Hi,
I'm questioning  how error of 1/P component of the covariance matrix is computed.

My starting point is your example use case:
geant4.9.6.p01/examples/extended/errorpropagation

I copied it, and make only one change: moved the target plane closer to the start, to 10cm and  later to 20 cm.

Set G4E to propagate forward:
  export G4ERROR_TARGET=PLANE_SURFACE
  export G4ERROR_MODE=FORWARDS
  export G4ERROR_PROP=UNTIL_TARGET
and printed position, momentum and momentum error (for the free state) at start and at this target location.
This is what I see (I run my code twice for 10 & 20 cm X-target location), still Air if the only material traversed.

---- At START ------
 Position (mm): (0,0,0)  phi/deg=0  Rxy/cm=0
 Momentum (MeV): (20000,0,0)  phi/deg=0  PT/MeV=20000
freeErr: |P| (MeV) 20000.00  +/- 0.00  
Looks good

--- at Target X=10 cm------
 Position (mm): (100,-0.07494821925585556,0)  phi/deg=-0.04294215841327705  Rxy/cm=10.00000280861739
 Momentum (MeV): (19999.94417956696,-29.97923330181213,0)  phi/deg=-0.08588435244242193  PT/MeV=19999.96664847778
freeErr: |P| (MeV) 19999.97  +/- 2440.97  


--- at Target X=20 cm------
 Position (mm): (200,-0.299793548487774,0)  phi/deg=-0.08588446094331748  Rxy/cm=20.00002246903031
 Momentum (MeV): (19999.84342105256,-59.95842910881996,0)  phi/deg=-0.1717690768833353  PT/MeV=19999.93329688478
freeErr: |P| (MeV) 19999.93  +/- 3452.04  



My conclusions:
 the mean energy=momentum  for initial 20 GeV muon are reduced by:
- 0.06 MeV  after 10 cm  in air.
- 0.16 MeV after 20 cm  in air.
what makes sense.

But the error of momentum , computed as shown below, is humongous and irrational:
err(P) =2.4 GeV  after 10 cm  in air.
err(P)=3.5 GeV after 20 cm in air.

It can't be the Landau energy loss width is of 3.5 GeV if the mean of it is just 0.16 MeV.

Sth is realy wrong .
Could you please have a look at it?
Thanks
Jan
 


 

--- code used ------
void printFreeTraj(G4ErrorFreeTrajState *g4ErrorTrajState, G4String txt){
  G4cout << "\n $$$---------------- FreeTraj  " << txt<<G4endl;
  // extract current state info
  G4Point3D posEnd = g4ErrorTrajState->GetPosition();
  G4Normal3D momEnd = g4ErrorTrajState->GetMomentum();
  G4ErrorTrajErr errorEnd = g4ErrorTrajState->GetError();

  G4cout << " Position (mm): " << posEnd <<"  phi/deg="<<posEnd.phi()/deg<<"  Rxy/cm="<<posEnd.perp()/cm<< G4endl
            << " Momentum (MeV): " << momEnd <<"  phi/deg="<<momEnd.phi()/deg<<"  PT/MeV="<<momEnd.perp()/MeV<< G4endl;

    double rsigP=   sqrt(errorEnd(1,1))* (momEnd.mag());
    printf("freeErr: |P| (MeV) %.2f  +/- %.2f  rsigP=%.3f\n",momEnd.mag(),rsigP*momEnd.mag() ,rsigP);
}