EvtJ2BB1.cc

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//--------------------------------------------------------------------------
//
// Environment:
//      This software is part of models developed at BES collaboration
//      based on the EvtGen framework.  If you use all or part
//      of it, please give an appropriate acknowledgement.
//
// Copyright Information: See EvtGen/BesCopyright
//      Copyright (A) 2006      Ping Rong-Gang, Pang Cai-Ying@IHEP
//
// Module: EvtJ2BB1.cc
//
// Description: Routine to decay J/psi-> B_8 bar B_8 using helicity amplitude.
//
// Modification history:
//
//    Pang C.-Y., Ping R.-G.    December, 2006       Module created
//
//------------------------------------------------------------------------
//
#include "EvtJ2BB1.hh"
#include "../EvtGenBase/EvtGenKine.hh"
#include "../EvtGenBase/EvtHelSys.hh"
#include "../EvtGenBase/EvtPDL.hh"
#include "../EvtGenBase/EvtParticle.hh"
#include "../EvtGenBase/EvtPatches.hh"
#include "../EvtGenBase/EvtReport.hh"
#include "../EvtGenBase/EvtTensor4C.hh"
#include "../EvtGenBase/EvtVector4C.hh"
#include "../EvtGenBase/EvtVector4R.hh"
#include "../EvtGenBase/EvtdFunction.hh"
#include <stdlib.h>
#include <string>
using std::endl;
 
EvtJ2BB1::~EvtJ2BB1() {}
 
void EvtJ2BB1::getName( std::string& model_name ) { model_name = "J2BB1"; }
 
EvtDecayBase* EvtJ2BB1::clone() { return new EvtJ2BB1; }
 
/*
void EvtJ2BB1::initProbMax() {
   double ProbMax=12.;
   setProbMax(ProbMax);
}
*/
 
void EvtJ2BB1::init() {
  checkNDaug( 2 );
  checkSpinParent( EvtSpinType::VECTOR );
  checkSpinDaughter( 0, EvtSpinType::DIRAC );
  checkSpinDaughter( 1, EvtSpinType::DIRAC );
}
 
void EvtJ2BB1::decay( EvtParticle* p ) {
 
  p->initializePhaseSpace( getNDaug(), getDaugs() );
 
  EvtParticle *v, *s1;
  EvtVector4R  pv, ps, ppr;
 
  v   = p->getDaug( 0 ); // Baryon
  s1  = p->getDaug( 1 ); // Baryon
  pv  = v->getP4();
  ps  = s1->getP4();
  ppr = p->getP4();
  //  Put phase space results into the daughters.
 
  EvtHelSys angles( ppr, pv ); // using helicity sys.angles
  double    theta    = angles.getHelAng( 1 );
  double    phi      = angles.getHelAng( 2 );
  double    gamma    = 0;
  double    costheta = cos( theta ); // using helicity angles in parent system
  //  double costheta=ppr.get(3)/ppr.d3mag();  //using Lab system
 
  if ( getNArg() > 0 ) { alpha = getArg( 0 ); }
  else
  {
    double mass_b1 = EvtPDL::getMass( getDaug( 0 ) );
    double mass_b2 = EvtPDL::getMass( getDaug( 1 ) );
    double mass_M  = EvtPDL::getMass( getParentId() );
    double c1      = pow( pv.d3mag(), 2.0 );
    double R       = ( mass_b1 + pv.get( 0 ) ) * ( mass_b2 + ps.get( 0 ) );
    u              = pow( ( R - c1 ) / ( R + c1 ), 2.0 );
    alpha          = ( pow( 4 + u, 2.0 ) - u * pow( 2 + 3 * u, 2.0 ) ) /
            ( pow( 4 + u, 2.0 ) + u * pow( 2 + 3 * u, 2.0 ) );
  }
 
  double F01 = sqrt( ( 1 + alpha ) / 2 );
  double F00 = sqrt( ( 1 - alpha ) / 4 );
  double F10 = F01;
  double F11 = F00;
 
  vertex( 0, 0, 0, Djmn( 1, 1, 0, phi, theta, gamma ) * F00 ); // J/psi helicity =1
                                                               // corresponding index=0
  vertex( 0, 0, 1, Djmn( 1, 1, 1, phi, theta, gamma ) * F01 );
  vertex( 0, 1, 0, Djmn( 1, 1, -1, phi, theta, gamma ) * F10 );
  vertex( 0, 1, 1, Djmn( 1, 1, 0, phi, theta, gamma ) * F11 );
  vertex( 1, 0, 0, Djmn( 1, -1, 0, phi, theta, gamma ) * F00 ); // J/psi helicity =-1
                                                                // corresponding index=1
  vertex( 1, 0, 1, Djmn( 1, -1, 1, phi, theta, gamma ) * F01 );
  vertex( 1, 1, 0, Djmn( 1, -1, -1, phi, theta, gamma ) * F10 );
  vertex( 1, 1, 1, Djmn( 1, -1, 0, phi, theta, gamma ) * F11 );
  vertex( 2, 0, 0, Djmn( 1, 0, 0, phi, theta, gamma ) * F00 ); // J/psi helicity =0
                                                               // corresponding index=2
  vertex( 2, 0, 1, Djmn( 1, 0, 1, phi, theta, gamma ) * F01 );
  vertex( 2, 1, 0, Djmn( 1, 0, -1, phi, theta, gamma ) * F10 );
  vertex( 2, 1, 1, Djmn( 1, 0, 0, phi, theta, gamma ) * F11 );
 
  return;
}