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Rivet analyses reference
BESIII_2022_I2033855
Analysis of $\psi(2S)\to\gamma\chi_{c(0,2)}$ decays with $\chi_{c(0,2)}\to \Xi^-\bar{\Xi}^+/\Xi^0\bar{\Xi}^0$
Experiment: BESIII (BEPC)
Inspire ID: 2033855
Status: VALIDATED NOHEPDATA
Authors:
References:
Beams: e- e+
Beam energies: (1.8, 1.8) GeV
Run details:
Analysis of the angular distribution of the photons and baryons produced in $\psi(2S)\to\gamma\chi_{c(0,2)}$ decays with $\chi_{c(0,2)}\to \Xi^-\bar{\Xi}^+/\Xi^0\bar{\Xi}^0$ Gives information about the decay and is useful for testing correlations in charmonium decays. N.B. the distributions were read from the figures in the paper and are not corrected and should only be used qualatively, however the $\alpha$ results are fully corrected.
Source code:
BESIII_2022_I2033855.cc
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195 | // -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"
namespace Rivet {
/// @brief psi(2S) -> gamma chi_c0,2 -> Xi Xibar
class BESIII_2022_I2033855 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2033855);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(Beam(), "Beams");
declare(UnstableParticles(Cuts::pid==10441 || Cuts::pid==445), "UFS");
declare(FinalState(), "FS");
// book hists
for(unsigned int ix=0;ix<3;++ix)
for(unsigned int iy=0;iy<2;++iy)
book(_h[ix][iy],4+ix,1,1+iy);
}
void findChildren(const Particle & p,map<long,int> & nRes, int &ncount) {
for( const Particle &child : p.children()) {
if(child.children().empty()) {
nRes[child.pid()]-=1;
--ncount;
}
else
findChildren(child,nRes,ncount);
}
}
/// 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();
// types of final state particles
const FinalState& fs = apply<FinalState>(event, "FS");
map<long,int> nCount;
int ntotal(0);
for (const Particle& p : fs.particles()) {
nCount[p.pid()] += 1;
++ntotal;
}
// loop over chi_c states
Particle chi;
bool matched = false;
const UnstableParticles & ufs = apply<UnstableParticles>(event, "UFS");
for (const Particle& p : ufs.particles()) {
if(p.children().empty()) continue;
map<long,int> nRes=nCount;
int ncount = ntotal;
findChildren(p,nRes,ncount);
if(ncount==1) {
matched = true;
for(auto const & val : nRes) {
if(val.first==PID::PHOTON) {
if(val.second!=1) {
matched = false;
break;
}
}
else if(val.second!=0) {
matched = false;
break;
}
}
if(matched) {
chi=p;
break;
}
}
}
if(!matched) vetoEvent;
// have chi_c find psi2S
if(chi.parents().empty() || chi.children().size()!=2 ||
chi.children()[0].pid() != -chi.children()[1].pid()) vetoEvent;
Particle psi2S = chi.parents()[0];
if(psi2S.pid()!=100443 || psi2S.children().size()!=2) vetoEvent;
// then the first photon
Particle gamma1;
if(psi2S.children()[0].pid()==PID::PHOTON)
gamma1 = psi2S.children()[0];
else if(psi2S.children()[1].pid()==PID::PHOTON)
gamma1 = psi2S.children()[1];
else
vetoEvent;
// now the decay products of the chi_c
Particle bPlus,bMinus;
bool foundBaryon=false;
for(unsigned int ix=0;ix<2;++ix) {
if(chi.children()[ix].pid()==PID::XIMINUS ||
chi.children()[ix].pid()==PID::XI0 ) {
foundBaryon=true;
bPlus=chi.children()[ix];
}
else if(chi.children()[ix].pid()==-PID::XIMINUS ||
chi.children()[ix].pid()==-PID::XI0 ) {
bMinus=chi.children()[ix];
}
}
if(!foundBaryon) vetoEvent;
// type chi state
unsigned int ichi = 0;
if(chi.pid()==20443) ichi = 1;
else if(chi.pid()==445) ichi = 2;
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(chi.momentum().betaVec());
Vector3 e1z = gamma1.momentum().p3().unit();
FourMomentum pBaryon = boost1.transform(bPlus.momentum());
Vector3 axis1 = pBaryon.p3().unit();
double cBaryon = e1z.dot(axis1);
if(bPlus.pid()==PID::XIMINUS)
_h[ichi][0]->fill(cBaryon);
else
_h[ichi][1]->fill(cBaryon);
}
pair<double,pair<double,double> > calcAlpha0(Histo1DPtr hist) {
if(hist->numEntries()==0.) return make_pair(0.,make_pair(0.,0.));
double d = 3./(pow(hist->xMax(),3)-pow(hist->xMin(),3));
double c = 3.*(hist->xMax()-hist->xMin())/(pow(hist->xMax(),3)-pow(hist->xMin(),3));
double sum1(0.),sum2(0.),sum3(0.),sum4(0.),sum5(0.);
for (auto bin : hist->bins() ) {
double Oi = bin.area();
if(Oi==0.) continue;
double a = d*(bin.xMax() - bin.xMin());
double b = d/3.*(pow(bin.xMax(),3) - pow(bin.xMin(),3));
double Ei = bin.areaErr();
sum1 += a*Oi/sqr(Ei);
sum2 += b*Oi/sqr(Ei);
sum3 += sqr(a)/sqr(Ei);
sum4 += sqr(b)/sqr(Ei);
sum5 += a*b/sqr(Ei);
}
// calculate alpha
double alpha = (-c*sum1 + sqr(c)*sum2 + sum3 - c*sum5)/(sum1 - c*sum2 + c*sum4 - sum5);
// and error
double cc = -pow((sum3 + sqr(c)*sum4 - 2*c*sum5),3);
double bb = -2*sqr(sum3 + sqr(c)*sum4 - 2*c*sum5)*(sum1 - c*sum2 + c*sum4 - sum5);
double aa = sqr(sum1 - c*sum2 + c*sum4 - sum5)*(-sum3 - sqr(c)*sum4 + sqr(sum1 - c*sum2 + c*sum4 - sum5) + 2*c*sum5);
double dis = sqr(bb)-4.*aa*cc;
if(dis>0.) {
dis = sqrt(dis);
return make_pair(alpha,make_pair(0.5*(-bb+dis)/aa,-0.5*(-bb-dis)/aa));
}
else {
return make_pair(alpha,make_pair(0.,0.));
}
}
/// Normalise histograms etc., after the run
void finalize() {
for(unsigned int ix=0;ix<3;++ix) {
for(unsigned int iy=0;iy<2;++iy) {
normalize(_h[ix][iy],1.,false);
pair<double,pair<double,double> > alpha0 = calcAlpha0(_h[ix][iy]);
Scatter2DPtr _h_alpha0;
book(_h_alpha0,1+ix,1,1+iy);
_h_alpha0->addPoint(0.5, alpha0.first, make_pair(0.5,0.5),
make_pair(alpha0.second.first,alpha0.second.second) );
}
}
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h[3][2];
/// @}
};
RIVET_DECLARE_PLUGIN(BESIII_2022_I2033855);
}
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