EvtJPE.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 @IHEP
//
// Module: EvtJPE.cc
//
// Description: Routine to decay Vector->photon +pseudoscalar, e.g. J/psi to gamma eta or eta_c
//
// Modification history:
//
//    Ping R.-G.       Apr., 2007       Module created
//
//------------------------------------------------------------------------
//
#include "EvtJPE.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;
 
EvtJPE::~EvtJPE() {}
 
void EvtJPE::getName( std::string& model_name ) { model_name = "JPE"; }
 
EvtDecayBase* EvtJPE::clone() { return new EvtJPE; }
 
void EvtJPE::init() {
  static EvtId GAM = EvtPDL::getId( "gamma" );
  // check that there are 0 arguments
  checkNArg( 0 );
  checkNDaug( 2 );
  checkSpinParent( EvtSpinType::VECTOR );
  checkSpinDaughter( 0, EvtSpinType::PHOTON );
  checkSpinDaughter( 1, EvtSpinType::SCALAR );
  if ( !( getDaug( 0 ) == GAM ) )
  {
    report( ERROR, "EvtGen" ) << "EvtJPE generator radiative decays expected " << endl;
    ::abort();
  }
}
 
/*
void EvtJPE::initProbMax() {
 
//Hard coded... should not be hard to calculate...
setProbMax(1.0);
 
}
*/
 
void EvtJPE::decay( EvtParticle* p ) {
 
  p->initializePhaseSpace( getNDaug(), getDaugs() );
 
  EvtParticle *v, *s1;
  EvtVector4R  pv, ps, ppr;
 
  v                = p->getDaug( 0 ); // gamma
  s1               = p->getDaug( 1 ); // eta_c
  pv               = v->getP4();
  ps               = s1->getP4();
  ppr              = p->getP4();
  EvtVector4R pvcm = v->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 mass_M = EvtPDL::getMass( getParentId() );
  double Mpsip  = fabs( mass_M - 3.686 );
  double Mpsipp = fabs( mass_M - 3.771 );
  double FP = pvcm.d3mag(); // for J/psi and psi(3770) decays, M1 transition  assumed with form
                            // factor =1. P-wave gives the p_gamma factor
  if ( Mpsip < 0.1 ) { FP = FP * FP; }
  else { FP = 1; } // for psi(2S) decays, M1 transition assumed with form factor=k_gamma^2
 
  double FM = -FP;
  vertex( 0, 0,
          Djmn( 1, 1, 1, phi, theta, gamma ) * FP ); // Jpsi helicity =1 corresponding index=0
  vertex( 0, 1, Djmn( 1, 1, -1, phi, theta, gamma ) * FM );
  vertex( 1, 0, Djmn( 1, -1, 1, phi, theta, gamma ) * FP ); // Jpsi helicity =-1 corresponding
                                                            // index=1
  vertex( 1, 1, Djmn( 1, -1, -1, phi, theta, gamma ) * FM );
  vertex( 2, 0,
          Djmn( 1, 0, 1, phi, theta, gamma ) * FP ); // Jpsi helicity =0 corresponding index=2
  vertex( 2, 1, Djmn( 1, 0, -1, phi, theta, gamma ) * FM );
 
  return;
}