gconfig/DecayConfig.C

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void DecayConfig() {
     
        // This script uses the external decayer TPythia6Decayer in place of the
        // concrete Monte Carlo native decay mechanisms only for the
        // specific types of decays defined below.
        
        // Access the external decayer singleton and initialize it  
        TPythia6Decayer* decayer = TPythia6Decayer::Instance();
        // The following just tells pythia6 to not decay particles only to
        // certain channels.
        
        decayer->SetForceDecay(TPythia6Decayer::kAll);
        //example:  Force the J/PSI decay channel e+e-
//        Int_t products[2];
//        Int_t mult[2]; 
//        Int_t npart=2;

          //decay products 
//        products[0]=11;
//        products[1]=-11;
         //multiplicity
//        mult[0]=1;
//        mult[1]=1;
         // force the decay channel
//        decayer->ForceParticleDecay(443,products,mult,npart);

        decayer->Init();
        
        // Tell the concrete monte carlo to use the external decayer.  The
        // external decayer will be used for:
        // i)particle decays not defined in concrete monte carlo, or
        //ii)particles for which the concrete monte carlo is told
        //   to use the external decayer for its type via:
        //     gMC->SetUserDecay(pdgId);
        //   If this is invoked, the external decayer will be used for particles 
        //   of type pdgId even if the concrete monte carlo has a decay mode 
        //   already defined for that particle type.
        gMC->SetExternalDecayer(decayer);
        
        TPythia6& pythia6 = *(TPythia6::Instance());
        
        // The pythia6 decayer is used in place of the concrete Monte Carlo
        // decay for the particles type mu+/-,pi+/-, K+/-, K0L in order to preserve
        // the decay product neutrino flavor, which is otherwise not preserved in
        // Geant3 decays. 
        const Int_t npartnf = 9;
        // mu-,mu+,pi+,pi-,K+,K-,K0L, Xi-
        Int_t pdgnf[npartnf] = {13,-13,211,-211,321,-321,130, 3312, 443};
        for ( Int_t ipartnf = 0; ipartnf < npartnf; ipartnf++ ) {
         Int_t ipdg = pdgnf[ipartnf];
         
        if (TString(gMC->GetName()) == "TGeant3") gMC->SetUserDecay(ipdg);// Force the decay to be done w/external decayer
        
         pythia6.SetMDCY(pythia6.Pycomp(ipdg),1,1); // Activate decay in pythia
        }
        
       
        // The following will print the decay modes
         pythia6.Pyupda(1,6);        
        
        // rho0 (113), rho+ (213), rho- (-213) and
        // D+(411) ,D-(-411),D0(421),D0bar(-421) have decay modes defined in 
        // TGeant3::DefineParticles, but for these particles
        // those decay modes are overridden to make use of pythia6.
        const Int_t nparthq = 3;    
        // rho0,rho+,rho-,D+,D-,D0,D0bar
        //Int_t pdghq[nparthq] = {113,213,-213,411,-411,421,-421};
        Int_t pdghq[nparthq] = {421,3122,-3122};
        for ( Int_t iparthq = 0; iparthq < nparthq; iparthq++ ) {
        Int_t ipdg = pdghq[iparthq];
        if (TString(gMC->GetName()) == "TGeant3") gMC->SetUserDecay(ipdg); // Force the decay to be done w/external decayer
        pythia6.SetMDCY(pythia6.Pycomp(ipdg),1,1); // Activate decay in pythia
        }
        // Set pi0 to be stable in pythia6 so that Geant3 can handle decay.
        // In general, TGeant3 is set up through TGeant3gu::gudcay to pass
        // all pythia6 decay products back to the G3 transport mechanism if they
        // have a lifetime > 1.E-15 sec for further transport.
        // Since the pi0 lifetime is less than this, if pi0 is produced as a decay
        // product in pythia6, e.g. KL0 -> pi0 pi+ pi-, the pi0 will be immediately
        // decayed by pythia6 to 2 gammas, and the KL0 decay product list passed
        // back to the transport mechanism will be "gamma gamma pi+ pi-", i.e.  
        // the pi0 will not be visible in the list of secondaries passed back to 
        // the transport mechanism and will not be pushed to the stack for possible 
        // storage to the stdhep output array.
        // To avoid this, the pi0 is set to stable in pythia6, and its decay
        // will be handled by Geant3. 
        //pythia6.SetMDCY(pythia6.Pycomp(111),1,0);
  //}
}