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The BaBar physics list contains the electromagnetic, hadronic and decay physics processes required to simulate B decay, Bhabha scattering and dimuon events in the BaBar detector. The code is listed here:
Unique to the BaBar simulation is the BgsTransportation process. It was developed as an alternative to the Geant4 standard transportation process in order to meet the speed and accuracy requirements of BaBar tracking. It is not currently part of the Geant4 distribution and must be added to the user's version of the toolkit. It is registered to Geant4 by the physics list method AddBgsTransportation(). A switch in the BaBar simulation framework allows the user to return to the standard Geant4 transportation process if desired.
Electromagnetic physics is built in the ConstructEM() method. Processes from the Geant4 standard electromagnetic package were chosen because of their greater speed and because the detail at low energies provided by the Geant4 Low Energy package is not required for BaBar.
This constructor treats gammas, electrons, positrons, muons, taus, charged hadrons and ions. The following processes are assigned to each particle:
Note that the ionization and bremsstrahlung processes for e+/e- are different from those for mu+/mu-. They are specially tuned for the mass difference and other effects. The hadron ionization process is used for the tau because of its large mass.
The ordering in the above list reflects the process ordering integers used in the phyiscs list (see code). This ordering is important due to the coupling between multiple scattering and energy loss processes.
No processes, except transportation, currently exist for the neutrinos or the Geantinos.
The method ConstructHad() assigns hadronic physics processes to the long-lived hadrons. For the hadronic processes an extra level of detail must be addressed. Cross sections and physics models must be assigned to the various processes before the processes are assigned to the particles. Default cross sections are provided by Geant4 and are used unless otherwise indicated.
Elastic scattering and inelastic scattering processes are assigned to all hadrons. For elastic scattering the same process, G4HadronElasticProcess, is assigned to all the long-lived hadrons. The hadronic model which implements this process is G4LElastic, which has its origins in the GHEISHA model of Geant3. It is used for all incident particle energies.
For hadron inelastic scattering, each long-lived hadron has its own process, for example, G4KaonPlusInelasticProcess. Each of these processes is typically implemented by one model, or two in the case of neutrons. The models chosen are dictated by the BaBar energy range. Most of the long-lived hadrons produced in the detector have kinetic energies below 1 GeV, with a few extending out to the multi-GeV range. Hence for BaBar, there is no need for the high energy fragmentation or quark-gluon string models. The models used for most hadrons at these energies are in the low energy parameterized family, which is a derivative of the GHEISHA model of Geant3. This model is fast but not very detailed. A better, but somewhat slower, model is the Bertini cascade, which has been applied here to protons, neutrons and pions. It is valid up to about 10 GeV.
Four processes apply to neutrons: elastic, inelastic, capture and fission. For neutrons below 20 MeV these processes are implemented by the high-precision neutron models. For neutrons above 20 MeV, the low energy parameterized models are used for elastic scattering, capture and fission, while the Bertini cascade is used for inelastic scattering.
The following hadronic processes, models and cross sections are assigned to each particle:
ConstructLeptHad() builds the models and processes required for gamma-nuclear and electro-nuclear physics. For gammas, there is a process which uses photo-nuclear cross sections to interact the gamma directly with the nucleus. Two models are registered to this process in order to cover the BaBar energy range. For electrons and positrons there are two special processes which convert these particles into photons before interacting the photons with the nucleus. The same hadronic model is assigned to each process. The processes and models assigned to each particle are given here: