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JetSplitter.cc
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JetSplitter.cc
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/**
* \authors The HEJ collaboration (see AUTHORS for details)
* \date 2019
* \copyright GPLv2 or later
*/
#include "HEJ/JetSplitter.hh"
#include <array>
#include <assert.h>
#include <numeric>
#include "fastjet/ClusterSequence.hh"
#include "fastjet/PseudoJet.hh"
#include "HEJ/Constants.hh"
#include "HEJ/exceptions.hh"
namespace HEJ {
namespace{
constexpr double ccut=HEJ::CMINPT; // min parton pt
template<class Iterator>
bool same_pt_and_rapidity(
Iterator begin, Iterator end,
fastjet::PseudoJet const & jet
){
constexpr double ep = 1e-2;
const fastjet::PseudoJet reconstructed_jet = std::accumulate(
begin, end, fastjet::PseudoJet{}
);
return
(std::abs(reconstructed_jet.pt() - jet.pt()) < ep)
&& (std::abs(reconstructed_jet.rapidity() - jet.rapidity()) < ep)
;
}
bool all_in_one_jet(
std::vector<fastjet::PseudoJet> const & partons,
fastjet::JetDefinition jet_def, double min_jet_pt
){
fastjet::ClusterSequence ev(partons, jet_def);
const std::vector<fastjet::PseudoJet> testjet = ev.inclusive_jets(min_jet_pt);
return testjet.size() == 1u
&& testjet[0].constituents().size() == partons.size();
}
}
using SplitResult = JetSplitter::SplitResult;
SplitResult JetSplitter::split(
fastjet::PseudoJet const & j2split, int ncons
) const{
if(ncons <= 0) {
throw std::invalid_argument{
"number of requested jet constituents less than 1"
};
}
double swt = 1.;
std::vector<fastjet::PseudoJet> jcons;
if(ncons == 1){
jcons.emplace_back(j2split);
jcons.back().set_user_index(0);
return {jcons, swt};
}
if(ncons == 2){
return Split2(j2split);
}
const double R_max = R_factor*R_;
assert(R_max < M_PI);
double pt_remaining = j2split.pt();
const double phi_jet = j2split.phi();
const double y_jet = j2split.rapidity();
for(int i = 0; i < ncons - 1; ++i){
/**
* Generate rapidity and azimuthal angle with a distance
* R = sqrt(delta_y^2 + delta_phi^2) < R_max
* from the jet centre
*/
const double R = R_max*ran_.get().flat();
const double theta = 2*M_PI*ran_.get().flat();
const double delta_phi = R*cos(theta);
const double delta_y = R*sin(theta);
/**
* Generate pt such that the total contribution of all partons
* along the jet pt axis does not exceed the jet pt
*/
const double pt_max = pt_remaining/cos(delta_phi);
assert(pt_max > 0);
if(pt_max < ccut) return {}; // no pt remaining for this parton
const double pt = (pt_max - ccut)*ran_.get().flat() + ccut;
pt_remaining -= pt*cos(delta_phi);
jcons.emplace_back(
pt*cos(phi_jet + delta_phi), pt*sin(phi_jet + delta_phi),
pt*sinh(y_jet + delta_y), pt*cosh(y_jet + delta_y)
);
jcons.back().set_user_index(i);
swt *= 2*M_PI*R*R_max*pt*(pt_max - ccut);
}
const fastjet::PseudoJet p_total = std::accumulate(
jcons.begin(), jcons.end(), fastjet::PseudoJet{}
);
// Calculate the pt of the last parton
const double last_px = j2split.px() - p_total.px();
const double last_py = j2split.py() - p_total.py();
const double last_pt = sqrt(last_px*last_px + last_py*last_py);
if(last_pt < ccut) return {};
// Calculate the rapidity of the last parton using the requirement that the
// new jet must have the same rapidity as the LO jet.
const double exp_2y_jet = (j2split.e() + j2split.pz())/(j2split.e() - j2split.pz());
const double bb = (p_total.e()+p_total.pz()) - exp_2y_jet*(p_total.e()-p_total.pz());
const double lasty = log((-bb+sqrt(bb*bb+4.*exp_2y_jet*last_pt*last_pt))/(2.*last_pt));
jcons.emplace_back(
last_px, last_py, last_pt*sinh(lasty), last_pt*cosh(lasty)
);
jcons.back().set_user_index(ncons-1);
assert(same_pt_and_rapidity(begin(jcons), end(jcons), j2split));
// Test that the last parton is not too far away from the jet centre.
if (jcons.back().delta_R(j2split) > R_max) return {};
if(! all_in_one_jet(jcons, jet_def_, min_jet_pt_)) return {};
return {jcons, swt};
}
double JetSplitter::sample_distance_2p(double & wt) const{
static constexpr double x_small = 0.1;
static constexpr double p_small = 0.4;
const double pR = ran_.get().flat();
if(pR < p_small){
wt *= x_small/p_small;
return x_small/p_small*pR;
}
wt *= (1-x_small)/(1-p_small);
return (1-x_small)/(1-p_small)*(pR-p_small) + x_small;
}
SplitResult JetSplitter::Split2(fastjet::PseudoJet const & j2split) const{
static constexpr size_t ncons = 2;
std::vector<fastjet::PseudoJet> jcons(ncons);
std::array<double, ncons> R, phi, y, pt;
double wt = 1;
const double theta = 2*M_PI*ran_.get().flat(); // angle in y-phi plane
// empiric observation: we are always within the jet radius
R[0] = sample_distance_2p(wt)*R_;
R[1] = -sample_distance_2p(wt)*R_;
for(size_t i = 0; i <= 1; ++i){
phi[i] = j2split.phi() + R[i]*cos(theta);
y[i] = j2split.rapidity() + R[i]*sin(theta);
}
for(size_t i = 0; i <= 1; ++i){
pt[i] = (j2split.py() - tan(phi[1-i])*j2split.px())/
(sin(phi[i]) - tan(phi[1-i])*cos(phi[i]));
if(pt[i] < ccut) return {};
jcons[i].reset_PtYPhiM(pt[i], y[i], phi[i]);
jcons[i].set_user_index(i);
}
assert(same_pt_and_rapidity(begin(jcons), end(jcons), j2split));
if(! all_in_one_jet(jcons, jet_def_, min_jet_pt_)) return {};
wt *= 2*M_PI*pt[0]*R[0]*R_*R_;
// from transformation of delta(R[1] - ...) to delta(pt[0] - ...)
const double dphi0 = phi[0] - j2split.phi();
const double ptJ = j2split.pt();
const double jacobian = cos(theta)*pt[1]*pt[1]/(ptJ*sin(dphi0));
return {jcons, jacobian*wt};
}
}

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