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Rivet analyses reference
BESIII_2018_I1689296
Radiative $J/\psi$ decays to $K^0_SK^0_S$
Experiment: BESIII (BEPC)
Inspire ID: 1689296
Status: VALIDATED NOHEPDATA
Authors:
References:
- Phys.Rev.D 98 (2018) 7, 072003
Beams: e- e+
Beam energies: (1.6, 1.6) GeV
Run details:
Measurement of mass and angular distributions in the decay $J/\psi\to\gamma K^0_SK^0S$. Plots were read from the paper and are not corrected for efficiency/acceptance, although the paper states the backgrounds are small.
Source code:
BESIII_2018_I1689296.cc
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103 | // -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#include "Rivet/Projections/DecayedParticles.hh"
namespace Rivet {
/// @brief J/psi -> gamma KS0 KS0
class BESIII_2018_I1689296 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2018_I1689296);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
UnstableParticles ufs = UnstableParticles(Cuts::abspid==443);
declare(ufs, "UFS");
DecayedParticles PSI(ufs);
PSI.addStable(PID::K0S);
declare(PSI, "PSI");
declare(Beam(), "Beams");
// hisotgrams
for(unsigned int ix=0;ix<3;++ix) {
if(ix<2) book(_h_mass[ix],1,1,1+ix);
book(_h_angle[ix],2,1,1+ix);
}
}
// angle cuts due regions of BES calorimeter
bool vetoPhoton(const double & cTheta) {
return cTheta>0.92 || (cTheta>0.8 && cTheta<0.86);
}
/// 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();
// find the J/psi decays
static const map<PdgId,unsigned int> & mode = { { 310,2},{ 22,1}};
DecayedParticles PSI = apply<DecayedParticles>(event, "PSI");
if( PSI.decaying().size()!=1) vetoEvent;
if(!PSI.modeMatches(0,3,mode)) vetoEvent;
// particles
const Particles & K0 = PSI.decayProducts()[0].at(310);
const Particle & gam = PSI.decayProducts()[0].at( 22)[0];
double mKK = (K0[0].momentum()+K0[1].momentum()).mass();
_h_mass[0]->fill(mKK);
for(unsigned int ix=0;ix<2;++ix)
_h_mass[1]->fill((gam.momentum()+K0[ix].momentum()).mass());
double cTheta = axis.dot(gam.p3().unit());
if(vetoPhoton(abs(cTheta))) vetoEvent;
_h_angle[0]->fill(cTheta);
// remaining angles
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(PSI.decaying()[0].momentum().betaVec());
FourMomentum pGamma = boost1.transform(gam.momentum());
FourMomentum pKK = boost1.transform(K0[0].momentum()+K0[1].momentum());
Vector3 e1z = pGamma.p3().unit();
Vector3 e1y = e1z.cross(axis).unit();
Vector3 e1x = e1y.cross(e1z).unit();
LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pKK.betaVec());
Vector3 axis2 = boost2.transform(boost1.transform(K0[0].momentum())).p3().unit();
_h_angle[1]->fill(e1z.dot(axis2));
double phi = atan2(e1y.dot(axis2),e1x.dot(axis2));
_h_angle[2]->fill(phi);
}
/// Normalise histograms etc., after the run
void finalize() {
for(unsigned int ix=0;ix<3;++ix) {
if(ix<2) normalize(_h_mass[ix],1.,false);
normalize(_h_angle[ix],1.,false);
}
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h_mass[2], _h_angle[3];
/// @}
};
RIVET_DECLARE_PLUGIN(BESIII_2018_I1689296);
}
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