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None Angular distribution of neutrinos from pions that decay in flight 

Forum: Particles
Date: 21 May, 2008
From: Bari Osmanov <Bari Osmanov>

Hello, everyone. I am simulating the interaction of 1 GeV proton beam with mercury target and have come to an unexpected result. When protons interact with mercury, positive and negative pions are produced. These pions further decay to muons and neutrinos. I am interested in getting angular distribution of neutrinos from pions that decayed in flight (not at rest). Normally, this distribution should be forward-directed (by forward I mean along the direction of incident protons). However, when I get the histogram of angular distribution, it looks like it is peaked at 90 degrees (perpendicular to the proton beam path). Here is how I do it in my SteppingAction:

 I define the direction to the detector (which is located far “behind” the target, about 150 degrees from proton beamline direction) as

  G4double dirX = -25.*m;
  G4double dirY = 0.*m;
  G4double dirZ = 54.*m;

  G4ThreeVector dir2det(dirX,dirY,dirZ);

Then I pick out the positive pions that decayed in flight with the emission of muon neutrino, obtain the momentum direction of neutrino, get the cosine of the angle between the two unit vectors, calculate the angle and fill the histogram:

if ((part == G4PionPlus::PionPlus()) && (proc_name == "Decay") && (sec_name == "nu_mu") && (stepstat == "NotAtRest") ){

     G4ThreeVector nu_dir = secondary_track->GetMomentumDirection();
     G4double cos2det = nu_dir*dir2det/dir2det.mag()/nu_dir.mag();
     G4double alpha = acos(cos2det)/CLHEP::degree;
     rm->FillHisto("angle_nu_mu_det",alpha);}

Once again, normally this histogram should peak around 180 degrees because proton beam is incident along Z axis in “negative” direction. Instead I get the histogram which peaks around 90 degrees...

 I will appreciate if someone helps me out, maybe I am making some simple mistake....

 Thank you in advance.

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1 None: Re: Angular distribution of neutrinos from pions that decay in flight   (Bari Osmanov - 12 Jun, 2008)
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