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MEee2Z.cc
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MEee2Z.cc
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// -*- C++ -*-
//
// MEee2Z.cc is a part of Herwig++ - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2007 The Herwig Collaboration
//
// Herwig++ is licenced under version 2 of the GPL, see COPYING for details.
// Please respect the MCnet academic guidelines, see GUIDELINES for details.
//
//
// This is the implementation of the non-inlined, non-templated member
// functions of the MEee2Z class.
//
#include "MEee2Z.h"
#include "ThePEG/Interface/ClassDocumentation.h"
#include "ThePEG/PDT/EnumParticles.h"
#include "ThePEG/MatrixElement/Tree2toNDiagram.h"
#include "ThePEG/Interface/ClassDocumentation.h"
#include "ThePEG/Interface/Switch.h"
#include "ThePEG/Helicity/WaveFunction/ScalarWaveFunction.h"
#include "ThePEG/Helicity/WaveFunction/VectorWaveFunction.h"
#include "ThePEG/Handlers/StandardXComb.h"
#include "ThePEG/Helicity/FermionSpinInfo.h"
#include "ThePEG/Helicity/VectorSpinInfo.h"
#include "Herwig++/MatrixElement/General/HardVertex.h"
#include "ThePEG/Persistency/PersistentOStream.h"
#include "ThePEG/Persistency/PersistentIStream.h"
#include "ThePEG/Cuts/Cuts.h"
namespace Herwig {
using namespace ThePEG;
using ThePEG::Helicity::FermionSpinPtr;
using ThePEG::Helicity::VectorSpinPtr;
using ThePEG::Helicity::VertexPtr;
using ThePEG::Helicity::FermionSpinInfo;
using ThePEG::Helicity::VectorSpinInfo;
using namespace ThePEG::Helicity;
using ThePEG::Helicity::DiracRep;
using ThePEG::Helicity::HaberDRep;
using ThePEG::Helicity::HELASDRep;
using ThePEG::Helicity::defaultDRep;
void MEee2Z::getDiagrams() const {
tcPDPtr Z0 = getParticleData(ParticleID::Z0);
tcPDPtr em = getParticleData(ParticleID::eminus);
tcPDPtr ep = getParticleData(ParticleID::eplus);
add(new_ptr((Tree2toNDiagram(2), em, ep, 1, Z0,-1)));
}
Energy2 MEee2Z::scale() const {
return sHat();
}
int MEee2Z::nDim() const {
return 1;
}
bool MEee2Z::generateKinematics(const double *) {
// Here you can use nDim() random numbers in the vector provided
// to generate the internal kinematics. Note that sHat() has
// already been given from the outside.
meMomenta()[2]=meMomenta()[0]+meMomenta()[1];
meMomenta()[2].rescaleMass();
jacobian(1.0);
// check passes all the cuts
vector<LorentzMomentum> out(1,meMomenta()[2]);
tcPDVector tout(1,mePartonData()[2]);
// return true if passes the cuts
return lastCuts().passCuts(tout, out, mePartonData()[0], mePartonData()[1]);
}
double MEee2Z::me2() const {
double aver=0.;
// the arrays for the wavefunction to be passed to the matrix element
vector<SpinorWaveFunction> fin;
vector<SpinorBarWaveFunction> ain;
vector<VectorWaveFunction> vin;
SpinorWaveFunction fwave(meMomenta()[0],mePartonData()[0],incoming);
SpinorBarWaveFunction awave(meMomenta()[1],mePartonData()[1],incoming);
for(unsigned int ihel=0;ihel<2;++ihel) {
fwave.reset(ihel);fin.push_back(fwave);
awave.reset(ihel);ain.push_back(awave);
}
VectorWaveFunction vwave(meMomenta()[2],mePartonData()[2],outgoing);
for(unsigned int ihel=0;ihel<3;++ihel) {
vwave.reset(ihel); vin.push_back(vwave);
}
ProductionMatrixElement temp=HelicityME(fin,ain,vin,aver);
// add the Breit-Wigner factors
Energy width=mePartonData()[2]->width();
Energy mass =mePartonData()[2]->mass();
InvEnergy2 fact = width*mass/(sqr(sHat()-mass*mass)+sqr(mass*width));
return aver*fact*sHat();
}
CrossSection MEee2Z::dSigHatDR() const {
return (me2()*jacobian()/sHat())*sqr(hbarc);
}
unsigned int MEee2Z::orderInAlphaS() const {
return 0;
}
unsigned int MEee2Z::orderInAlphaEW() const {
return 1;
}
Selector<MEBase::DiagramIndex>
MEee2Z::diagrams(const DiagramVector &) const {
Selector<DiagramIndex> sel;
sel.insert(1.0, 0);
return sel;
}
Selector<const ColourLines *>
MEee2Z::colourGeometries(tcDiagPtr) const {
static const ColourLines neutral ( " " );
Selector<const ColourLines *> sel;
sel.insert(1.,&neutral);
return sel;
}
void MEee2Z::persistentOutput(PersistentOStream & os) const {
os << _theFFZVertex;
}
void MEee2Z::persistentInput(PersistentIStream & is, int) {
is >> _theFFZVertex;
}
ClassDescription<MEee2Z> MEee2Z::initMEee2Z;
// Definition of the static class description member.
void MEee2Z::Init() {
static ClassDocumentation<MEee2Z> documentation
("The MEee2Z class implements the e+e- -> Z as a 1->2 process for testing"
" of spin correlations etc..");
}
void MEee2Z::constructVertex(tSubProPtr sub) {
// extract the particles in the hard process
ParticleVector hard;
hard.push_back(sub->incoming().first);hard.push_back(sub->incoming().second);
hard.push_back(sub->outgoing()[0]);
if(hard[0]->id()<hard[1]->id()) swap(hard[0],hard[1]);
vector<SpinorWaveFunction> fin;
vector<SpinorBarWaveFunction> ain;
vector<VectorWaveFunction> vin;
SpinorWaveFunction( fin,hard[0],incoming,false,true);
SpinorBarWaveFunction(ain,hard[1],incoming,false,true);
VectorWaveFunction (vin,hard[2],outgoing,true,false,true);
double dummy;
ProductionMatrixElement prodme=HelicityME(fin,ain,vin,dummy);
// construct the vertex
HardVertexPtr hardvertex=new_ptr(HardVertex());
// set the matrix element for the vertex
hardvertex->ME(prodme);
// set the pointers to and from the vertex
for(unsigned int ix=0;ix<3;++ix) {
dynamic_ptr_cast<SpinfoPtr>(hard[ix]->spinInfo())->
setProductionVertex(hardvertex);
}
}
// the helicity amplitude matrix element
ProductionMatrixElement MEee2Z::HelicityME(vector<SpinorWaveFunction> fin,
vector<SpinorBarWaveFunction> ain,
vector<VectorWaveFunction> vout,
double & aver) const
{
ProductionMatrixElement output(PDT::Spin1Half,PDT::Spin1Half,PDT::Spin1);
Complex product;
// sum over helicities to get the matrix element
unsigned int inhel1,inhel2,outhel1;
double me(0.);
LorentzPolarizationVector vec;
Complex ii(0.,1.);
for(inhel1=0;inhel1<2;++inhel1) {
for(inhel2=0;inhel2<2;++inhel2) {
for(outhel1=0;outhel1<3;++outhel1) {
product=_theFFZVertex->evaluate(sHat(),fin[inhel1],ain[inhel2],
vout[outhel1]);
output(inhel1,inhel2,outhel1)=product;
me+=real(product*conj(product));
}
}
}
aver=me/4.;
return output;
}
}

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