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Rivet analyses reference
BESIII_2022_I2099126
Measurement of $\Lambda\to n\gamma$ decay asymmetry using $J/\psi$ decays to $\Lambda^0\bar\Lambda^0$
Experiment: BESIII (BEPC)
Inspire ID: 2099126
Status: VALIDATED NOHEPDATA
Authors:
References:
Beams: e- e+
Beam energies: (1.6, 1.6) GeV
Analysis of the angular distribution of the baryons, and decay products, produced in $e^+e^-\to J/\psi \to \Lambda^0\bar\Lambda^0$ with the decay $\Lambda\to n\gamma$. Gives information about the decay and is useful for testing correlations in hadron decays. N.B. the moment data is not corrected and should only be used qualatively.
Source code:
BESIII_2022_I2099126.cc
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236 | // -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/Beam.hh"
#include "Rivet/Projections/FinalState.hh"
#include "Rivet/Projections/UnstableParticles.hh"
namespace Rivet {
/// @brief JPsi > Lambda, Lambdabar with Lambda -> n gamma
class BESIII_2022_I2099126 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2022_I2099126);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(Beam(), "Beams");
declare(UnstableParticles(), "UFS");
declare(FinalState(), "FS");
for(unsigned int ix=0;ix<2;++ix) {
book(_n[ix],"TMP/n_" + toString(ix+1));
book(_t[ix],"TMP/t_" + toString(ix+1));
for(unsigned int iy=0;iy<2;++iy) {
book(_h_mu[ix][iy],1,1,2*ix+iy+1);
}
}
book(_n[2],"TMP/n_3");
book(_t[2],"TMP/t_3");
}
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 lambda0 baryons
const UnstableParticles & ufs = apply<UnstableParticles>(event, "UFS");
Particle Lambda,LamBar;
bool matched(false);
for (const Particle& p : ufs.particles(Cuts::abspid==3122)) {
if(p.children().empty()) continue;
map<long,int> nRes=nCount;
int ncount = ntotal;
findChildren(p,nRes,ncount);
matched=false;
// check for antiparticle
for (const Particle& p2 : ufs.particles(Cuts::pid==-p.pid())) {
if(p2.children().empty()) continue;
map<long,int> nRes2=nRes;
int ncount2 = ncount;
findChildren(p2,nRes2,ncount2);
if(ncount2==0) {
matched = true;
for(auto const & val : nRes2) {
if(val.second!=0) {
matched = false;
break;
}
}
// found baryon and antibaryon
if(matched) {
if(p.pid()>0) {
Lambda = p;
LamBar = p2;
}
else {
Lambda = p2;
LamBar = p;
}
break;
}
}
}
if(matched) break;
}
if(!matched) vetoEvent;
// check the Lambda decay mode
bool radiative[2]={false,false};
// identifyt Lambda decay
Particle baryon1;
if ( (Lambda.children()[0].pid()==PID::PROTON &&
Lambda.children()[1].pid()==PID::PIMINUS ) ) {
radiative[0]=false;
baryon1 = Lambda.children()[0];
}
else if ( (Lambda.children()[1].pid()==PID::PROTON &&
Lambda.children()[0].pid()==PID::PIMINUS ) ) {
radiative[0]=false;
baryon1 = Lambda.children()[1];
}
else if ( (Lambda.children()[0].pid()==PID::NEUTRON &&
Lambda.children()[1].pid()==PID::PHOTON ) ) {
radiative[0]=true;
baryon1 = Lambda.children()[0];
}
else if ( (Lambda.children()[1].pid()==PID::NEUTRON &&
Lambda.children()[0].pid()==PID::PHOTON ) ) {
radiative[0]=true;
baryon1 = Lambda.children()[1];
}
else
vetoEvent;
Particle baryon2;
if ( (LamBar.children()[0].pid()==PID::ANTIPROTON &&
LamBar.children()[1].pid()==PID::PIPLUS ) ) {
radiative[1]=false;
baryon2 = LamBar.children()[0];
}
else if ( (LamBar.children()[1].pid()==PID::ANTIPROTON &&
LamBar.children()[0].pid()==PID::PIPLUS ) ) {
radiative[1]=false;
baryon2 = LamBar.children()[1];
}
else if ( (LamBar.children()[0].pid()==PID::ANTINEUTRON &&
LamBar.children()[1].pid()==PID::PHOTON ) ) {
radiative[1]=true;
baryon2 = LamBar.children()[0];
}
else if ( (LamBar.children()[1].pid()==PID::ANTINEUTRON &&
LamBar.children()[0].pid()==PID::PHOTON ) ) {
radiative[1]=true;
baryon2 = LamBar.children()[1];
}
else
vetoEvent;
if (radiative[0] == radiative[1]) vetoEvent;
// boost to the Lambda rest frame
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambda.momentum().betaVec());
Vector3 e1z = Lambda.momentum().p3().unit();
Vector3 e1y = e1z.cross(axis).unit();
Vector3 e1x = e1y.cross(e1z).unit();
Vector3 axis1 = boost1.transform(baryon1.momentum()).p3().unit();
double n1x(e1x.dot(axis1)),n1y(e1y.dot(axis1)),n1z(e1z.dot(axis1));
// boost to the Lambda bar
LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(LamBar.momentum().betaVec());
Vector3 axis2 = boost2.transform(baryon2.momentum()).p3().unit();
double n2x(e1x.dot(axis2)),n2y(e1y.dot(axis2)),n2z(e1z.dot(axis2));
double cosL = axis.dot(Lambda.momentum().p3().unit());
double sinL = sqrt(1.-sqr(cosL));
double T1 = sqr(sinL)*n1x*n2x+sqr(cosL)*n1z*n2z;
// lambda -> n gamma
if(radiative[0]) {
_h_mu[0][0]->fill( cosL,n2y);
_h_mu[0][1]->fill( cosL,n1y);
_n[0]->fill();
_n[2]->fill();
_t[0]->fill(T1);
_t[2]->fill(T1);
}
// lambdabar -> nbar gamma
else {
_h_mu[1][0]->fill( cosL,n1y);
_h_mu[1][1]->fill( cosL,n2y);
_n[1]->fill();
_n[2]->fill();
_t[1]->fill(T1);
_t[2]->fill(T1);
}
}
/// Normalise histograms etc., after the run
void finalize() {
// values of constants
double aPsi = 0.461;
double aPlus =-0.758;
double factor = 45.*(3. +aPsi)/(11. + 5.*aPsi)/aPlus;
// plots
for(unsigned int ix=0;ix<2;++ix) {
for(unsigned int iy=0;iy<2;++iy) {
scale(_h_mu[ix][iy],10.*0.2/ *_n[ix]);
}
}
// alpha from the moments
for(unsigned int ix=0;ix<3;++ix) {
double value = _t[ix]->val()/_n[ix]->val();
double error = _t[ix]->err()/_n[ix]->val();
value *= factor;
error *= abs(factor);
if(ix==1) value *=-1.;
Scatter2DPtr alpha;
book(alpha,2,1,1+ix);
alpha->addPoint(0.5, value, make_pair(0.5,0.5),
make_pair(error,error) );
}
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h_mu[2][2];
CounterPtr _n[3];
Histo1DPtr _h_ctheta[3];
CounterPtr _t[3];
/// @}
};
RIVET_DECLARE_PLUGIN(BESIII_2022_I2099126);
}
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