EvtPhotonParticle.cc

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//--------------------------------------------------------------------------
//
// Environment:
//      This software is part of the EvtGen package developed jointly
//      for the BaBar and CLEO collaborations.  If you use all or part
//      of it, please give an appropriate acknowledgement.
//
// Copyright Information: See EvtGen/COPYRIGHT
//      Copyright (C) 1998      Caltech, UCSB
//
// Module: EvtPhotonParticle.cc
//
// Description: Class to describe massless vectors
//
// Modification history:
//
//    DJL/RYD   September 25, 1996           Module created
//
//------------------------------------------------------------------------
//
#include "EvtPhotonParticle.hh"
#include "EvtComplex.hh"
#include "EvtPatches.hh"
#include "EvtReport.hh"
#include "EvtVector4C.hh"
#include <iostream>
#include <math.h>
#include <stdlib.h>
using std::endl;
 
EvtPhotonParticle::~EvtPhotonParticle() {}
 
void EvtPhotonParticle::init( EvtId part_n, const EvtVector4R& p4 ) {
 
  init( part_n, p4.get( 0 ), p4.get( 1 ), p4.get( 2 ), p4.get( 3 ) );
}
 
void EvtPhotonParticle::init( EvtId part_n, double e, double px, double py, double pz ) {
 
  _validP4 = true;
  setp( e, px, py, pz );
  setpart_num( part_n );
 
  setLifetime();
 
  // defere calculation of basis vectors untill they are needed!
  _evalBasis = 0;
}
 
EvtVector4C EvtPhotonParticle::epsParentPhoton( int i ) {
 
  if ( !_evalBasis )
  {
 
    _evalBasis = 1;
    eps1.set( EvtComplex( 0.0, 0.0 ), EvtComplex( -1.0 / sqrt( 2.0 ), 0.0 ),
              EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), EvtComplex( 0.0, 0.0 ) );
    eps2.set( EvtComplex( 0.0, 0.0 ), EvtComplex( 1.0 / sqrt( 2.0 ), 0.0 ),
              EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), EvtComplex( 0.0, 0.0 ) );
 
    // These are for photon along z axis.  Rotate to get
    // correct direction...
 
    double phi, theta;
 
    EvtVector4R p = this->getP4();
 
    double px = p.get( 1 );
    double py = p.get( 2 );
    double pz = p.get( 3 );
 
    phi   = atan2( py, px );
    theta = acos( pz / sqrt( px * px + py * py + pz * pz ) );
    eps1.applyRotateEuler( phi, theta, -phi );
    eps2.applyRotateEuler( phi, theta, -phi );
  }
 
  EvtVector4C temp;
 
  switch ( i )
  {
 
  case 0: temp = eps1; break;
  case 1: temp = eps2; break;
  default:
    report( ERROR, "EvtGen" ) << "EvtPhotonParticle.cc: Asked "
                              << "for state:" << i << endl;
    ::abort();
    break;
  }
 
  return temp;
}
 
EvtVector4C EvtPhotonParticle::epsPhoton( int i ) {
 
  report( ERROR, "EvtGen" ) << "EvtPhotonParticle.cc: Can not get "
                            << "state in photons restframe." << endl;
  ;
  ::abort();
  return EvtVector4C();
}
 
EvtSpinDensity EvtPhotonParticle::rotateToHelicityBasis() const {
 
  EvtVector4C eplus( 0.0, -1.0 / sqrt( 2.0 ), EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), 0.0 );
  EvtVector4C eminus( 0.0, 1.0 / sqrt( 2.0 ), EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), 0.0 );
 
  // Really uggly have to cast away constness because the
  // function epsParentPhoton caches the state vectors...
  EvtVector4C e1 = ( (EvtParticle*)this )->epsParentPhoton( 0 );
  EvtVector4C e2 = ( (EvtParticle*)this )->epsParentPhoton( 1 );
 
  EvtSpinDensity R;
  R.SetDim( 2 );
 
  R.Set( 0, 0, ( eplus.conj() ) * e1 );
  R.Set( 0, 1, ( eplus.conj() ) * e2 );
 
  R.Set( 1, 0, ( eminus.conj() ) * e1 );
  R.Set( 1, 1, ( eminus.conj() ) * e2 );
 
  return R;
}
 
EvtSpinDensity EvtPhotonParticle::rotateToHelicityBasis( double alpha, double beta,
                                                         double gamma ) const {
 
  EvtVector4C eplus( 0.0, -1.0 / sqrt( 2.0 ), EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), 0.0 );
  EvtVector4C eminus( 0.0, 1.0 / sqrt( 2.0 ), EvtComplex( 0.0, -1.0 / sqrt( 2.0 ) ), 0.0 );
 
  eplus.applyRotateEuler( alpha, beta, gamma );
  eminus.applyRotateEuler( alpha, beta, gamma );
 
  // Really uggly have to cast away constness because the
  // function epsParentPhoton caches the state vectors...
  EvtVector4C e1 = ( (EvtParticle*)this )->epsParentPhoton( 0 );
  EvtVector4C e2 = ( (EvtParticle*)this )->epsParentPhoton( 1 );
 
  EvtSpinDensity R;
  R.SetDim( 2 );
 
  R.Set( 0, 0, ( eplus.conj() ) * e1 );
  R.Set( 0, 1, ( eplus.conj() ) * e2 );
 
  R.Set( 1, 0, ( eminus.conj() ) * e1 );
  R.Set( 1, 1, ( eminus.conj() ) * e2 );
 
  return R;
}