Use of geant4_vmc with neutrinos

Hello,

I am trying to use the vmc framework in order to modelize the production of a neutrino beam by colliding protons on a target (more precisely, considering here Geant4(_vmc))) I am interested in getting the whole kinematics of produced neutrinos, but also of their parents (pions, kaons, muons...)

I am no sure to understand correctly the use of the vmc stack particles but I was standing in the preliminary idea that, when one searches for the genealogy chain of a particle (e.g. nu mu<-pi<-proton) , the associated kinematics of all the family members was registered in the stack a their *creation*, before energy loss in further steps and other things. Looking at the G4 physics book, I was also thinking that kinematics of a decay was defined, in the end, in the laboratory frame (in the opposite of the parent rest frame), but I am puzzled now by some preliminary tests (using geant_vmc 2.8 with a an application blindly derived for the examples) :

• For example, if I get from stack the decay mu+ -> nu_vu_bar nu_e e+

The attached file (decay.txt) summarizes the dedicated verbosity information.
With a naive counting if I sum the kinetic energy of the 3 products at the beginning step [41.2 +23.7 39.7 MeV] (If I understand well, supposed to stand at the decay time/place of the parent), I find (adding the mass energy of the e+): 105.11 MeV which is ~the rest mass of the parent muon, that suggests the considered frame to be the rest frame of the parent ...(in the range of energy could the mu/pi be always stopped before decaying or is the boosting in lab frame hidden by the level of verbosity ...?) I observe also that most pi+/- products have "back to back" momenta.

Is there something wrong or missing in options/config ? If not by default, and if the previous assumption is not an artefact of the physics, is there a way to set the whole final kinematics in lab frame ?

• But more generally, how to reach the kinematics and ID(+parentID) (in vmc stack) of a particle *after all energy loss, just before it decays* (or at the end of the story for the neutrinos).

Thanks in advance for the help,

Best regards,

Arnaud Robert