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DrellYanMECorrection.cc
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// -*- C++ -*-
//
// DrellYanMECorrection.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 DrellYanMECorrection class.
//
#include "DrellYanMECorrection.h"
#include "ThePEG/Repository/UseRandom.h"
#include "ThePEG/PDT/EnumParticles.h"
#include "ThePEG/Repository/CurrentGenerator.h"
#include "ThePEG/Interface/Parameter.h"
#include "ThePEG/Interface/ClassDocumentation.h"
#include "ThePEG/Persistency/PersistentOStream.h"
#include "ThePEG/Persistency/PersistentIStream.h"
#include "Herwig++/Shower/Base/Evolver.h"
#include "Herwig++/Shower/Base/KinematicsReconstructor.h"
#include "Herwig++/Shower/Base/PartnerFinder.h"
using namespace Herwig;
DrellYanMECorrection::DrellYanMECorrection()
: _channelwgtA(0.12), _channelwgtB(2.00), _nover(0), _maxwgt(0.) {}
IBPtr DrellYanMECorrection::clone() const {
return new_ptr(*this);
}
IBPtr DrellYanMECorrection::fullclone() const {
return new_ptr(*this);
}
void DrellYanMECorrection::doinit() {
MECorrectionBase::doinit();
_channelweights.push_back(_channelwgtA/(1.+_channelwgtA));
_channelweights.push_back(_channelweights[0]+1./(1.+_channelwgtA)/(1+_channelwgtB));
_channelweights.push_back(1.0);
}
void DrellYanMECorrection::dofinish() {
MECorrectionBase::dofinish();
if(_nover==0) return;
generator()->log() << "DrellYanMECorrection when applying the hard correction "
<< _nover << " weights larger than one were generated of which"
<< " the largest was " << _maxwgt << "\n";
}
void DrellYanMECorrection::persistentOutput(PersistentOStream & os) const {
os << _channelwgtA << _channelwgtB << _channelweights;
}
void DrellYanMECorrection::persistentInput(PersistentIStream & is, int) {
is >> _channelwgtA >> _channelwgtB >> _channelweights;
}
ClassDescription<DrellYanMECorrection> DrellYanMECorrection::initDrellYanMECorrection;
// Definition of the static class description member.
void DrellYanMECorrection::Init() {
static ClassDocumentation<DrellYanMECorrection> documentation
("The DrellYanMECorrection class implements the soft and hard"
" matrix element correction for the Drell-Yan process. This is"
" a technical parameter for the phase-space generation and "
"should not affect the results only the efficiency and fraction"
" of events with weight > 1.");
static Parameter<DrellYanMECorrection,double> interfaceQQbarChannelWeight
("QQbarChannelWeight",
"The relative weights of the q qbar abd q g channels for selection."
" This is a technical parameter for the phase-space generation and "
"should not affect the results only the efficiency and fraction"
" of events with weight > 1.",
&DrellYanMECorrection::_channelwgtA, 0.12, 0.01, 100.,
false, false, Interface::limited);
static Parameter<DrellYanMECorrection,double> interfaceQGChannelWeight
("QGChannelWeight",
"The relative weights of the qg abd qbar g channels for selection."
" This is a technical parameter for the phase-space generation and "
"should not affect the results only the efficiency and fraction",
&DrellYanMECorrection::_channelwgtB, 2., 0.01, 100.,
false, false, Interface::limited);
}
bool DrellYanMECorrection::canHandle(ShowerTreePtr tree, double & initial,
double & final, EvolverPtr evolver) {
// check on type of radiation
if(!evolver->isISRadiationON()) return false;
// two incoming particles
if(tree->incomingLines().size()!=2) return false;
// should be a quark and an antiquark
unsigned int ix(0);
ShowerParticlePtr part[2];
map<ShowerProgenitorPtr,ShowerParticlePtr>::const_iterator cit;
for(cit=tree->incomingLines().begin();cit!=tree->incomingLines().end();++cit)
{part[ix]=cit->first->progenitor();++ix;}
// check quark and antiquark
if(!(part[0]->id()>0&&part[0]->id()<6&&part[1]->id()<0&&part[1]->id()>-6)&&
!(part[1]->id()>0&&part[1]->id()<6&&part[0]->id()<0&&part[0]->id()>-6))
return false;
// one or two outgoing particles
if(tree->outgoingLines().size()>2) return false;
// find the outgoing particles
ix=0;
map<ShowerProgenitorPtr,tShowerParticlePtr>::const_iterator cjt;
for(cjt=tree->outgoingLines().begin();cjt!=tree->outgoingLines().end();++cjt)
{part[ix]=cjt->first->progenitor();++ix;}
// outgoing particles (1 which is W/Z)
if(tree->outgoingLines().size()==1&&
!(part[0]->id()!=ParticleID::gamma||part[0]->id()!=ParticleID::Z0||
abs(part[0]->id())==ParticleID::Wplus))
return false;
else if(tree->outgoingLines().size()==2)
{
if(part[0]->parents().empty()||part[1]->parents().empty()) return false;
if(part[0]->parents()[0]!=part[1]->parents()[0]) return false;
if(!(part[0]->parents()[0]->id()==ParticleID::gamma||
part[0]->parents()[0]->id()==ParticleID::Z0||
abs(part[0]->parents()[0]->id())==ParticleID::Wplus)) return false;
}
// extract the boson mass and store
if(tree->outgoingLines().size()==1) _mb2=sqr(part[0]->mass());
else _mb2=sqr(part[0]->parents()[0]->mass());
// can handle it
initial = 1.;
final = 1.;
return true;
}
void DrellYanMECorrection::applyHardMatrixElementCorrection(ShowerTreePtr tree) {
assert(tree->outgoingLines().size());
// get the quark,antiquark and the gauge boson
// get the quarks
map<ShowerProgenitorPtr,ShowerParticlePtr>::const_iterator cit;
ShowerParticleVector incoming;
vector<tcBeamPtr> beams;
for(cit=tree->incomingLines().begin();cit!=tree->incomingLines().end();++cit) {
incoming.push_back(cit->first->progenitor());
beams.push_back(cit->first->beam());
}
// get the gauge bosons
PPtr boson;
if(tree->outgoingLines().size()==1) {
boson=tree->outgoingLines().begin()->first->copy();
}
else {
boson=tree->outgoingLines().begin()->first->copy()->parents()[0];
}
// ensure that the quark is first
if(incoming[0]->id()<incoming[1]->id()) {
swap(incoming[0],incoming[1]);
swap(beams[0],beams[1]);
}
// calculate the momenta
unsigned int iemit,itype;
vector<Lorentz5Momentum> pnew;
LorentzRotation trans;
pair<double,double> xnew;
// if not accepted return
if(!applyHard(incoming,beams,boson,iemit,itype,pnew,trans,xnew)) return;
// if applying ME correction create the new particles
if(itype==0) {
// get the momenta of the new particles
Lorentz5Momentum pquark(pnew[0]),panti(pnew[1]),pgluon(pnew[2]);
if(iemit==2) swap(pquark,panti);
// ensure gluon can be put on shell
Lorentz5Momentum ptest(pgluon);
if(ptest.boost(-(pquark+panti).boostVector()).e() <
getParticleData(ParticleID::g)->constituentMass()) return;
// create the new gluon
PPtr newg= getParticleData(ParticleID::g)->produceParticle(pgluon);
PPtr newq,newa;
ColinePtr col;
// make the new particles
if(iemit==1) {
col=incoming[0]->colourLine();
newq = getParticleData(incoming[0]->id())->produceParticle(pquark);
newa = new_ptr(Particle(*incoming[1]));
col->removeAntiColoured(newa);
newa->set5Momentum(panti);
}
else {
col=incoming[1]->antiColourLine();
newa = getParticleData(incoming[1]->id())->produceParticle(panti);
newq = new_ptr(Particle(*incoming[0]));
col->removeColoured(newq);
newq->set5Momentum(pquark);
}
// set the colour lines
ColinePtr newline=new_ptr(ColourLine());
if(iemit==1) {
newline->addColoured(newq);
newline->addColoured(newg);
col->addAntiColoured(newg);
col->addAntiColoured(newa);
}
else {
newline->addAntiColoured(newa);
newline->addAntiColoured(newg);
col->addColoured(newg);
col->addColoured(newq);
}
// change the existing quark and antiquark
PPtr orig;
for(cit=tree->incomingLines().begin();cit!=tree->incomingLines().end();++cit) {
if(cit->first->progenitor()->id()==newq->id()) {
// remove old particles from colour line
col->removeColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newq);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newq,1,false)));
sp->x(xnew.first);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(iemit!=1);
if(iemit==1) orig=cit->first->original();
}
else {
// remove old particles from colour line
col->removeAntiColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newa);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newa,1,false)));
sp->x(xnew.second);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(iemit==1);
if(iemit==2) orig=cit->first->original();
}
}
// fix the momentum of the gauge boson
map<ShowerProgenitorPtr,tShowerParticlePtr>::const_iterator
cjt=tree->outgoingLines().begin();
Boost boostv=cjt->first->progenitor()->momentum().findBoostToCM();
trans *=LorentzRotation(boostv);
cjt->first->progenitor()->transform(trans);
cjt->first->copy()->transform(trans);
tree->hardMatrixElementCorrection(true);
// add the gluon
ShowerParticlePtr sg=new_ptr(ShowerParticle(*newg,1,true));
ShowerProgenitorPtr gluon=new_ptr(ShowerProgenitor(orig,newg,sg));
gluon->perturbative(false);
tree->outgoingLines().insert(make_pair(gluon,sg));
}
else if(itype==1) {
Lorentz5Momentum pin(pnew[0]),pout(pnew[1]),pgluon(pnew[2]);
if(iemit==2) swap(pin,pout);
// ensure outgoing quark can be put on-shell
Lorentz5Momentum ptest(pout);
if(ptest.boost(-(pin+pgluon).boostVector()).e() <
incoming[1]->dataPtr()->constituentMass()) return;
// create the new gluon
PPtr newg = getParticleData(ParticleID::g)->produceParticle(pgluon);
// create the new outgoing quark
PPtr newout= getParticleData(-incoming[1]->id())->produceParticle(pout);
// create the new incoming quark
PPtr newin = new_ptr(Particle(*incoming[0]));
newin->set5Momentum(pin);
// colour info
ColinePtr col=incoming[0]->colourLine();
col->removeColoured(newin);
ColinePtr newline=new_ptr(ColourLine());
newline->addColoured(newout);
newline->addColoured(newg);
col->addAntiColoured(newg);
col->addColoured(newin);
// change the existing incoming partons
PPtr orig;
for(cit=tree->incomingLines().begin();cit!=tree->incomingLines().end();++cit) {
if(cit->first->progenitor()->id()==newin->id()) {
// remove old particles from colour line
col->removeColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newin);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newin,1,false)));
sp->x(xnew.first);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(true);
}
else {
// remove old particles from colour line
col->removeAntiColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newg);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newg,1,false)));
sp->x(xnew.second);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(false);
orig=cit->first->original();
}
}
// fix the momentum of the gauge boson
map<ShowerProgenitorPtr,tShowerParticlePtr>::const_iterator
cjt=tree->outgoingLines().begin();
Boost boostv=cjt->first->progenitor()->momentum().findBoostToCM();
trans *=LorentzRotation(boostv);
cjt->first->progenitor()->transform(trans);
cjt->first->copy()->transform(trans);
tree->hardMatrixElementCorrection(true);
// add the outgoing quark
ShowerParticlePtr sout=new_ptr(ShowerParticle(*newout,1,true));
ShowerProgenitorPtr out=new_ptr(ShowerProgenitor(orig,newout,sout));
out->perturbative(false);
tree->outgoingLines().insert(make_pair(out,sout));
}
else if(itype==2) {
Lorentz5Momentum pin(pnew[0]),pout(pnew[1]),pgluon(pnew[2]);
if(iemit==2) swap(pin,pout);
// ensure outgoing antiquark can be put on-shell
Lorentz5Momentum ptest(pout);
if(ptest.boost(-(pin+pgluon).boostVector()).e() <
incoming[0]->dataPtr()->constituentMass()) return;
// create the new gluon
PPtr newg = getParticleData(ParticleID::g)->produceParticle(pgluon);
// create the new outgoing antiquark
PPtr newout= getParticleData(-incoming[0]->id())->produceParticle(pout);
// create the new incoming antiquark
PPtr newin = new_ptr(Particle(*incoming[1]));
newin->set5Momentum(pin);
// colour info
ColinePtr col=incoming[0]->colourLine();
col->removeAntiColoured(newin);
ColinePtr newline=new_ptr(ColourLine());
newline->addAntiColoured(newout);
newline->addAntiColoured(newg);
col->addColoured(newg);
col->addAntiColoured(newin);
// change the existing incoming partons
PPtr orig;
for(cit=tree->incomingLines().begin();cit!=tree->incomingLines().end();++cit) {
if(cit->first->progenitor()->id()==newin->id()) {
// remove old particles from colour line
col->removeAntiColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newin);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newin,1,false)));
sp->x(xnew.second);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(true);
}
else {
// remove old particles from colour line
col->removeColoured(cit->first->copy());
col->removeColoured(cit->first->progenitor());
// insert new particles
cit->first->copy(newg);
ShowerParticlePtr sp(new_ptr(ShowerParticle(*newg,1,false)));
sp->x(xnew.first);
cit->first->progenitor(sp);
tree->incomingLines()[cit->first]=sp;
cit->first->perturbative(false);
orig=cit->first->original();
}
}
// fix the momentum of the gauge boson
map<ShowerProgenitorPtr,tShowerParticlePtr>::const_iterator
cjt=tree->outgoingLines().begin();
Boost boostv=cjt->first->progenitor()->momentum().findBoostToCM();
trans *=LorentzRotation(boostv);
cjt->first->progenitor()->transform(trans);
cjt->first->copy()->transform(trans);
tree->hardMatrixElementCorrection(true);
// add the outgoing antiquark
ShowerParticlePtr sout=new_ptr(ShowerParticle(*newout,1,true));
ShowerProgenitorPtr out=new_ptr(ShowerProgenitor(orig,newout,sout));
out->perturbative(false);
tree->outgoingLines().insert(make_pair(out,sout));
}
}
bool DrellYanMECorrection::softMatrixElementVeto(ShowerProgenitorPtr initial,
ShowerParticlePtr parent,Branching br) {
if(parent->isFinalState()) return false;
// check if me correction should be applied
long id[2]={initial->id(),parent->id()};
if(id[0]!=id[1]||id[1]==ParticleID::g) return false;
// get the pT
Energy pT=br.kinematics->pT();
// check if hardest so far
if(pT<initial->highestpT()) return false;
// compute the invariants
double kappa(sqr(br.kinematics->scale())/_mb2),z(br.kinematics->z());
Energy2 shat(_mb2/z*(1.+(1.-z)*kappa)),that(-(1.-z)*kappa*_mb2),uhat(-(1.-z)*shat);
// check which type of process
// g qbar
double wgt(1.);
if(id[0]>0&&br.ids[0]==ParticleID::g)
wgt=_mb2/(shat+uhat)*(sqr(_mb2-that)+sqr(_mb2-shat))/(sqr(shat+uhat)+sqr(uhat));
else if(id[0]>0&&br.ids[0]==id[0])
wgt=_mb2/(shat+uhat)*(sqr(_mb2-uhat)+sqr(_mb2-that))/(sqr(shat)+sqr(shat+uhat));
else if(id[0]<0&&br.ids[0]==ParticleID::g)
wgt=_mb2/(shat+uhat)*(sqr(_mb2-that)+sqr(_mb2-shat))/(sqr(shat+uhat)+sqr(uhat));
else if(id[0]<0&&br.ids[0]==-id[0])
wgt=_mb2/(shat+uhat)*(sqr(_mb2-uhat)+sqr(_mb2-that))/(sqr(shat)+sqr(shat+uhat));
else return false;
if(wgt<.0||wgt>1.) generator()->log() << "Soft ME correction weight too large or "
<< "negative in DrellYanMECorrection::"
<< "softMatrixElementVeto()soft weight "
<< " sbar = " << shat/_mb2
<< " tbar = " << that/_mb2
<< "weight = " << wgt << "\n";
// if not vetoed
if(UseRandom::rndbool(wgt)) {
initial->highestpT(pT);
return false;
}
// otherwise
parent->setEvolutionScale(br.kinematics->scale());
return true;
}
bool DrellYanMECorrection::applyHard(ShowerParticleVector quarks,
vector<tcBeamPtr> beams,PPtr boson,
unsigned int & iemit,unsigned int & itype,
vector<Lorentz5Momentum> & pnew,
LorentzRotation & trans,
pair<double,double> & xout) {
// check that quark along +z and qbar along -z
bool quarkplus=quarks[0]->momentum().z()>quarks[1]->momentum().z();
// calculate the limits on s
Energy mb(boson->mass());
_mb2=sqr(mb);
Energy2 smin=_mb2;
Energy2 s=
(CurrentGenerator::current().currentEvent()->incoming().first->momentum()+
CurrentGenerator::current().currentEvent()->incoming().second->momentum()).m2();
Energy2 smax(s);
// calculate the rapidity of the boson
double yB=0.5*log((boson->momentum().e()+boson->momentum().z())/
(boson->momentum().e()-boson->momentum().z()));
if(!quarkplus) yB=-yB;
// if no phase-space return
if(smax<smin) return false;
// get the evolution scales (this needs improving)
double kappa[2]={1.,1.};
// get the momentum fractions for the leading order process
// and the values of the PDF's
double x[2],fx[2];
tcPDFPtr pdf[2];
for(unsigned int ix=0;ix<quarks.size();++ix) {
x[ix]=quarks[ix]->x();
assert(beams[ix]);
pdf[ix]=beams[ix]->pdf();
assert(pdf[ix]);
fx[ix]=pdf[ix]->xfx(beams[ix],quarks[ix]->dataPtr(),_mb2,x[ix]);
}
// select the type of process and generate the kinematics
double rn(UseRandom::rnd());
Energy2 shat(ZERO),uhat(ZERO),that(ZERO);
double weight(0.),xnew[2]={1.,1.};
// generate the value of s according to 1/s^2
shat = smax*smin/(smin+UseRandom::rnd()*(smax-smin));
Energy2 jacobian = sqr(shat)*(1./smin-1./smax);
double sbar=shat/_mb2;
// calculate limits on that
Energy2 tmax=_mb2*kappa[0]*(1.-sbar)/(kappa[0]+sbar);
Energy2 tmin=shat*(1.-sbar)/(kappa[1]+sbar);
// calculate the limits on uhat
Energy2 umax(_mb2-shat-tmin),umin(_mb2-shat-tmax);
// check inside phase space
if(tmax<tmin||umax<umin) return false;
// q qbar -> g V
if(rn<_channelweights[0]) {
// generate t and u according to 1/t+1/u
// generate in 1/t
if(UseRandom::rndbool(0.5)) {
that=tmax*pow(tmin/tmax,UseRandom::rnd());
uhat=_mb2-shat-that;
jacobian *=log(tmin/tmax);
}
// generate in 1/u
else {
uhat=umax*pow(umin/umax,UseRandom::rnd());
that=_mb2-shat-uhat;
jacobian *=log(umin/umax);
}
Energy4 jacobian2 = jacobian * 2.*uhat*that/(shat-_mb2);
// new scale (this is mt^2=pt^2+mb^2)
Energy2 scale(uhat*that/shat+_mb2);
// the PDF's with the emitted gluon
double fxnew[2];
xnew[0]=exp(yB)/sqrt(s)*sqrt(shat*(_mb2-uhat)/(_mb2-that));
xnew[1]=shat/(s*xnew[0]);
for(unsigned int ix=0;ix<2;++ix)
{fxnew[ix]=pdf[ix]->xfx(beams[ix],quarks[ix]->dataPtr(),scale,xnew[ix]);}
// jacobian and me parts of the weight
weight=jacobian2*(sqr(_mb2-that)+sqr(_mb2-uhat))/(sqr(shat)*that*uhat);
// pdf part of the weight
weight *=fxnew[0]*fxnew[1]*x[0]*x[1]/(fx[0]*fx[1]*xnew[0]*xnew[1]);
// finally coupling, colour factor and different channel pieces
weight *= 2./3./Constants::pi/_channelweights[0]*coupling()->value(scale);
// select the emiting particle
iemit=1;
if(UseRandom::rnd()<sqr(_mb2-uhat)/(sqr(_mb2-uhat)+sqr(_mb2-that))) iemit=2;
itype=0;
}
// incoming gluon
else {
// generate t
if(rn>_channelweights[1]) {
swap(tmax,tmin);
tmax=_mb2-shat-tmax;
tmin=_mb2-shat-tmin;
}
that=tmax*pow(tmin/tmax,UseRandom::rnd());
uhat=_mb2-shat-that;
Energy4 jacobian2 = jacobian * that*log(tmax/tmin);
// new scale (this is mt^2=pt^2+mb^2)
Energy2 scale(uhat*that/shat+_mb2);
// g qbar -> qbar V
double fxnew[2];
if(rn<_channelweights[1]) {
itype=2;
xnew[0]=exp(yB)/sqrt(s)*sqrt(shat*(_mb2-uhat)/(_mb2-that));
xnew[1]=shat/(s*xnew[0]);
fxnew[0]=pdf[0]->xfx(beams[0],getParticleData(ParticleID::g),scale,xnew[0]);
fxnew[1]=pdf[1]->xfx(beams[1],quarks[1]->dataPtr(),scale,xnew[1]);
jacobian2/=(_channelweights[1]-_channelweights[0]);
}
// q g -> q V
else {
itype=1;
xnew[0]=exp(yB)/sqrt(s)*sqrt(shat*(_mb2-that)/(_mb2-uhat));
xnew[1]=shat/(s*xnew[0]);
fxnew[0]=pdf[0]->xfx(beams[0],quarks[0]->dataPtr(),scale,xnew[0]);
fxnew[1]=pdf[1]->xfx(beams[1],getParticleData(ParticleID::g),scale,xnew[1]);
jacobian2/=(_channelweights[2]-_channelweights[1]);
}
// jacobian and me parts of the weight
weight=-jacobian2*(sqr(_mb2-that)+sqr(_mb2-shat))/(sqr(shat)*shat*that);
// pdf part of the weight
weight *=fxnew[0]*fxnew[1]*x[0]*x[1]/(fx[0]*fx[1]*xnew[0]*xnew[1]);
// finally coupling, colour factor and different channel pieces
weight *= 0.25/Constants::pi*coupling()->value(scale);
// select the emiting particle
iemit=1;
if(UseRandom::rnd()<sqr(_mb2-that)/(sqr(_mb2-that)+sqr(_mb2-shat))) iemit=2;
}
// if me correction should be applied
if(weight>1.) {
++_nover;
_maxwgt=max(_maxwgt,weight);
weight=1.;
}
if(UseRandom::rnd()>weight) return false;
// construct the momenta in the rest frame of the boson
Lorentz5Momentum pb(ZERO,ZERO,ZERO,mb,mb),pspect,pg,pemit;
double cos3 = 0.0;
if(itype==0)
{
pg = Lorentz5Momentum(ZERO,ZERO,ZERO,0.5*(shat-_mb2)/mb,ZERO);
Energy2 tp(that),up(uhat);
double zsign(-1.);
if(iemit==2)
{
tp=uhat;
up=that;
zsign=1.;
}
pspect = Lorentz5Momentum(ZERO,ZERO,zsign*0.5*(_mb2-tp)/mb,
0.5*(_mb2-tp)/mb,ZERO);
Energy eemit=0.5*(_mb2-up)/mb;
cos3 = 0.5/pspect.z()/pg.e()*(sqr(pspect.e())+sqr(pg.e())-sqr(eemit));
}
else
{
pg=Lorentz5Momentum(ZERO,ZERO,ZERO,0.5*(_mb2-uhat)/mb,ZERO);
double zsign(1.);
if(iemit==1)
{
if(itype==1) zsign=-1.;
pspect=Lorentz5Momentum(ZERO,ZERO,0.5*zsign*(shat-_mb2)/mb,
0.5*(shat-_mb2)/mb);
Energy eemit=0.5*(_mb2-that)/mb;
cos3 = 0.5/pspect.z()/pg.e()*(sqr(pspect.e())+sqr(pg.e())-sqr(eemit));
}
else
{
if(itype==2) zsign=-1.;
pspect=Lorentz5Momentum(ZERO,ZERO,0.5*zsign*(_mb2-that)/mb,
0.5*(_mb2-that)/mb);
Energy eemit=0.5*(shat-_mb2)/mb;
cos3 = 0.5/pspect.z()/pg.e()*(-sqr(pspect.e())-sqr(pg.e())+sqr(eemit));
}
}
// rotate the gluon
double sin3(sqrt(1.-sqr(cos3)));
double phi(Constants::twopi*UseRandom::rnd());
pg.setX(pg.e()*sin3*cos(phi));
pg.setY(pg.e()*sin3*sin(phi));
pg.setZ(pg.e()*cos3);
if(itype==0) {
pemit=pb+pg-pspect;
}
else {
if(iemit==1) pemit=pb+pspect-pg;
else pemit=pspect+pg-pb;
}
pemit.rescaleMass();
// find the new CMF
Lorentz5Momentum pp[2];
if(itype==0) {
if(iemit==1) {
pp[0]=pemit;
pp[1]=pspect;
}
else {
pp[0]=pspect;
pp[1]=pemit;
}
}
else if(itype==1) {
pp[1]=pg;
if(iemit==1) pp[0]=pemit;
else pp[0]=pspect;
}
else {
pp[0]=pg;
if(iemit==1) pp[1]=pemit;
else pp[1]=pspect;
}
Lorentz5Momentum pz= quarkplus ? pp[0] : pp[1];
pp[0]/=xnew[0];
pp[1]/=xnew[1];
Lorentz5Momentum plab(pp[0]+pp[1]);
plab.rescaleMass();
// construct the boost to rest frame of plab
trans=LorentzRotation(plab.findBoostToCM());
pz.transform(trans);
// rotate so emitting particle along z axis
// rotate so in x-z plane
trans.rotateZ(-atan2(pz.y(),pz.x()));
// rotate so along
trans.rotateY(-acos(pz.z()/pz.vect().mag()));
// undo azimuthal rotation
trans.rotateZ(atan2(pz.y(),pz.x()));
// perform the transforms
pb .transform(trans);
pspect.transform(trans);
pg .transform(trans);
pemit .transform(trans);
// momenta to be returned
pnew.push_back(pemit);
pnew.push_back(pspect);
pnew.push_back(pg);
pnew.push_back(pb);
xout.first=xnew[0];
xout.second=xnew[1];
return true;
}

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