Re: Use of geant4_vmc with neutrinos

Hi Arnaud,

See my replies below in your text.

Best regards,

Ivana

Arnaud Robert wrote:
> 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.

The VMC stack provides only the minimum interface (see TVirtualMCStack class) so it may also depend on the user implementation of this interface class. But in most implementations in the VMC examples, and also in the ALICE stack implementation, the genealogy chain of a particle is kept in the stack.

> 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 your attached file, the parent muon first lost its kinetic energy in ionization process and then it decayed at rest. So there is nothing contradictory with your observations.

(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.

It depends on the parent particle lifetime and initial energy whether it decays on flight or at rest or if it leaves the detector without a decay. With activating Geant4 verbosity you can see the particle properties at each step so you can see easily the particle properties at the step when it decayed.

>
> 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 ?

There is no need to set any option as the whole kinematics you get is in the 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).

The particle is saved in the stack only with its initial kinetic energy, so you have to access to particle properties during tracking via MCApplication::Stepping function to get their energy just before it decays.

>
>
> Thanks in advance for the help,
>
> Best regards,
>
> Arnaud Robert
>

-- 
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e-mail:  Ivana.Hrivnacova_at_cern.ch
address: Institut de Physique Nucléaire, 91406 Orsay, France
phone:   +33 169156594
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