272 lines
8.5 KiB
C++
272 lines
8.5 KiB
C++
// Geant4 simulation for MuSR
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// AUTHOR: Toni SHIROKA, Paul Scherrer Institut, PSI
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// DATE : 2008-05
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//
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//
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// ********************************************************************
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// * License and Disclaimer *
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// * *
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// * The Geant4 software is copyright of the Copyright Holders of *
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// * the Geant4 Collaboration. It is provided under the terms and *
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// * conditions of the Geant4 Software License, included in the file *
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// * LICENSE and available at http://cern.ch/geant4/license . These *
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// * include a list of copyright holders. *
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// * *
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// * Neither the authors of this software system, nor their employing *
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// * institutes,nor the agencies providing financial support for this *
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// * work make any representation or warranty, express or implied, *
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// * regarding this software system or assume any liability for its *
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// * use. Please see the license in the file LICENSE and URL above *
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// * for the full disclaimer and the limitation of liability. *
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// * *
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// * This code implementation is the result of the scientific and *
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// * technical work of the GEANT4 collaboration. *
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// * By using, copying, modifying or distributing the software (or *
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// * any work based on the software) you agree to acknowledge its *
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// * use in resulting scientific publications, and indicate your *
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// * acceptance of all terms of the Geant4 Software license. *
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// ********************************************************************
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//
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// ------------------------------------------------------------
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// GEANT 4 class header file
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//
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// History:
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// 17 August 2004 P.Gumplinger and T.MacPhail
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// samples Michel spectrum including 1st order
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// radiative corrections
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// Reference: Florian Scheck "Muon Physics", in Physics Reports
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// (Review Section of Physics Letters) 44, No. 4 (1978)
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// 187-248. North-Holland Publishing Company, Amsterdam
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// at page 210 cc.
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//
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// W.E. Fisher and F. Scheck, Nucl. Phys. B83 (1974) 25.
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//
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// ------------------------------------------------------------
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//
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#include "MuDecayChannelWithSpin.hh"
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#include "Randomize.hh"
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#include "G4DecayProducts.hh"
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#include "G4LorentzVector.hh"
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MuDecayChannelWithSpin::MuDecayChannelWithSpin(const G4String& theParentName,
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G4double theBR)
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: MuDecayChannel(theParentName,theBR)
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{
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}
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MuDecayChannelWithSpin::~MuDecayChannelWithSpin()
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{
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}
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G4DecayProducts *MuDecayChannelWithSpin::DecayIt(G4double)
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{
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// This version assumes V-A coupling with 1st order radiative correctons,
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// the standard model Michel parameter values, but
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// gives incorrect energy spectrum for neutrinos
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#ifdef G4VERBOSE
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if (GetVerboseLevel()>1) G4cout << "MuDecayChannelWithSpin::DecayIt ";
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#endif
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if (parent == 0) FillParent();
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if (daughters == 0) FillDaughters();
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// parent mass
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G4double parentmass = parent->GetPDGMass();
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EMMU = parentmass;
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//daughters'mass
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G4double daughtermass[3];
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G4double sumofdaughtermass = 0.0;
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for (G4int index=0; index<3; index++){
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daughtermass[index] = daughters[index]->GetPDGMass();
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sumofdaughtermass += daughtermass[index];
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}
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EMASS = daughtermass[0];
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//create parent G4DynamicParticle at rest
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G4ThreeVector dummy;
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G4DynamicParticle * parentparticle = new G4DynamicParticle( parent, dummy, 0.0);
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//create G4Decayproducts
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G4DecayProducts *products = new G4DecayProducts(*parentparticle);
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delete parentparticle;
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// calcurate electron energy
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G4double michel_rho = 0.75; //Standard Model Michel rho
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G4double michel_delta = 0.75; //Standard Model Michel delta
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G4double michel_xsi = 1.00; //Standard Model Michel xsi
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G4double michel_eta = 0.00; //Standard Model eta
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G4double rndm, x, ctheta;
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G4double FG;
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G4double FG_max = 2.00;
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G4double W_mue = (EMMU*EMMU+EMASS*EMASS)/(2.*EMMU);
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G4double x0 = EMASS/W_mue;
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G4double x0_squared = x0*x0;
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// ***************************************************
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// x0 <= x <= 1. and -1 <= y <= 1
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//
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// F(x,y) = f(x)*g(x,y); g(x,y) = 1.+g(x)*y
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// ***************************************************
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// ***** sampling F(x,y) directly (brute force) *****
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do{
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// Sample the positron energy by sampling from F
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rndm = G4UniformRand();
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x = x0 + rndm*(1.-x0);
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G4double x_squared = x*x;
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G4double F_IS, F_AS, G_IS, G_AS;
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F_IS = 1./6.*(-2.*x_squared+3.*x-x0_squared);
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F_AS = 1./6.*std::sqrt(x_squared-x0_squared)*(2.*x-2.+std::sqrt(1.-x0_squared));
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G_IS = 2./9.*(michel_rho-0.75)*(4.*x_squared-3.*x-x0_squared);
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G_IS = G_IS + michel_eta*(1.-x)*x0;
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G_AS = 3.*(michel_xsi-1.)*(1.-x);
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G_AS = G_AS+2.*(michel_xsi*michel_delta-0.75)*(4.*x-4.+std::sqrt(1.-x0_squared));
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G_AS = 1./9.*std::sqrt(x_squared-x0_squared)*G_AS;
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F_IS = F_IS + G_IS;
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F_AS = F_AS + G_AS;
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// *** Radiative Corrections ***
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G4double R_IS = F_c(x,x0);
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G4double F = 6.*F_IS + R_IS/std::sqrt(x_squared-x0_squared);
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// *** Radiative Corrections ***
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G4double R_AS = F_theta(x,x0);
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rndm = G4UniformRand();
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ctheta = 2.*rndm-1.;
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G4double G = 6.*F_AS - R_AS/std::sqrt(x_squared-x0_squared);
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FG = std::sqrt(x_squared-x0_squared)*F*(1.+(G/F)*ctheta);
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if(FG>FG_max){
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G4cout<<"***Problem in Muon Decay *** : FG > FG_max"<<G4endl;
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FG_max = FG;
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}
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rndm = G4UniformRand();
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}while(FG<rndm*FG_max);
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G4double energy = x * W_mue;
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rndm = G4UniformRand();
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G4double phi = twopi * rndm;
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if(energy < EMASS) energy = EMASS;
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// calculate daughter momentum
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G4double daughtermomentum[3];
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daughtermomentum[0] = std::sqrt(energy*energy - EMASS*EMASS);
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G4double stheta = std::sqrt(1.-ctheta*ctheta);
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G4double cphi = std::cos(phi);
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G4double sphi = std::sin(phi);
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//Coordinates of the decay positron with respect to the muon spin
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G4double px = stheta*cphi;
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G4double py = stheta*sphi;
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G4double pz = ctheta;
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G4ThreeVector direction0(px,py,pz);
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direction0.rotateUz(parent_polarization);
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G4DynamicParticle * daughterparticle0
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= new G4DynamicParticle( daughters[0], daughtermomentum[0]*direction0);
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products->PushProducts(daughterparticle0);
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// daughter 1 ,2 (neutrinos)
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// create neutrinos in the C.M frame of two neutrinos
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G4double energy2 = parentmass*(1.0 - x/2.0);
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G4double vmass = std::sqrt((energy2-daughtermomentum[0])*(energy2+daughtermomentum[0]));
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G4double beta = -1.0*daughtermomentum[0]/energy2;
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G4double costhetan = 2.*G4UniformRand()-1.0;
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G4double sinthetan = std::sqrt((1.0-costhetan)*(1.0+costhetan));
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G4double phin = twopi*G4UniformRand()*rad;
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G4double sinphin = std::sin(phin);
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G4double cosphin = std::cos(phin);
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G4ThreeVector direction1(sinthetan*cosphin,sinthetan*sinphin,costhetan);
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G4DynamicParticle * daughterparticle1
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= new G4DynamicParticle( daughters[1], direction1*(vmass/2.));
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G4DynamicParticle * daughterparticle2
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= new G4DynamicParticle( daughters[2], direction1*(-1.0*vmass/2.));
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// boost to the muon rest frame
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G4LorentzVector p4;
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p4 = daughterparticle1->Get4Momentum();
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p4.boost( direction0.x()*beta, direction0.y()*beta, direction0.z()*beta);
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daughterparticle1->Set4Momentum(p4);
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p4 = daughterparticle2->Get4Momentum();
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p4.boost( direction0.x()*beta, direction0.y()*beta, direction0.z()*beta);
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daughterparticle2->Set4Momentum(p4);
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products->PushProducts(daughterparticle1);
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products->PushProducts(daughterparticle2);
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daughtermomentum[1] = daughterparticle1->GetTotalMomentum();
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daughtermomentum[2] = daughterparticle2->GetTotalMomentum();
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// output message
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#ifdef G4VERBOSE
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if (GetVerboseLevel()>1) {
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G4cout << "MuDecayChannelWithSpin::DecayIt ";
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G4cout << " create decay products in rest frame " <<G4endl;
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products->DumpInfo();
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}
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#endif
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return products;
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}
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G4double MuDecayChannelWithSpin::R_c(G4double x){
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G4int n_max = (int)(100.*x);
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if(n_max<10)n_max=10;
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G4double L2 = 0.0;
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for(G4int n=1; n<=n_max; n++){
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L2 += std::pow(x,n)/(n*n);
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}
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G4double omega = std::log(EMMU/EMASS);
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G4double r_c;
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r_c = 2.*L2-(pi*pi/3.)-2.;
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r_c = r_c + omega * (1.5+2.*std::log((1.-x)/x));
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r_c = r_c - std::log(x)*(2.*std::log(x)-1.);
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r_c = r_c + (3.*std::log(x)-1.-1./x)*std::log(1.-x);
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return r_c;
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}
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