Re: tracing back to primaries

[Richard Jones <richard.t.jones@uconn.edu>]

from Ed Brash:

>One particular physics process that we are looking at right now is gamma + p --> omega + delta+.  The omega then decays to pi0 + gamma, and the delta+ decays to proton + pi0.  (I realize these may not be the most probably decay modes, but the channel is interesting since one gets all neutral particles in the final state, except for the proton).  This is the way  that we have modelled it in the genr8 input file.  That is, we have just specified the decay down to the pi0's.  Now, of course, the pi0's will each decay quite quickly to two photons.  My initial impression was that these decays would be allowed to proceed in the geant simulation, as presumably the DCAY data
>switch would be turned on when geant ran.
>
>My question has to do with how such things would be specified in the resulting geant .xml event.  I notice that always the reaction is just specified with the four vertex particles (gamma, pi0, pi0, p).  If the pi0's decay, do each of the decay photons get associated, via the cheat tag, with the initial pi0 from which they came?  And have I made the correction assumption that the pi0's are allowed to decay?  If not, should I be specifying the decay of the pi0's in the genr8 input file as well?
>
The answer is that only the particles that you specify as "final state 
particles" whole momentum 4-vectors appear in the genr8 output are 
considered "primaries" by Geant.  It is true that at our energies the 
pi0s do not go very far before they decay, but as far as Geant is 
concerned any photons that come from decays that it generates are 
secondaries, much the same as positron number 153 in shower 2 in the 
EMcal. Secondaries are not enumerated in the cheat tags, so if you 
specify pi0's as final-state particles in genr8 then the cheat tags in 
the xml output from HDGeant will just point back to the pi0 from which 
they originate.  That is, the cheat tags will tell you which pi0 a given 
shower came from, but will not help you to differentiate which of the 
two gammas.  I have found that genr8 (and Scott's mcwrap) always output 
the final-state particles in the order of the reaction specification, so 
if I want to know the ancestry of some final-state gamma I just 
reference its primary track index against a list for that reaction.

In Radphi we have always asked our generator to decay the pi0s for us so 
we only pass long-lived particles to Geant.  In that case, you use the 
order of the particle in the final-state list to decide which pi0 (or 
higher resonance) it came from, eg. gammas 1 and 2 come from the pi0 
from the omega, gamma 3 is the bachelor (spinster?) from the omega, 
gammas 4 and 5 are from the pi0 from the baryon decay, and particle 6 is 
a proton.  You should check the reaction spec file you gave to genr8 to 
get the exact order.

Richard Jones