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diff --git a/FixedOrderGen/src/PhaseSpacePoint.cc b/FixedOrderGen/src/PhaseSpacePoint.cc
index d5d80d5..0246290 100644
--- a/FixedOrderGen/src/PhaseSpacePoint.cc
+++ b/FixedOrderGen/src/PhaseSpacePoint.cc
@@ -1,742 +1,777 @@
/**
* \authors The HEJ collaboration (see AUTHORS for details)
* \date 2019
* \copyright GPLv2 or later
*/
#include "PhaseSpacePoint.hh"
#include <algorithm>
#include "CLHEP/Vector/LorentzVector.h"
#include "HEJ/Constants.hh"
#include "HEJ/exceptions.hh"
#include "HEJ/kinematics.hh"
#include "HEJ/Particle.hh"
#include "HEJ/utility.hh"
#include "Process.hh"
#include "Subleading.hh"
using namespace HEJ;
namespace HEJFOG{
static_assert(
std::numeric_limits<double>::has_quiet_NaN,
"no quiet NaN for double"
);
constexpr double NaN = std::numeric_limits<double>::quiet_NaN();
HEJ::Event::EventData to_EventData(PhaseSpacePoint const & psp){
HEJ::Event::EventData result;
result.incoming = psp.incoming();
assert(result.incoming.size() == 2);
result.outgoing=psp.outgoing();
// technically Event::EventData doesn't have to be sorted,
// but PhaseSpacePoint should be anyway
assert(
std::is_sorted(
begin(result.outgoing), end(result.outgoing),
HEJ::rapidity_less{}
)
);
assert(result.outgoing.size() >= 2);
result.decays=psp.decays();
result.parameters.central= {NaN, NaN, psp.weight() };
return result;
}
namespace{
bool can_swap_to_uno(
HEJ::Particle const & p1, HEJ::Particle const & p2
){
return is_parton(p1)
&& p1.type != pid::gluon
&& p2.type == pid::gluon;
}
size_t count_gluons(std::vector<Particle>::const_iterator first,
std::vector<Particle>::const_iterator last){
return std::count_if(first, last, [](Particle const & p)
{return p.type == pid::gluon;});
}
/** assumes FKL configurations between first and last,
* else there can be a quark in a non-extreme position
* e.g. uno configuration gqg would pass
*/
bool can_change_to_qqx(
std::vector<Particle>::const_iterator first,
std::vector<Particle>::const_iterator last){
return 1 < count_gluons(first,last);
}
bool is_AWZ_proccess(Process const & proc){
return proc.boson && is_AWZ_boson(*proc.boson);
}
bool is_up_type(Particle const & part){
return HEJ::is_anyquark(part) && !(abs(part.type)%2);
}
bool is_down_type(Particle const & part){
return HEJ::is_anyquark(part) && abs(part.type)%2;
}
bool can_couple_to_W(Particle const & part, pid::ParticleID const W_id){
const int W_charge = W_id>0?1:-1;
return abs(part.type)<5
&& ( (W_charge*part.type > 0 && is_up_type(part))
|| (W_charge*part.type < 0 && is_down_type(part)) );
}
}
void PhaseSpacePoint::maybe_turn_to_subl(
double chance,
unsigned int const channels,
Process const & proc,
HEJ::RNG & ran
){
if(proc.njets <= 2) return;
assert(outgoing_.size() >= 2);
// decide what kind of subleading process is allowed
bool allow_uno = false;
bool allow_strange = true;
const size_t nout = outgoing_.size();
const bool can_be_uno_backward = (channels&Subleading::uno)
&& can_swap_to_uno(outgoing_[0], outgoing_[1]);
const bool can_be_uno_forward = (channels&Subleading::uno)
&& can_swap_to_uno(outgoing_[nout-1], outgoing_[nout-2]);
allow_uno = can_be_uno_backward || can_be_uno_forward;
bool allow_qqx = false;
if(is_AWZ_proccess(proc)) {
allow_qqx = (channels&Subleading::qqx)
&& can_change_to_qqx(outgoing_.cbegin(), outgoing_.cend());
if(std::none_of(outgoing_.cbegin(), outgoing_.cend(),
[&proc](Particle const & p){ return can_couple_to_W(p, *proc.boson);})) {
// enforce qqx if A/W/Z can't couple somewhere else
assert(allow_qqx);
allow_uno = false;
chance = 1.;
// strange not allowed for W
if(abs(*proc.boson)== pid::Wp) allow_strange = false;
}
}
if(!allow_uno && !allow_qqx) return;
if(ran.flat() < chance){
weight_ /= chance;
if(allow_uno && !allow_qqx){
turn_to_uno(can_be_uno_backward, can_be_uno_forward, ran);
} else if (!allow_uno && allow_qqx) {
turn_to_qqx(allow_strange, ran);
} else {
assert( allow_uno && allow_qqx);
if(ran.flat() < 0.5) turn_to_uno(can_be_uno_backward, can_be_uno_forward, ran);
else turn_to_qqx(allow_strange, ran);
weight_ *= 2.;
}
} else weight_ /= 1 - chance;
}
void PhaseSpacePoint::turn_to_uno(
const bool can_be_uno_backward, const bool can_be_uno_forward,
HEJ::RNG & ran
){
if(!can_be_uno_backward && !can_be_uno_forward) return;
const size_t nout = outgoing_.size();
if(can_be_uno_backward && can_be_uno_forward){
if(ran.flat() < 0.5){
std::swap(outgoing_[0].type, outgoing_[1].type);
} else {
std::swap(outgoing_[nout-1].type, outgoing_[nout-2].type);
}
weight_ *= 2.;
} else if(can_be_uno_backward){
std::swap(outgoing_[0].type, outgoing_[1].type);
} else {
assert(can_be_uno_forward);
std::swap(outgoing_[nout-1].type, outgoing_[nout-2].type);
}
}
void PhaseSpacePoint::turn_to_qqx(const bool allow_strange, HEJ::RNG & ran){
/// find first and last gluon in FKL chain
auto first = std::find_if(outgoing_.begin(), outgoing_.end(),
[](Particle const & p){return p.type == pid::gluon;});
std::vector<Particle*> FKL_gluons;
for(auto p = first; p!=outgoing_.end(); ++p){
if(p->type == pid::gluon) FKL_gluons.push_back(&*p);
else if(is_anyquark(*p)) break;
}
const size_t ng = FKL_gluons.size();
if(ng < 2)
throw std::logic_error("not enough gluons to create qqx");
// select flavour of quark
const double r1 = 2.*ran.flat()-1.;
const double max_flavour = allow_strange?n_f:n_f-1;
weight_ *= max_flavour*2;
int flavour = pid::down + std::floor(std::abs(r1)*max_flavour);
flavour*=r1<0.?-1:1;
// select gluon for switch
const size_t idx = floor((ng-1) * ran.flat());
weight_ *= (ng-1);
FKL_gluons[idx]->type = ParticleID(flavour);
FKL_gluons[idx+1]->type = ParticleID(-flavour);
}
template<class ParticleMomenta>
fastjet::PseudoJet PhaseSpacePoint::gen_last_momentum(
ParticleMomenta const & other_momenta,
const double mass_square, const double y
) const {
std::array<double,2> pt{0.,0.};
for (auto const & p: other_momenta) {
pt[0]-= p.px();
pt[1]-= p.py();
}
const double mperp = sqrt(pt[0]*pt[0]+pt[1]*pt[1]+mass_square);
const double pz=mperp*sinh(y);
const double E=mperp*cosh(y);
return {pt[0], pt[1], pz, E};
}
namespace {
//! adds a particle to target (in correct rapidity ordering)
//! @returns positon of insertion
auto insert_particle(std::vector<HEJ::Particle> & target,
HEJ::Particle && particle
){
const auto pos = std::upper_bound(
begin(target),end(target),particle,rapidity_less{}
);
target.insert(pos, std::move(particle));
return pos;
}
}
+ namespace {
+ Particle reconstruct_boson(std::vector<Particle> const & leptons) {
+ HEJ::Particle decayed_boson;
+ decayed_boson.p = leptons[0].p + leptons[1].p;
+ const int pidsum = leptons[0].type + leptons[1].type;
+ if(pidsum == +1) {
+ assert(is_antilepton(leptons[0]));
+ if(is_antineutrino(leptons[0])) {
+ throw HEJ::not_implemented{"lepton-flavour violating final state"};
+ }
+ assert(is_neutrino(leptons[1]));
+ // charged antilepton + neutrino means we had a W+
+ decayed_boson.type = HEJ::pid::Wp;
+ }
+ else if(pidsum == -1) {
+ assert(is_antilepton(leptons[0]));
+ if(is_neutrino(leptons[1])) {
+ throw HEJ::not_implemented{"lepton-flavour violating final state"};
+ }
+ assert(is_antineutrino(leptons[0]));
+ // charged lepton + antineutrino means we had a W-
+ decayed_boson.type = HEJ::pid::Wm;
+ }
+ else {
+ throw HEJ::not_implemented{
+ "final state with leptons "
+ + HEJ::name(leptons[0].type)
+ + " and "
+ + HEJ::name(leptons[1].type)
+ };
+ }
+ return decayed_boson;
+ }
+ }
+
PhaseSpacePoint::PhaseSpacePoint(
- Process const & proc,
+ Process const & proc_in,
JetParameters const & jet_param,
HEJ::PDF & pdf, double E_beam,
double const subl_chance,
unsigned int const subl_channels,
ParticlesPropMap const & particles_properties,
HEJ::RNG & ran
)
{
+ auto proc{ proc_in };
assert(proc.njets >= 2);
if(proc.boson
&& particles_properties.find(*(proc.boson))
== particles_properties.end())
throw HEJ::missing_option("Boson "
+std::to_string(*(proc.boson))+" can't be generated: missing properties");
status_ = good;
weight_ = 1;
const int nout = proc.njets + (proc.boson?1:0) + proc.leptons.size();
outgoing_.reserve(nout);
// generate parton momenta
const bool is_pure_jets = (nout == proc.njets);
auto partons = gen_LO_partons(
proc.njets, is_pure_jets, jet_param, E_beam, ran
);
// pre fill flavour with gluons
for(auto&& p_out: partons) {
outgoing_.emplace_back(Particle{pid::gluon, std::move(p_out), {}});
}
if(status_ != good) return;
// create phase space point for (lepton,neutrino)-pair of W+/W-
// @TODO this have massive overlap with the "create boson" section
if(proc.leptons.size()==2){
- auto pair(gen_enu(proc.leptons,ran));
- for(auto&& p: pair)
- insert_particle(outgoing_, std::move(p));
- }
-
- // create boson
- if(proc.boson){
+ auto leptons(gen_enu(proc.leptons,ran));
+ const auto pos{insert_particle(outgoing_, reconstruct_boson(leptons))};
+ const size_t boson_idx = std::distance(begin(outgoing_), pos);
+ decays_.emplace(boson_idx, std::move(leptons));
+ proc.boson = pos->type;
+ } else if(proc.boson){ // decay boson
const auto & boson_prop = particles_properties.at(*proc.boson);
auto boson(gen_boson(*proc.boson, boson_prop.mass, boson_prop.width, ran));
const auto pos{insert_particle(outgoing_, std::move(boson))};
if(! boson_prop.decays.empty()){
const size_t boson_idx = std::distance(begin(outgoing_), pos);
decays_.emplace(
boson_idx,
decay_boson(outgoing_[boson_idx], boson_prop.decays, ran)
);
}
}
// normalisation of momentum-conserving delta function
weight_ *= pow(2*M_PI, 4);
/** @TODO
* uf (jet_param.min_pt) doesn't correspond to our final scale choice.
* The HEJ scale generators currently expect a full event as input,
* so fixing this is not completely trivial
*/
reconstruct_incoming(proc, subl_channels, pdf, E_beam, jet_param.min_pt, ran);
if(status_ != good) return;
// set outgoing states
most_backward_FKL(outgoing_).type = incoming_[0].type;
most_forward_FKL(outgoing_).type = incoming_[1].type;
maybe_turn_to_subl(subl_chance, subl_channels, proc, ran);
if(proc.boson) couple_boson(*proc.boson, ran);
}
double PhaseSpacePoint::gen_hard_pt(
int np , double ptmin, double ptmax, double y,
HEJ::RNG & ran
) {
// heuristic parameters for pt sampling
const double ptpar = ptmin + np/5.;
const double arg_small_y = atan((ptmax - ptmin)/ptpar);
const double y_cut = 3.;
const double r1 = ran.flat();
if(y < y_cut){
const double pt = ptmin + ptpar*tan(r1*arg_small_y);
const double temp = cos(r1*arg_small_y);
weight_ *= pt*ptpar*arg_small_y/(temp*temp);
return pt;
}
const double ptpar2 = ptpar/(1 + 5*(y-y_cut));
const double temp = 1. - std::exp((ptmin-ptmax)/ptpar2);
const double pt = ptmin - ptpar2*std::log(1-r1*temp);
weight_ *= pt*ptpar2*temp/(1-r1*temp);
return pt;
}
double PhaseSpacePoint::gen_soft_pt(int np, double max_pt, HEJ::RNG & ran) {
constexpr double ptpar = 4.;
const double r = ran.flat();
const double pt = max_pt + ptpar/np*std::log(r);
weight_ *= pt*ptpar/(np*r);
return pt;
}
double PhaseSpacePoint::gen_parton_pt(
int count, JetParameters const & jet_param, double max_pt, double y,
HEJ::RNG & ran
) {
constexpr double p_small_pt = 0.02;
if(! jet_param.peak_pt) {
return gen_hard_pt(count, jet_param.min_pt, max_pt, y, ran);
}
const double r = ran.flat();
if(r > p_small_pt) {
weight_ /= 1. - p_small_pt;
return gen_hard_pt(count, *jet_param.peak_pt, max_pt, y, ran);
}
weight_ /= p_small_pt;
const double pt = gen_soft_pt(count, *jet_param.peak_pt, ran);
if(pt < jet_param.min_pt) {
weight_=0.0;
status_ = not_enough_jets;
return jet_param.min_pt;
}
return pt;
}
std::vector<fastjet::PseudoJet> PhaseSpacePoint::gen_LO_partons(
int np, bool is_pure_jets,
JetParameters const & jet_param,
double max_pt,
HEJ::RNG & ran
){
if (np<2) throw std::invalid_argument{"Not enough partons in gen_LO_partons"};
weight_ /= pow(16.*pow(M_PI,3),np);
weight_ /= std::tgamma(np+1); //remove rapidity ordering
std::vector<fastjet::PseudoJet> partons;
partons.reserve(np);
for(int i = 0; i < np; ++i){
const double y = -jet_param.max_y + 2*jet_param.max_y*ran.flat();
weight_ *= 2*jet_param.max_y;
const bool is_last_parton = i+1 == np;
if(is_pure_jets && is_last_parton) {
constexpr double parton_mass_sq = 0.;
partons.emplace_back(gen_last_momentum(partons, parton_mass_sq, y));
break;
}
const double phi = 2*M_PI*ran.flat();
weight_ *= 2.0*M_PI;
const double pt = gen_parton_pt(np, jet_param, max_pt, y, ran);
if(weight_ == 0.0) return {};
partons.emplace_back(fastjet::PtYPhiM(pt, y, phi));
assert(jet_param.min_pt <= partons[i].pt());
assert(partons[i].pt() <= max_pt+1e-5);
}
// Need to check that at LO, the number of jets = number of partons;
fastjet::ClusterSequence cs(partons, jet_param.def);
auto cluster_jets=cs.inclusive_jets(jet_param.min_pt);
if (cluster_jets.size()!=unsigned(np)){
weight_=0.0;
status_ = not_enough_jets;
return {};
}
std::sort(begin(partons), end(partons), rapidity_less{});
return partons;
}
std::vector<Particle> PhaseSpacePoint::gen_enu(
std::vector<HEJ::pid::ParticleID> const & pair, HEJ::RNG & ran
){
assert(pair.size() == 2);
// Now we know the transverse momentum of the W, given by the array kt
// double pW[4];
// Choose its mass according to Breit-Wigner
double r1=ran.flat();
double sW=HEJ::MW*(HEJ::MW + HEJ::GammaW*tan((M_PI*r1)/2. + (-1. + r1)*atan(HEJ::MW/HEJ::GammaW)));
// Multiply by derivate (d sap)/(d r)
{
static double temp=atan(HEJ::MW/HEJ::GammaW);
weight_*=(HEJ::GammaW*HEJ::MW*(M_PI+2.*temp))/(1.+cos(M_PI*r1+2.*(-1.+r1)*temp));
}
// Generate a yW Gaussian distributed around 0
double yW;
{
double lninvr1,r1,r2,temp,a;
r1=ran.flat();
r2=ran.flat();
lninvr1=-log(r1);
a=0.7; // tuned number
temp=a*sqrt(2.*lninvr1)*cos(2.*M_PI*r2);
yW=temp;
weight_=weight_*(exp(temp*temp/2./a/a))*sqrt(2.*M_PI)*a;
}
auto p = gen_last_momentum(outgoing_, sW, yW);
CLHEP::HepLorentzVector pWv(p.px(),p.py(),p.pz(),p.e());
double ppW[4],ppWs;
CLHEP::HepLorentzVector ppWv,appWv,pd1,pd2;
ppWs=sqrt(sW)/2.;
// Choose theta and phi
double pptheta=2.*M_PI*ran.flat();
double cosppphi=2.*ran.flat()-1.;
weight_*=2.*M_PI*2.;
double sinphi=sqrt(1.-cosppphi*cosppphi); // Know 0 < phi < pi
weight_*=1./(pow(2.*M_PI,3)*2.)/(4.); // Divide by 8, *2, see WPhaseSpace.tex @TODO where?!?
// (phi -> cosphi now so no sinphi in J)
// construct 4-vector in W rest frame
ppW[0]=ppWs;
ppW[1]=ppWs*cos(pptheta)*sinphi;
ppW[2]=ppWs*sin(pptheta)*sinphi;
ppW[3]=ppWs*cosppphi;
ppWv.set(ppW[1],ppW[2],ppW[3],ppW[0]);
appWv.set(-ppW[1],-ppW[2],-ppW[3],ppW[0]);
// translate to lab frame
pd1=ppWv.boost(pWv.boostVector()); //particle
pd2=appWv.boost(pWv.boostVector()); //anti-particle
// assign particle and anti-particle momentum
// find the particle, assuming the pair is ordered (anti-lepton,lepton)
HEJ::Particle alepton{pair[0],
fastjet::PseudoJet{pd2.px(),pd2.py(),pd2.pz(),pd2.e()}, {}};
HEJ::Particle lepton{pair[1],
fastjet::PseudoJet{pd1.px(),pd1.py(),pd1.pz(),pd1.e()}, {}};
return std::vector<HEJ::Particle>{alepton,lepton};
}
Particle PhaseSpacePoint::gen_boson(
HEJ::ParticleID bosonid, double mass, double width,
HEJ::RNG & ran
){
// Usual phase space measure
weight_ /= 16.*pow(M_PI, 3);
// Generate a y Gaussian distributed around 0
/// @TODO: magic number only for Higgs
/// @TODO better sampling for W
const double y = random_normal(1.6, ran);
const double r1 = ran.flat();
const double sH = mass*(
mass + width*tan(M_PI/2.*r1 + (r1-1.)*atan(mass/width))
);
auto p = gen_last_momentum(outgoing_, sH, y);
return Particle{bosonid, std::move(p), {}};
}
Particle const & PhaseSpacePoint::most_backward_FKL(
std::vector<Particle> const & partons
) const{
if(!HEJ::is_parton(partons[0])) return partons[1];
return partons[0];
}
Particle const & PhaseSpacePoint::most_forward_FKL(
std::vector<Particle> const & partons
) const{
const size_t last_idx = partons.size() - 1;
if(!HEJ::is_parton(partons[last_idx])) return partons[last_idx-1];
return partons[last_idx];
}
Particle & PhaseSpacePoint::most_backward_FKL(
std::vector<Particle> & partons
) const{
if(!HEJ::is_parton(partons[0])) return partons[1];
return partons[0];
}
Particle & PhaseSpacePoint::most_forward_FKL(
std::vector<Particle> & partons
) const{
const size_t last_idx = partons.size() - 1;
if(!HEJ::is_parton(partons[last_idx])) return partons[last_idx-1];
return partons[last_idx];
}
namespace {
/// partons are ordered: even = anti, 0 = gluon
ParticleID index_to_pid(size_t i){
if(!i) return pid::gluon;
return static_cast<ParticleID>(i%2?(i+1)/2:-i/2);
}
/// partons are ordered: even = anti, 0 = gluon
size_t pid_to_index(ParticleID id){
if(id==pid::gluon) return 0;
return id>0?id*2-1:abs(id)*2;
}
std::bitset<11> init_allowed(ParticleID const id){
if(abs(id) == pid::proton)
return ~0;
std::bitset<11> out = 0;
if(is_parton(id))
out[pid_to_index(id)] = 1;
return out;
}
/// decides which "index" (see index_to_pid) are allowed for process
std::bitset<11> allowed_quarks(ParticleID const boson){
std::bitset<11> allowed = ~0;
if(abs(boson) == pid::Wp){
// special case W:
// Wp: anti-down or up-type quark, no b/t -> 0001100110(1) = 205
// Wm: down or anti-up-type quark, no b/t -> 0010011001(1) = 307
allowed = boson>0?205:307;
}
return allowed;
}
}
/**
* checks which partons are allowed as initial state:
* 1. only allow what is given in the Runcard (p -> all)
* 2. A/W/Z require something to couple to
* a) no qqx => no incoming gluon
* b) 2j => no incoming gluon
* c) 3j => can couple OR is gluon => 2 gluons become qqx later
*/
std::array<std::bitset<11>,2> PhaseSpacePoint::filter_partons(
Process const & proc, unsigned int const subl_channels, HEJ::RNG & ran
){
std::array<std::bitset<11>,2> allowed_partons{
init_allowed(proc.incoming[0]),
init_allowed(proc.incoming[1])
};
bool const allow_qqx = subl_channels&Subleading::qqx;
// special case A/W/Z
if(is_AWZ_proccess(proc) && ((proc.njets < 4) || !allow_qqx)){
// all possible incoming states
auto allowed(allowed_quarks(*proc.boson));
if(proc.njets == 2 || !allow_qqx) allowed[0]=0;
// possible states per leg
std::array<std::bitset<11>,2> const maybe_partons{
allowed_partons[0]&allowed, allowed_partons[1]&allowed};
if(maybe_partons[0].any() && maybe_partons[1].any()){
// two options to get allowed initial state => choose one at random
const size_t idx = ran.flat() < 0.5;
allowed_partons[idx] = maybe_partons[idx];
// else choose the possible
} else if(maybe_partons[0].any()) {
allowed_partons[0] = maybe_partons[0];
} else if(maybe_partons[1].any()) {
allowed_partons[1] = maybe_partons[1];
} else{
throw std::invalid_argument{"Incoming state not allowed."};
}
}
return allowed_partons;
}
void PhaseSpacePoint::reconstruct_incoming(
Process const & proc, unsigned int const subl_channels,
HEJ::PDF & pdf, double E_beam,
double uf,
HEJ::RNG & ran
){
std::tie(incoming_[0].p, incoming_[1].p) = incoming_momenta(outgoing_);
// calculate xa, xb
const double sqrts=2*E_beam;
const double xa=(incoming_[0].p.e()-incoming_[0].p.pz())/sqrts;
const double xb=(incoming_[1].p.e()+incoming_[1].p.pz())/sqrts;
// abort if phase space point is outside of collider energy reach
if (xa>1. || xb>1.){
weight_=0;
status_ = too_much_energy;
return;
}
auto const & ids = proc.incoming;
std::array<std::bitset<11>,2> allowed_partons(
filter_partons(proc, subl_channels, ran));
for(size_t i = 0; i < 2; ++i){
if(ids[i] == pid::proton || ids[i] == pid::p_bar){
// pick ids according to pdfs
incoming_[i].type =
generate_incoming_id(i, i?xb:xa, uf, pdf, allowed_partons[i], ran);
} else {
assert(allowed_partons[i][pid_to_index(ids[i])]);
incoming_[i].type = ids[i];
}
}
assert(momentum_conserved(1e-7));
}
HEJ::ParticleID PhaseSpacePoint::generate_incoming_id(
size_t const beam_idx, double const x, double const uf,
HEJ::PDF & pdf, std::bitset<11> allowed_partons, HEJ::RNG & ran
){
std::array<double,11> pdf_wt;
pdf_wt[0] = allowed_partons[0]?fabs(pdf.pdfpt(beam_idx,x,uf,pid::gluon)):0.;
double pdftot = pdf_wt[0];
for(size_t i = 1; i < pdf_wt.size(); ++i){
pdf_wt[i] = allowed_partons[i]?4./9.*fabs(pdf.pdfpt(beam_idx,x,uf,index_to_pid(i))):0;
pdftot += pdf_wt[i];
}
const double r1 = pdftot * ran.flat();
double sum = 0;
for(size_t i=0; i < pdf_wt.size(); ++i){
if (r1 < (sum+=pdf_wt[i])){
weight_*= pdftot/pdf_wt[i];
return index_to_pid(i);
}
}
std::cerr << "Error in choosing incoming parton: "<<x<<" "<<uf<<" "
<<sum<<" "<<pdftot<<" "<<r1<<std::endl;
throw std::logic_error{"Failed to choose parton flavour"};
}
void PhaseSpacePoint::couple_boson(
HEJ::ParticleID const boson, HEJ::RNG & ran
){
if(abs(boson) != pid::Wp) return; // only matters for W
/// @TODO this could be use to sanity check gamma and Z
// find all possible quarks
std::vector<Particle*> allowed_parts;
for(auto & part: outgoing_){
// Wp -> up OR anti-down, Wm -> anti-up OR down, no bottom
if ( can_couple_to_W(part, boson) )
allowed_parts.push_back(&part);
}
if(allowed_parts.size() == 0){
throw std::logic_error{"Found no parton for coupling with boson"};
}
// select one and flip it
size_t idx = 0;
if(allowed_parts.size() > 1){
/// @TODO more efficient sampling
/// old code: probability[i] = exp(parton[i].y - W.y)
idx = floor(ran.flat()*allowed_parts.size());
weight_ *= allowed_parts.size();
}
const int W_charge = boson>0?1:-1;
allowed_parts[idx]->type =
static_cast<ParticleID>( allowed_parts[idx]->type - W_charge );
}
double PhaseSpacePoint::random_normal(
double stddev,
HEJ::RNG & ran
){
const double r1 = ran.flat();
const double r2 = ran.flat();
const double lninvr1 = -log(r1);
const double result = stddev*sqrt(2.*lninvr1)*cos(2.*M_PI*r2);
weight_ *= exp(result*result/(2*stddev*stddev))*sqrt(2.*M_PI)*stddev;
return result;
}
bool PhaseSpacePoint::momentum_conserved(double ep) const{
fastjet::PseudoJet diff;
for(auto const & in: incoming()) diff += in.p;
for(auto const & out: outgoing()) diff -= out.p;
return nearby_ep(diff, fastjet::PseudoJet{}, ep);
}
Decay PhaseSpacePoint::select_decay_channel(
std::vector<Decay> const & decays,
HEJ::RNG & ran
){
double br_total = 0.;
for(auto const & decay: decays) br_total += decay.branching_ratio;
// adjust weight
// this is given by (channel branching ratio)/(chance to pick channel)
// where (chance to pick channel) =
// (channel branching ratio)/(total branching ratio)
weight_ *= br_total;
const double r1 = br_total*ran.flat();
double br_sum = 0.;
for(auto const & decay: decays){
br_sum += decay.branching_ratio;
if(r1 < br_sum) return decay;
}
throw std::logic_error{"unreachable"};
}
std::vector<Particle> PhaseSpacePoint::decay_boson(
HEJ::Particle const & parent,
std::vector<Decay> const & decays,
HEJ::RNG & ran
){
const auto channel = select_decay_channel(decays, ran);
if(channel.products.size() != 2){
throw HEJ::not_implemented{
"only decays into two particles are implemented"
};
}
std::vector<Particle> decay_products(channel.products.size());
for(size_t i = 0; i < channel.products.size(); ++i){
decay_products[i].type = channel.products[i];
}
// choose polar and azimuth angle in parent rest frame
const double E = parent.m()/2;
const double theta = 2.*M_PI*ran.flat();
const double cos_phi = 2.*ran.flat()-1.;
const double sin_phi = sqrt(1. - cos_phi*cos_phi); // Know 0 < phi < pi
const double px = E*cos(theta)*sin_phi;
const double py = E*sin(theta)*sin_phi;
const double pz = E*cos_phi;
decay_products[0].p.reset(px, py, pz, E);
decay_products[1].p.reset(-px, -py, -pz, E);
for(auto & particle: decay_products) particle.p.boost(parent.p);
return decay_products;
}
}

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