Problem 1977

Summary: G4PiMinusAbsorptionAtRest in hydrocarbons gives too many captures on hydrogen
Product: Geant4 Reporter: Matthew Strait <straitm>
Component: processes/hadronic/stoppingAssignee: dennis.herbert.wright
Status: RESOLVED WONTFIX    
Severity: normal CC: straitm
Priority: P4    
Version: other   
Hardware: All   
OS: All   

Description Matthew Strait 2017-06-06 23:43:34 CEST 
According to [1], when a pi- stops in a hydrocarbon, it should be captured on hydrogen only 1-2% of the time.  In our simulation of the NOvA detector, which is mostly hydrocarbons, pi- are instead absorbed by hydrogen 20% of the time.  In any such capture process, hydrogen is a special case since the, e.g., pionic hydrogen atom is a neutral system whereas other pionic atoms are charged.

For mu- capture, Geant4 correctly simulates that the muon is never (or nearly never) captured by hydrogen, so I suspect that the pion capture and muon capture processes don't use the same code to handle attaching the particle to an atom.

Please let me know what further detail would be useful.

[1] D. F. Measday. The nuclear physics of muon capture. Phys.Rept., 354:243­409, 2001.
Comment 1 dennis.herbert.wright 2017-06-22 02:14:16 CEST 
In Geant4 no pionic atom is formed, as it is for muons.   The pion is directly absorbed into the nucleus without an atomic cascade.  This is likely the cause of the difference you see.

It would be rather straightforward to develop such a pion capture model based on the muon case, but there is currently no manpower to do it.