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MEee2gZ2ll.cc
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// -*- C++ -*-
//
// MEee2gZ2ll.cc is a part of Herwig - A multi-purpose Monte Carlo event generator
// Copyright (C) 2002-2019 The Herwig Collaboration
//
// Herwig is licenced under version 3 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 MEee2gZ2ll class.
//
#include "MEee2gZ2ll.h"
#include "ThePEG/Interface/ClassDocumentation.h"
#include "ThePEG/Interface/Switch.h"
#include "ThePEG/Interface/Parameter.h"
#include "ThePEG/Interface/Reference.h"
#include "ThePEG/Persistency/PersistentOStream.h"
#include "ThePEG/Persistency/PersistentIStream.h"
#include "ThePEG/PDT/EnumParticles.h"
#include "ThePEG/MatrixElement/Tree2toNDiagram.h"
#include "ThePEG/Helicity/WaveFunction/VectorWaveFunction.h"
#include "ThePEG/Handlers/StandardXComb.h"
#include "Herwig/MatrixElement/HardVertex.h"
#include "ThePEG/PDF/PolarizedBeamParticleData.h"
#include "ThePEG/Utilities/DescribeClass.h"
#include <numeric>
#include "Herwig/Shower/RealEmissionProcess.h"
using namespace Herwig;
void MEee2gZ2ll::getDiagrams() const {
// specific the diagrams
tcPDPtr ep = getParticleData(ParticleID::eplus);
tcPDPtr em = getParticleData(ParticleID::eminus);
// setup the processes
for( int i =11;i<=16;++i) {
if(allowed_==0 || (allowed_==1 && i%2==1) || (allowed_==2&&i==11)
|| (allowed_==3&&i==13) || (allowed_==4&&i==15)) {
tcPDPtr lm = getParticleData(i);
tcPDPtr lp = lm->CC();
add(new_ptr((Tree2toNDiagram(2), em, ep, 1, gamma_, 3, lm, 3, lp, -1)));
add(new_ptr((Tree2toNDiagram(2), em, ep, 1, Z0_, 3, lm, 3, lp, -2)));
}
}
}
Energy2 MEee2gZ2ll::scale() const {
return sHat();
}
unsigned int MEee2gZ2ll::orderInAlphaS() const {
return 0;
}
unsigned int MEee2gZ2ll::orderInAlphaEW() const {
return 2;
}
Selector<MEBase::DiagramIndex>
MEee2gZ2ll::diagrams(const DiagramVector & diags) const {
double lastCont(0.5),lastBW(0.5);
if ( lastXCombPtr() ) {
lastCont = meInfo()[0];
lastBW = meInfo()[1];
}
Selector<DiagramIndex> sel;
for ( DiagramIndex i = 0; i < diags.size(); ++i ) {
if ( diags[i]->id() == -1 ) sel.insert(lastCont, i);
else if ( diags[i]->id() == -2 ) sel.insert(lastBW, i);
}
return sel;
}
Selector<const ColourLines *>
MEee2gZ2ll::colourGeometries(tcDiagPtr) const {
static ColourLines ctST(" ");
Selector<const ColourLines *> sel;
sel.insert(1.0, &ctST);
return sel;
}
void MEee2gZ2ll::persistentOutput(PersistentOStream & os) const {
os << allowed_ << FFZVertex_ << FFPVertex_ << gamma_ << Z0_
<< alphaQED_ << ounit(pTmin_,GeV) << preFactor_;
}
void MEee2gZ2ll::persistentInput(PersistentIStream & is, int) {
is >> allowed_ >> FFZVertex_ >> FFPVertex_ >> gamma_ >> Z0_
>> alphaQED_ >> iunit(pTmin_,GeV) >> preFactor_;
}
// *** Attention *** The following static variable is needed for the type
// description system in ThePEG. Please check that the template arguments
// are correct (the class and its base class), and that the constructor
// arguments are correct (the class name and the name of the dynamically
// loadable library where the class implementation can be found).
DescribeClass<MEee2gZ2ll,HwMEBase>
describeMEee2gZ2ll("Herwig::MEee2gZ2ll", "HwMELepton.so");
void MEee2gZ2ll::Init() {
static ClassDocumentation<MEee2gZ2ll> documentation
("The MEee2gZ2ll class implements the matrix element for"
"e+e- to leptons via Z and photon exchange using helicity amplitude"
"techniques");
static Switch<MEee2gZ2ll,int> interfaceallowed
("Allowed",
"Allowed outgoing leptons",
&MEee2gZ2ll::allowed_, 0, false, false);
static SwitchOption interfaceallowedAll
(interfaceallowed,
"All",
"Allow all leptons as outgoing particles",
0);
static SwitchOption interfaceallowedCharged
(interfaceallowed,
"Charged",
"Only charged leptons as outgoing particles",
1);
static SwitchOption interfaceallowedElectron
(interfaceallowed,
"Electron",
"Only the electron and positron as outgoing leptons",
2);
static SwitchOption interfaceallowedMuon
(interfaceallowed,
"Muon",
"Only muons as outgoing particles",
3);
static SwitchOption interfaceallowedTau
(interfaceallowed,
"Tau",
"Only taus as outgoing particles",
4);
static Parameter<MEee2gZ2ll,Energy> interfacepTMin
("pTMin",
"Minimum pT for hard radiation",
&MEee2gZ2ll::pTmin_, GeV, 1.0*GeV, 0.001*GeV, 10.0*GeV,
false, false, Interface::limited);
static Parameter<MEee2gZ2ll,double> interfacePrefactor
("Prefactor",
"Prefactor for the overestimate of the emission probability",
&MEee2gZ2ll::preFactor_, 6.0, 1.0, 100.0,
false, false, Interface::limited);
static Reference<MEee2gZ2ll,ShowerAlpha> interfaceEMCoupling
("AlphaQED",
"Pointer to the object to calculate the EM coupling for the correction",
&MEee2gZ2ll::alphaQED_, false, false, true, false, false);
}
double MEee2gZ2ll::me2() const {
return loME(mePartonData(),rescaledMomenta(),true);
}
double MEee2gZ2ll::loME(const vector<cPDPtr> & partons,
const vector<Lorentz5Momentum> & momenta,
bool first) const {
vector<SpinorWaveFunction> fin,aout;
vector<SpinorBarWaveFunction> ain,fout;
SpinorWaveFunction ein (momenta[0],partons[0],incoming);
SpinorBarWaveFunction pin (momenta[1],partons[1],incoming);
SpinorBarWaveFunction ilmout(momenta[2],partons[2],outgoing);
SpinorWaveFunction ilpout(momenta[3],partons[3],outgoing);
for(unsigned int ix=0;ix<2;++ix) {
ein.reset(ix) ;fin.push_back( ein );
pin.reset(ix) ;ain.push_back( pin );
ilmout.reset(ix);fout.push_back(ilmout);
ilpout.reset(ix);aout.push_back(ilpout);
}
// compute the matrix element
double me,lastCont,lastBW;
HelicityME(fin,ain,fout,aout,me,lastCont,lastBW);
// save the components
if(first) {
DVector save;
save.push_back(lastCont);
save.push_back(lastBW);
meInfo(save);
}
// return the answer
return me;
}
ProductionMatrixElement MEee2gZ2ll::HelicityME(vector<SpinorWaveFunction> & fin,
vector<SpinorBarWaveFunction> & ain,
vector<SpinorBarWaveFunction> & fout,
vector<SpinorWaveFunction> & aout,
double & me,double & cont,
double & BW ) const {
// the particles should be in the order
// for the incoming
// 0 incoming fermion (u spinor)
// 1 incoming antifermion (vbar spinor)
// for the outgoing
// 0 outgoing fermion (ubar spinor)
// 1 outgoing antifermion (v spinor)
// me to be returned
ProductionMatrixElement output(PDT::Spin1Half,PDT::Spin1Half,
PDT::Spin1Half,PDT::Spin1Half);
ProductionMatrixElement gamma (PDT::Spin1Half,PDT::Spin1Half,
PDT::Spin1Half,PDT::Spin1Half);
ProductionMatrixElement Zboson(PDT::Spin1Half,PDT::Spin1Half,
PDT::Spin1Half,PDT::Spin1Half);
// // wavefunctions for the intermediate particles
VectorWaveFunction interZ,interG;
// temporary storage of the different diagrams
Complex diag1,diag2;
// sum over helicities to get the matrix element
unsigned int inhel1,inhel2,outhel1,outhel2;
double total[3]={0.,0.,0.};
for(inhel1=0;inhel1<2;++inhel1) {
for(inhel2=0;inhel2<2;++inhel2) {
// intermediate Z
interZ = FFZVertex_->evaluate(sHat(),1,Z0_,fin[inhel1],ain[inhel2]);
// intermediate photon
interG = FFPVertex_->evaluate(sHat(),1,gamma_,fin[inhel1],ain[inhel2]);
for(outhel1=0;outhel1<2;++outhel1) {
for(outhel2=0;outhel2<2;++outhel2) {
// first the Z exchange diagram
diag1 = FFZVertex_->evaluate(sHat(),aout[outhel2],fout[outhel1],
interZ);
// then the photon exchange diagram
diag2 = FFPVertex_->evaluate(sHat(),aout[outhel2],fout[outhel1],
interG);
// add up squares of individual terms
total[1] += norm(diag1);
Zboson(inhel1,inhel2,outhel1,outhel2) = diag1;
total[2] += norm(diag2);
gamma (inhel1,inhel2,outhel1,outhel2) = diag2;
// the full thing including interference
diag1 += diag2;
total[0] += norm(diag1);
output(inhel1,inhel2,outhel1,outhel2) = diag1;
}
}
}
}
// results
for(int ix=0;ix<3;++ix) total[ix] *= 0.25;
tcPolarizedBeamPDPtr beam[2] =
{dynamic_ptr_cast<tcPolarizedBeamPDPtr>(mePartonData()[0]),
dynamic_ptr_cast<tcPolarizedBeamPDPtr>(mePartonData()[1])};
if( beam[0] || beam[1] ) {
RhoDMatrix rho[2] = {beam[0] ? beam[0]->rhoMatrix() : RhoDMatrix(mePartonData()[0]->iSpin()),
beam[1] ? beam[1]->rhoMatrix() : RhoDMatrix(mePartonData()[1]->iSpin())};
total[0] = output.average(rho[0],rho[1]);
total[1] = Zboson.average(rho[0],rho[1]);
total[2] = gamma .average(rho[0],rho[1]);
}
cont = total[2];
BW = total[1];
me = total[0];
return output;
}
void MEee2gZ2ll::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]);hard.push_back(sub->outgoing()[1]);
if(hard[0]->id()<hard[1]->id()) swap(hard[0],hard[1]);
if(hard[2]->id()<hard[3]->id()) swap(hard[2],hard[3]);
vector<SpinorWaveFunction> fin,aout;
vector<SpinorBarWaveFunction> ain,fout;
SpinorWaveFunction( fin ,hard[0],incoming,false,true);
SpinorBarWaveFunction(ain ,hard[1],incoming,false,true);
SpinorBarWaveFunction(fout,hard[2],outgoing,true ,true);
SpinorWaveFunction( aout,hard[3],outgoing,true ,true);
// calculate the matrix element
double me,cont,BW;
ProductionMatrixElement prodme=HelicityME(fin,ain,fout,aout,me,cont,BW);
// construct the vertex
HardVertexPtr hardvertex=new_ptr(HardVertex());
// set the matrix element for the vertex
hardvertex->ME(prodme);
// set the pointers and to and from the vertex
for(unsigned int ix=0;ix<4;++ix) {
tSpinPtr spin = hard[ix]->spinInfo();
if(ix<2) {
tcPolarizedBeamPDPtr beam =
dynamic_ptr_cast<tcPolarizedBeamPDPtr>(hard[ix]->dataPtr());
if(beam) spin->rhoMatrix() = beam->rhoMatrix();
}
spin->productionVertex(hardvertex);
}
}
void MEee2gZ2ll::doinit() {
HwMEBase::doinit();
// set the particle data objects
Z0_=getParticleData(ThePEG::ParticleID::Z0);
gamma_=getParticleData(ThePEG::ParticleID::gamma);
// cast the SM pointer to the Herwig SM pointer
tcHwSMPtr hwsm= dynamic_ptr_cast<tcHwSMPtr>(standardModel());
// do the initialisation
if(!hwsm) throw InitException() << "Wrong type of StandardModel object in "
<< "MEee2gZ2ll::doinit() the Herwig"
<< " version must be used"
<< Exception::runerror;
FFZVertex_ = hwsm->vertexFFZ();
FFPVertex_ = hwsm->vertexFFP();
}
void MEee2gZ2ll::rebind(const TranslationMap & trans) {
FFZVertex_ = trans.translate(FFZVertex_);
FFPVertex_ = trans.translate(FFPVertex_);
Z0_ = trans.translate(Z0_);
gamma_ = trans.translate(gamma_);
HwMEBase::rebind(trans);
}
IVector MEee2gZ2ll::getReferences() {
IVector ret = HwMEBase::getReferences();
ret.push_back(FFZVertex_);
ret.push_back(FFPVertex_);
ret.push_back(Z0_ );
ret.push_back(gamma_ );
return ret;
}
RealEmissionProcessPtr MEee2gZ2ll::generateHardest(RealEmissionProcessPtr born,
ShowerInteraction inter) {
// check if QED switched on
if(inter==ShowerInteraction::QCD) return RealEmissionProcessPtr();
// generate the momenta for the hard emission
vector<Lorentz5Momentum> emission;
unsigned int iemit,ispect;
Energy pTveto = generateHard(born,emission,iemit,ispect,false);
// check if charged
if(!partons_[2]->charged()) return RealEmissionProcessPtr();
// maximum pT of emission
if(pTveto<=ZERO) {
born->pT()[ShowerInteraction::QED] = pTmin_;
return born;
}
else {
born->pT()[ShowerInteraction::QED] = pTveto;
}
born->interaction(ShowerInteraction::QED);
// get the quark and antiquark
ParticleVector qq;
for(unsigned int ix=0;ix<2;++ix) qq.push_back(born->bornOutgoing()[ix]);
bool order = qq[0]->id()>0;
if(!order) swap(qq[0],qq[1]);
// create the new quark, antiquark and gluon
PPtr newq = qq[0]->dataPtr()->produceParticle(emission[2]);
PPtr newa = qq[1]->dataPtr()->produceParticle(emission[3]);
PPtr newg = gamma_->produceParticle(emission[4]);
// create the output real emission process
for(unsigned int ix=0;ix<born->bornIncoming().size();++ix) {
born->incoming().push_back(born->bornIncoming()[ix]->dataPtr()->
produceParticle(born->bornIncoming()[ix]->momentum()));
}
if(order) {
born->outgoing().push_back(newq);
born->outgoing().push_back(newa);
}
else {
born->outgoing().push_back(newa);
born->outgoing().push_back(newq);
swap(iemit,ispect);
}
born->outgoing().push_back(newg);
// set emitter and spectator
born->emitter (iemit);
born->spectator(ispect);
born->emitted(4);
return born;
}
double MEee2gZ2ll::meRatio(vector<cPDPtr> partons,
vector<Lorentz5Momentum> momenta,
unsigned int iemitter, bool subtract) const {
Lorentz5Momentum q = momenta[2]+momenta[3]+momenta[4];
Energy2 Q2=q.m2();
Energy2 lambda = sqrt((Q2-sqr(momenta[2].mass()+momenta[3].mass()))*
(Q2-sqr(momenta[2].mass()-momenta[3].mass())));
InvEnergy2 D[2];
double lome[2];
for(unsigned int iemit=0;iemit<2;++iemit) {
unsigned int ispect = iemit==0 ? 1 : 0;
Energy2 pipj = momenta[4 ] * momenta[2+iemit ];
Energy2 pipk = momenta[4 ] * momenta[2+ispect];
Energy2 pjpk = momenta[2+iemit] * momenta[2+ispect];
double y = pipj/(pipj+pipk+pjpk);
double z = pipk/( pipk+pjpk);
Energy mij = sqrt(2.*pipj+sqr(momenta[2+iemit].mass()));
Energy2 lamB = sqrt((Q2-sqr(mij+momenta[2+ispect].mass()))*
(Q2-sqr(mij-momenta[2+ispect].mass())));
Energy2 Qpk = q*momenta[2+ispect];
Lorentz5Momentum pkt =
lambda/lamB*(momenta[2+ispect]-Qpk/Q2*q)
+0.5/Q2*(Q2+sqr(momenta[2+ispect].mass())-sqr(momenta[2+ispect].mass()))*q;
Lorentz5Momentum pijt =
q-pkt;
double muj = momenta[2+iemit ].mass()/sqrt(Q2);
double muk = momenta[2+ispect].mass()/sqrt(Q2);
double vt = sqrt((1.-sqr(muj+muk))*(1.-sqr(muj-muk)))/(1.-sqr(muj)-sqr(muk));
double v = sqrt(sqr(2.*sqr(muk)+(1.-sqr(muj)-sqr(muk))*(1.-y))-4.*sqr(muk))
/(1.-y)/(1.-sqr(muj)-sqr(muk));
// dipole term
D[iemit] = 0.5/pipj*(2./(1.-(1.-z)*(1.-y))
-vt/v*(2.-z+sqr(momenta[2+iemit].mass())/pipj));
// matrix element
vector<Lorentz5Momentum> lomom(4);
lomom[0] = momenta[0];
lomom[1] = momenta[1];
if(iemit==0) {
lomom[2] = pijt;
lomom[3] = pkt ;
}
else {
lomom[3] = pijt;
lomom[2] = pkt ;
}
lome[iemit] = loME(partons,lomom,false);
}
InvEnergy2 ratio = realME(partons,momenta)
*abs(D[iemitter])/(abs(D[0]*lome[0])+abs(D[1]*lome[1]));
double output = Q2*ratio;
if(subtract) output -= 2.*Q2*D[iemitter];
return output;
}
InvEnergy2 MEee2gZ2ll::realME(const vector<cPDPtr> & partons,
const vector<Lorentz5Momentum> & momenta) const {
// compute the spinors
vector<SpinorWaveFunction> fin,aout;
vector<SpinorBarWaveFunction> ain,fout;
vector<VectorWaveFunction> gout;
SpinorWaveFunction ein (momenta[0],partons[0],incoming);
SpinorBarWaveFunction pin (momenta[1],partons[1],incoming);
SpinorBarWaveFunction qkout(momenta[2],partons[2],outgoing);
SpinorWaveFunction qbout(momenta[3],partons[3],outgoing);
VectorWaveFunction photon(momenta[4],partons[4],outgoing);
for(unsigned int ix=0;ix<2;++ix) {
ein.reset(ix) ;
fin.push_back( ein );
pin.reset(ix) ;
ain.push_back( pin );
qkout.reset(ix);
fout.push_back(qkout);
qbout.reset(ix);
aout.push_back(qbout);
photon.reset(2*ix);
gout.push_back(photon);
}
vector<Complex> diag(4,0.);
ProductionMatrixElement output(PDT::Spin1Half,PDT::Spin1Half,
PDT::Spin1Half,PDT::Spin1Half,
PDT::Spin1);
double total(0.);
for(unsigned int inhel1=0;inhel1<2;++inhel1) {
for(unsigned int inhel2=0;inhel2<2;++inhel2) {
// intermediate Z
VectorWaveFunction interZ =
FFZVertex_->evaluate(scale(),1,Z0_,fin[inhel1],ain[inhel2]);
// intermediate photon
VectorWaveFunction interG =
FFPVertex_->evaluate(scale(),1,gamma_,fin[inhel1],ain[inhel2]);
for(unsigned int outhel1=0;outhel1<2;++outhel1) {
for(unsigned int outhel2=0;outhel2<2;++outhel2) {
for(unsigned int outhel3=0;outhel3<2;++outhel3) {
SpinorBarWaveFunction off1 =
FFPVertex_->evaluate(scale(),3,partons[2],fout[outhel1],gout[outhel3]);
diag[0] = FFZVertex_->evaluate(scale(),aout[outhel2],off1,interZ);
diag[1] = FFPVertex_->evaluate(scale(),aout[outhel2],off1,interG);
SpinorWaveFunction off2 =
FFPVertex_->evaluate(scale(),3,partons[3],aout[outhel2],gout[outhel3]);
diag[2] = FFZVertex_->evaluate(scale(),off2,fout[outhel1],interZ);
diag[3] = FFPVertex_->evaluate(scale(),off2,fout[outhel1],interG);
// sum of diagrams
Complex sum = std::accumulate(diag.begin(),diag.end(),Complex(0.));
// matrix element
output(inhel1,inhel2,outhel1,outhel2,outhel3)=sum;
// me2
total += norm(sum);
}
}
}
}
}
// spin average
total *= 0.25;
tcPolarizedBeamPDPtr beam[2] =
{dynamic_ptr_cast<tcPolarizedBeamPDPtr>(partons[0]),
dynamic_ptr_cast<tcPolarizedBeamPDPtr>(partons[1])};
if( beam[0] || beam[1] ) {
RhoDMatrix rho[2] =
{beam[0] ? beam[0]->rhoMatrix() : RhoDMatrix(mePartonData()[0]->iSpin()),
beam[1] ? beam[1]->rhoMatrix() : RhoDMatrix(mePartonData()[1]->iSpin())};
total = output.average(rho[0],rho[1]);
}
// divide out the coupling and charge
total /= norm(FFPVertex_->norm())*
sqr(double(mePartonData()[2]->iCharge())/3.);
// return the total
return total*UnitRemoval::InvE2;
}
Energy MEee2gZ2ll::generateHard(RealEmissionProcessPtr born,
vector<Lorentz5Momentum> & emmision,
unsigned int & iemit, unsigned int & ispect,
bool applyVeto) {
// get the momenta of the incoming and outgoing particles
// incoming
tPPtr em = born->bornIncoming()[0];
tPPtr ep = born->bornIncoming()[1];
if(em->id()<0) swap(em,ep);
// outgoing
tPPtr qk = born->bornOutgoing()[0];
tPPtr qb = born->bornOutgoing()[1];
if(qk->id()<0) swap(qk,qb);
// extract the momenta
loMomenta_.clear();
loMomenta_.push_back(em->momentum());
loMomenta_.push_back(ep->momentum());
loMomenta_.push_back(qk->momentum());
loMomenta_.push_back(qb->momentum());
// and ParticleData objects
partons_.resize(5);
partons_[0]=em->dataPtr();
partons_[1]=ep->dataPtr();
partons_[2]=qk->dataPtr();
partons_[3]=qb->dataPtr();
partons_[4]=gamma_;
// boost from lab to CMS frame with outgoing particles
// along the z axis
LorentzRotation eventFrame( ( loMomenta_[2] + loMomenta_[3] ).findBoostToCM() );
Lorentz5Momentum spectator = eventFrame*loMomenta_[2];
eventFrame.rotateZ( -spectator.phi() );
eventFrame.rotateY( -spectator.theta() );
eventFrame.invert();
// mass of the final-state system
Energy2 M2 = (loMomenta_[2]+loMomenta_[3]).m2();
Energy M = sqrt(M2);
double mu1 = loMomenta_[2].mass()/M;
double mu2 = loMomenta_[3].mass()/M;
double mu12 = sqr(mu1), mu22 = sqr(mu2);
double lambda = sqrt(1.+sqr(mu12)+sqr(mu22)-2.*mu12-2.*mu22-2.*mu12*mu22);
// max pT
Energy pTmax = 0.5*sqrt(M2)*
(1.-sqr(loMomenta_[2].mass()+loMomenta_[3].mass())/M2);
// max y
double ymax = acosh(pTmax/pTmin_);
double a = alphaQED_->overestimateValue()/Constants::twopi*
2.*ymax*preFactor_*sqr(double(mePartonData()[2]->iCharge())/3.);
// variables for the emission
Energy pT[2];
double y[2],phi[2],x3[2],x1[2][2],x2[2][2];
double contrib[2][2];
// storage of the real emission momenta
vector<Lorentz5Momentum> realMomenta[2][2]=
{{vector<Lorentz5Momentum>(5),vector<Lorentz5Momentum>(5)},
{vector<Lorentz5Momentum>(5),vector<Lorentz5Momentum>(5)}};
for(unsigned int ix=0;ix<2;++ix)
for(unsigned int iy=0;iy<2;++iy)
for(unsigned int iz=0;iz<2;++iz)
realMomenta[ix][iy][iz] = loMomenta_[iz];
// generate the emission
for(unsigned int ix=0;ix<2;++ix) {
if(ix==1) {
swap(mu1 ,mu2 );
swap(mu12,mu22);
}
pT[ix] = pTmax;
y [ix] = 0.;
bool reject = true;
do {
// generate pT
pT[ix] *= pow(UseRandom::rnd(),1./a);
if(pT[ix]<pTmin_) {
pT[ix] = -GeV;
break;
}
// generate y
y[ix] = -ymax+2.*UseRandom::rnd()*ymax;
// generate phi
phi[ix] = UseRandom::rnd()*Constants::twopi;
// calculate x3 and check in allowed region
x3[ix] = 2.*pT[ix]*cosh(y[ix])/M;
if(x3[ix] < 0. || x3[ix] > 1. -sqr( mu1 + mu2 ) ) continue;
// find the possible solutions for x1
double xT2 = sqr(2./M*pT[ix]);
double root = (-sqr(x3[ix])+xT2)*
(xT2*mu22+2.*x3[ix]-sqr(mu12)+2.*mu22+2.*mu12-sqr(x3[ix])-1.
+2.*mu12*mu22-sqr(mu22)-2.*mu22*x3[ix]-2.*mu12*x3[ix]);
double c1=2.*sqr(x3[ix])-4.*mu22-6.*x3[ix]+4.*mu12-xT2*x3[ix]
+2.*xT2-2.*mu12*x3[ix]+2.*mu22*x3[ix]+4.;
if(root<0.) continue;
x1[ix][0] = 1./(4.-4.*x3[ix]+xT2)*(c1-2.*sqrt(root));
x1[ix][1] = 1./(4.-4.*x3[ix]+xT2)*(c1+2.*sqrt(root));
// change sign of y if 2nd particle emits
if(ix==1) y[ix] *=-1.;
// loop over the solutions
for(unsigned int iy=0;iy<2;++iy) {
contrib[ix][iy]=0.;
// check x1 value allowed
if(x1[ix][iy]<2.*mu1||x1[ix][iy]>1.+mu12-mu22) continue;
// calculate x2 value and check allowed
x2[ix][iy] = 2.-x3[ix]-x1[ix][iy];
double root = max(0.,sqr(x1[ix][iy])-4.*mu12);
root = sqrt(root);
double x2min = 1.+mu22-mu12
-0.5*(1.-x1[ix][iy]+mu12-mu22)/(1.-x1[ix][iy]+mu12)*(x1[ix][iy]-2.*mu12+root);
double x2max = 1.+mu22-mu12
-0.5*(1.-x1[ix][iy]+mu12-mu22)/(1.-x1[ix][iy]+mu12)*(x1[ix][iy]-2.*mu12-root);
if(x2[ix][iy]<x2min||x2[ix][iy]>x2max) continue;
// check the z components
double z1 = sqrt(sqr(x1[ix][iy])-4.*mu12-xT2);
double z2 = -sqrt(sqr(x2[ix][iy])-4.*mu22);
double z3 = pT[ix]*sinh(y[ix])*2./M;
if(ix==1) z3 *=-1.;
if(abs(-z1+z2+z3)<1e-9) z1 *= -1.;
if(abs(z1+z2+z3)>1e-5) continue;
// if using as an ME correction the veto
if(applyVeto) {
// double xb = x1[ix][iy], xc = x2[ix][iy];
// double b = mu12, c = mu22;
// double r = 0.5*(1.+b/(1.+c-xc));
// double z1 = r + (xb-(2.-xc)*r)/sqrt(sqr(xc)-4.*c);
// double kt1 = (1.-b+c-xc)/z1/(1.-z1);
// r = 0.5*(1.+c/(1.+b-xb));
// double z2 = r + (xc-(2.-xb)*r)/sqrt(sqr(xb)-4.*b);
// double kt2 = (1.-c+b-xb)/z2/(1.-z2);
// if(ix==1) {
// swap(z1 ,z2);
// swap(kt1,kt2);
// }
// // veto the shower region
// if( kt1 < d_kt1_ || kt2 < d_kt2_ ) continue;
}
// construct the momenta
realMomenta[ix][iy][4] =
Lorentz5Momentum(pT[ix]*cos(phi[ix]),pT[ix]*sin(phi[ix]),
pT[ix]*sinh(y[ix]) ,pT[ix]*cosh(y[ix]),ZERO);
if(ix==0) {
realMomenta[ix][iy][2] =
Lorentz5Momentum(-pT[ix]*cos(phi[ix]),-pT[ix]*sin(phi[ix]),
z1*0.5*M,x1[ix][iy]*0.5*M,M*mu1);
realMomenta[ix][iy][3] =
Lorentz5Momentum(ZERO,ZERO, z2*0.5*M,x2[ix][iy]*0.5*M,M*mu2);
}
else {
realMomenta[ix][iy][2] =
Lorentz5Momentum(ZERO,ZERO,-z2*0.5*M,x2[ix][iy]*0.5*M,M*mu2);
realMomenta[ix][iy][3] =
Lorentz5Momentum(-pT[ix]*cos(phi[ix]),-pT[ix]*sin(phi[ix]),
-z1*0.5*M,x1[ix][iy]*0.5*M,M*mu1);
}
// boost the momenta back to the lab
for(unsigned int iz=2;iz<5;++iz)
realMomenta[ix][iy][iz] *= eventFrame;
// jacobian and prefactors for the weight
Energy J = M/sqrt(xT2)*abs(-x1[ix][iy]*x2[ix][iy]+2.*mu22*x1[ix][iy]
+x2[ix][iy]+x2[ix][iy]*mu12+mu22*x2[ix][iy]
-sqr(x2[ix][iy]))
/pow(sqr(x2[ix][iy])-4.*mu22,1.5);
// prefactors etc
contrib[ix][iy] = 0.5*pT[ix]/J/preFactor_/lambda;
// matrix element piece
contrib[ix][iy] *= meRatio(partons_,realMomenta[ix][iy],ix,false);
// coupling piece
contrib[ix][iy] *= alphaQED_->ratio(sqr(pT[ix]));
}
if(contrib[ix][0]+contrib[ix][1]>1.) {
ostringstream s;
s << "MEee2gZ2qq::generateHardest weight for channel " << ix
<< "is " << contrib[ix][0]+contrib[ix][1]
<< " which is greater than 1";
generator()->logWarning( Exception(s.str(), Exception::warning) );
}
reject = UseRandom::rnd() > contrib[ix][0] + contrib[ix][1];
}
while (reject);
if(pT[ix]<pTmin_)
pT[ix] = -GeV;
}
// now pick the emmision with highest pT
Energy pTemit(ZERO);
// no emission
if(pT[0]<ZERO&&pT[1]<ZERO) return -GeV;
// which one emitted
if(pT[0]>pT[1]) {
iemit = 2;
ispect = 3;
pTemit = pT[0];
if(UseRandom::rnd()<contrib[0][0]/(contrib[0][0]+contrib[0][1]))
emmision = realMomenta[0][0];
else
emmision = realMomenta[0][1];
}
else {
iemit = 3;
ispect = 2;
pTemit = pT[1];
if(UseRandom::rnd()<contrib[1][0]/(contrib[1][0]+contrib[1][1]))
emmision = realMomenta[1][0];
else
emmision = realMomenta[1][1];
}
// return pT of emmision
return pTemit;
}

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