EvtVll.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: EvtVll.cc
//
// Description: The decay of a vector meson to two leptons,
//              or generally, two spin 1/2 particles.
//              E.g., J/psi -> e+ e-
//
// Modification history:
//
//    RYD       January 17, 1997       Module created
//
//------------------------------------------------------------------------
//
#include "EvtVll.hh"
#include "../EvtGenBase/EvtDiracSpinor.hh"
#include "../EvtGenBase/EvtGenKine.hh"
#include "../EvtGenBase/EvtPDL.hh"
#include "../EvtGenBase/EvtParticle.hh"
#include "../EvtGenBase/EvtPatches.hh"
#include "../EvtGenBase/EvtReport.hh"
#include "../EvtGenBase/EvtVector4C.hh"
#include <iostream>
#include <stdlib.h>
#include <string>
 
EvtVll::~EvtVll() {}
 
void EvtVll::getName( std::string& model_name ) { model_name = "VLL"; }
 
EvtDecayBase* EvtVll::clone() { return new EvtVll; }
 
void EvtVll::init() {
 
  // check that there are 0 arguments
  checkNArg( 0 );
  checkNDaug( 2 );
 
  checkSpinParent( EvtSpinType::VECTOR );
 
  checkSpinDaughter( 0, EvtSpinType::DIRAC );
  checkSpinDaughter( 1, EvtSpinType::DIRAC );
}
 
void EvtVll::initProbMax() { setProbMax( 1.0 ); }
 
void EvtVll::decay( EvtParticle* p ) {
 
  p->initializePhaseSpace( getNDaug(), getDaugs() );
 
  EvtParticle *l1, *l2;
  l1 = p->getDaug( 0 );
  l2 = p->getDaug( 1 );
 
  EvtVector4C l11, l12, l21, l22;
  l11 = EvtLeptonVCurrent( l1->spParent( 0 ), l2->spParent( 0 ) );
  l12 = EvtLeptonVCurrent( l1->spParent( 0 ), l2->spParent( 1 ) );
  l21 = EvtLeptonVCurrent( l1->spParent( 1 ), l2->spParent( 0 ) );
  l22 = EvtLeptonVCurrent( l1->spParent( 1 ), l2->spParent( 1 ) );
 
  EvtVector4C eps0 = p->eps( 0 );
  EvtVector4C eps1 = p->eps( 1 );
  EvtVector4C eps2 = p->eps( 2 );
 
  double M2 = p->mass();
  M2 *= M2;
  double m2 = l1->mass();
  m2 *= m2;
 
  double norm = 1.0 / sqrt( 2 * M2 + 4 * m2 - 4 * m2 * m2 / M2 );
 
  vertex( 0, 0, 0, norm * ( eps0 * l11 ) );
  vertex( 0, 0, 1, norm * ( eps0 * l12 ) );
  vertex( 0, 1, 0, norm * ( eps0 * l21 ) );
  vertex( 0, 1, 1, norm * ( eps0 * l22 ) );
 
  vertex( 1, 0, 0, norm * ( eps1 * l11 ) );
  vertex( 1, 0, 1, norm * ( eps1 * l12 ) );
  vertex( 1, 1, 0, norm * ( eps1 * l21 ) );
  vertex( 1, 1, 1, norm * ( eps1 * l22 ) );
 
  vertex( 2, 0, 0, norm * ( eps2 * l11 ) );
  vertex( 2, 0, 1, norm * ( eps2 * l12 ) );
  vertex( 2, 1, 0, norm * ( eps2 * l21 ) );
  vertex( 2, 1, 1, norm * ( eps2 * l22 ) );
 
  return;
}