Rivet analyses
ψ(2S) → e+e−χc(1, 2) and χc(1, 2) → e+e−J/ψ decays
Experiment: BESIII (BEPC)
Inspire ID: 1509920
Status: VALIDATED NOHEPDATA
Authors: - Peter Richardson
References: - Phys.Rev.Lett. 118 (2017) 22, 221802
Beams: e- e+
Beam energies: (1.8, 1.8)GeV
Run details: - e+e- > psi 2s.
Measurement of the e+e− mass distribution and the helicity angle in the decays ψ(2S) → e+e−χc(1, 2) and χc(1, 2) → e+e−J/ψ decays. Data was read from the plots in the paper and the mass distributions are not corrected, although the angle distributions are
Source
code:BESIII_2017_I1509920.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/DecayedParticles.hh"
namespace Rivet {
/// @brief psi(2S) -> e+e- chi_c and chi_c -> e+e- Jpsi$
class BESIII_2017_I1509920 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2017_I1509920);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(Beam(), "Beams");
UnstableParticles ufs_psi(Cuts::abspid==PID::PSI2S);
DecayedParticles psi(ufs_psi);
psi.addStable(20443);
psi.addStable(445);
declare(psi, "PSI");
UnstableParticles ufs_chi(Cuts::abspid==445 or Cuts::abspid==20443);
DecayedParticles chi(ufs_chi);
chi.addStable(PID::JPSI);
declare(chi, "CHI");
// Histograms
for(unsigned int ix=0;ix<2;++ix)
for(unsigned int iy=0;iy<4;++iy)
book(_h[ix][iy],1+ix,1,1+iy);
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// get the axis, direction of incoming electron
const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
Vector3 axis;
if(beams.first.pid()>0)
axis = beams.first .momentum().p3().unit();
else
axis = beams.second.momentum().p3().unit();
// decaying particles
static const map<PdgId,unsigned int> & mode1 = { {20443,1},{ 11,1}, { -11,1}};
static const map<PdgId,unsigned int> & mode2 = { { 445,1},{ 11,1}, { -11,1}};
static const map<PdgId,unsigned int> & mode3 = { { 443,1},{ 11,1}, { -11,1}};
unsigned int iproj=0;
for(const DecayedParticles & in : {apply<DecayedParticles>(event, "PSI"),apply<DecayedParticles>(event, "CHI")} ) {
for(unsigned int ix=0;ix<in.decaying().size();++ix) {
int imode=-1,ichi=0;
if(iproj==0) {
if(in.modeMatches(ix,3,mode1)) {
imode=0;
ichi=20443;
}
else if(in.modeMatches(ix,3,mode2)) {
imode=1;
ichi=445;
}
else
continue;
}
else {
if(in.modeMatches(ix,3,mode3)) {
imode = in.decaying()[ix].pid()==20443 ? 2 : 3;
ichi=443;
}
else
continue;
}
// extract particles
const Particle & em = in.decayProducts()[ix].at( 11)[0];
const Particle & ep = in.decayProducts()[ix].at( -11)[0];
const Particle & out = in.decayProducts()[ix].at(ichi)[0];
FourMomentum qq = ep.momentum()+em.momentum();
double q = qq.mass();
_h[0][imode]->fill(q);
if(iproj==0) {
_h[1][imode]->fill(axis.dot((em.momentum()+ep.momentum()).p3().unit()));
}
else if(iproj==1) {
// boost everything to decaying particle frame
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(in.decaying()[ix].momentum().betaVec());
FourMomentum pout = boost1.transform(out.momentum());
FourMomentum pe = boost1.transform(em.momentum());
qq = boost1.transform(qq);
Vector3 axis1 = pout.p3().unit();
LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(qq.betaVec());
pe = boost2.transform(pe);
_h[1][imode]->fill(pe.p3().unit().dot(axis1));
}
}
++iproj;
}
}
/// Normalise histograms etc., after the run
void finalize() {
for(unsigned int ix=0;ix<2;++ix)
for(unsigned int iy=0;iy<4;++iy)
normalize(_h[ix][iy],1.,false);
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h[2][4];
/// @}
};
RIVET_DECLARE_PLUGIN(BESIII_2017_I1509920);
}