/factory/addNeutrino
/physics_lists/nu/NeETotXscActivation  true
/physics_lists/nu/NuDetectorName       gTarget # G4Region name
/physics_lists/nu/NuNucleusBias        1.e12
...
/run/initialize
...

The \(\nu A\) total cross-sections (class G4MuNeutrinoNucleusTotXsc) will be biased (\(\cdot 10^{12}\)) in the G4Region named gTarget. The process G4MuNeutrinoNucleusProcess manages charge and neutral current models implemented in classes G4NuMuNucleusCc(Nc)Model with x and \(Q^2\) distributions stored in G4PARTICLEXSDATA->/neutrino/nu_mu data set. The biasing has to provide \(\lesssim\) 0.1 neutrino detection with approximately flat distribution, (\(1-\exp[-\sigma_{bias}n_{at}L]\sim\sigma_{bias}n_{at}L\)).

Example

All neutrino processes are activated and biased:

/physics_lists/factory/addNeutrino
/physics_lists/nu/NuOscDistanceName   rBaseLine  # oscillation L name
/physics_lists/nu/NuOscDistanceBias   5.0e4      # L is biased
...
/physics_lists/nu/NuEleCcBias          1.0     # charged current is not biased
/physics_lists/nu/NuEleNcBias          5.0e14  # neutral current is biased
/physics_lists/nu/NuDetectorName       lTarget # detector region name
...
/physics_lists/nu/NeETotXscActivation  true
/physics_lists/nu/NuNucleusBias        1.e12
...
/run/initialize
...

Another way to add neutrino physics is to add it in the main test program:

G4VModularPhysicsList* phys = nullptr;
...
// define neutrino physics
phys->AddPhysics( new G4NeutrinoPhysics() );
  runManager->SetUserInitialization(phys);
...

More details can be found in [Gri19, Gri23] (however they refer to the old interface via /em/ rather than /nu/).

Bibliography

Gri19

V.M. Grichine. Geant4 neutrino-electron interaction model. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 942:162403, October 2019. URL: http://dx.doi.org/10.1016/j.nima.2019.162403, doi:10.1016/j.nima.2019.162403.

Gri23

V.M. Grichine. Geant4model for neutrino nucleon/nucleus integral cross sections. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 1053:168394, August 2023. URL: http://dx.doi.org/10.1016/j.nima.2023.168394, doi:10.1016/j.nima.2023.168394.