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Question shift in angular distribution 

Forum: Event and Track Management
Date: 31 May, 2014
From: Summit <Summit>

Dear G4Users,

I am simulating 12C + C and trying to get angular distribution of secondary particles. In order to get this, I have following modifications in Hadr02.

void HistoManager::ScoreNewTrack(const G4Track* track) {

      G4double ZZ = pd->GetPDGCharge()/eplus;
-----
-----
      G4ThreeVector dir = track->GetMomentumDirection();
      G4double x = dir.x();
      G4double y = dir.y();
      G4double z = dir.z(); 
      G4double r = std::sqrt(x*x + y*y + z*z);

      //cos theta
      G4double acostheta = std::acos(z/r);
      if(ZZ == 2)
      histo->fill(4,acostheta/deg,1.0);
----
----
     if(ZZ == 6)
      histo->fill(11,acostheta/deg,1.0);
----

}

Here, for atomic number, Z = 6, I am getting the Gaussian distribution with centred almost at origin. But for Z = 2, it is expected that the centre of the Gaussian distribution must be at origin but it shifted and found at ~ 4 deg.

Can anyone suggest me why I am getting shift in the peak???

Thanks.

Summit.

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/05/31/01.57-86683-Z2_acostheta.png
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2014/05/31/01.57-16936-Z6_acostheta.png

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