Rivet analyses
Decay asymmetries in Ξ0 → Λ0γ, Λ0π0 and Σ0γ
Experiment: NA48 ()
Inspire ID: 868871
Status: VALIDATED
Authors: - Peter Richardson
References: - Phys.Lett. B693 (2010) 241-248
Beams: * *
Beam energies: ANY
Run details: - Any process producing Xi0 baryons
Measurement of the decay asymmetries in Ξ0 → Λ0γ, Λ0π0 and Σ0γ by the NA48 experiment. The asymmetry parameter is extracted by fitting to normalised angular distribution. This analysis is useful for testing spin correlations in hadron decays.
Source
code:NA48_2010_I868871.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"
#include <sstream>
namespace Rivet {
/// @brief asymmetrics in Xi0 decays
class NA48_2010_I868871 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(NA48_2010_I868871);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(UnstableParticles(), "UFS" );
// Book histograms
book(_h_ctheta_pi0 , "ctheta_pi0" , 20,-1,1);
book(_h_ctheta_gamma, "ctheta_gamma", 20,-1,1);
book(_h_ctheta_Sigma, {-1., -0.9, -0.8, -0.7, -0.6, -0.5, -0.4, -0.3, -0.2, -0.1,
0., 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.});
for (auto& b : _h_ctheta_Sigma->bins()) {
const string name = "ctheta_Sigma_"+std::to_string(b.index()-1);
book(b, name, 20, -1.0, 1.0);
}
book(_nSigma,"TMP/nSigma");
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// loop over Omega baryons
for (const Particle& Xi : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==3322)) {
int sign = Xi.pid()/3322;
if(Xi.children().size()!=2) continue;
Particle baryon1,meson1;
unsigned int mode(0);
if(Xi.children()[0].pid()==sign*3122 &&
Xi.children()[1].pid()==111) {
baryon1 = Xi.children()[0];
meson1 = Xi.children()[1];
mode=1;
}
else if(Xi.children()[1].pid()==sign*3122 &&
Xi.children()[0].pid()==111) {
baryon1 = Xi.children()[1];
meson1 = Xi.children()[0];
mode=1;
}
else if(Xi.children()[0].pid()==sign*3122 &&
Xi.children()[1].pid()==22) {
baryon1 = Xi.children()[0];
meson1 = Xi.children()[1];
mode=2;
}
else if(Xi.children()[1].pid()==sign*3122 &&
Xi.children()[0].pid()==22) {
baryon1 = Xi.children()[1];
meson1 = Xi.children()[0];
mode=2;
}
else if(Xi.children()[0].pid()==sign*3212 &&
Xi.children()[1].pid()==22) {
baryon1 = Xi.children()[0];
meson1 = Xi.children()[1];
mode=3;
}
else if(Xi.children()[1].pid()==sign*3212 &&
Xi.children()[0].pid()==22) {
baryon1 = Xi.children()[1];
meson1 = Xi.children()[0];
mode=3;
}
else
continue;
if(baryon1.children().size()!=2) continue;
Particle baryon2,meson2,baryon3,meson3;
if(mode==1 || mode ==2) {
if(baryon1.children()[0].pid()== sign*2212 &&
baryon1.children()[1].pid()==-sign*211) {
baryon2 = baryon1.children()[0];
meson2 = baryon1.children()[1];
}
else if(baryon1.children()[1].pid()== sign*2212 &&
baryon1.children()[0].pid()==-sign*211) {
baryon2 = baryon1.children()[1];
meson2 = baryon1.children()[0];
}
else
continue;
}
else if(mode==3) {
if(baryon1.children()[0].pid()== sign*3122 &&
baryon1.children()[1].pid()== 22) {
baryon2 = baryon1.children()[0];
meson2 = baryon1.children()[1];
}
else if(baryon1.children()[1].pid()== sign*3122 &&
baryon1.children()[0].pid()== 22) {
baryon2 = baryon1.children()[1];
meson2 = baryon1.children()[0];
}
else
continue;
if(baryon2.children()[0].pid()== sign*2212 &&
baryon2.children()[1].pid()==-sign*211) {
baryon3 = baryon2.children()[0];
meson3 = baryon2.children()[1];
}
else if(baryon2.children()[1].pid()== sign*2212 &&
baryon2.children()[0].pid()==-sign*211) {
baryon3 = baryon2.children()[1];
meson3 = baryon2.children()[0];
}
else
continue;
}
// first boost to the Xi rest frame
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xi.momentum().betaVec());
FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
// to lambda rest frame
LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
Vector3 axis = pbaryon1.p3().unit();
FourMomentum pp = boost2.transform(pbaryon2);
// calculate angle
double cTheta = pp.p3().unit().dot(axis);
if(mode==1) {
_h_ctheta_pi0->fill(cTheta,1.);
}
else if(mode==2) {
_h_ctheta_gamma->fill(cTheta,1.);
}
else if(mode==3) {
FourMomentum pbaryon3 = boost1.transform(baryon3.momentum());
FourMomentum pp2 = boost2.transform(pbaryon3);
Vector3 axis2 = pp.p3().unit();
double cTheta2 = pp2.p3().unit().dot(axis2);
_h_ctheta_Sigma->fill(cTheta,cTheta2);
_nSigma->fill();
}
}
}
pair<double,double> calcAlpha(Histo1DPtr& hist) {
if (hist->numEntries()==0.) return make_pair(0.,0.);
double sum1(0.),sum2(0.);
for (const auto& bin : hist->bins() ) {
double Oi = bin.sumW();
if(Oi==0.) continue;
double ai = 0.5*(bin.xMax()-bin.xMin());
double bi = 0.5*ai*(bin.xMax()+bin.xMin());
double Ei = bin.errW();
sum1 += sqr(bi/Ei);
sum2 += bi/sqr(Ei)*(Oi-ai);
}
return make_pair(sum2/sum1,sqrt(1./sum1));
}
pair<double,double> calcAlpha(Histo1DGroupPtr& hist) {
double sum1(0.),sum2(0.);
for (auto& h2 : hist->bins()) {
double xsum=2.*h2.xMin() + h2.xWidth();
for (const auto& bin : h2->bins() ) {
double Oi = bin.sumW();
if(Oi==0.) continue;
double ai = 0.25*(bin.xMax()-bin.xMin())*h2.xWidth();
double bi = 0.25*ai*(bin.xMax()+bin.xMin())*xsum;
double Ei = bin.errW();
sum1 += sqr(bi/Ei);
sum2 += bi/sqr(Ei)*(Oi-ai);
}
}
return make_pair(sum2/sum1,sqrt(1./sum1));
}
/// Normalise histograms etc., after the run
void finalize() {
// Xi0 -> Lambda0 pi0
normalize(_h_ctheta_pi0);
Estimate0DPtr _h_alpha_pi0;
book(_h_alpha_pi0,1,1,1);
pair<double,double> alpha = calcAlpha(_h_ctheta_pi0);
_h_alpha_pi0->set(alpha.first, alpha.second);
// Xi0 -> Lambda gamma (N.B. sign due defns)
normalize(_h_ctheta_gamma);
Estimate0DPtr _h_alpha_gamma;
book(_h_alpha_gamma,1,1,2);
alpha = calcAlpha(_h_ctheta_gamma);
_h_alpha_gamma->set(-alpha.first, alpha.second);
// Xi0 -> Sigma gamma
scale(_h_ctheta_Sigma, 1./ *_nSigma);
divByGroupWidth(_h_ctheta_Sigma);
Estimate0DPtr _h_alpha_Sigma;
book(_h_alpha_Sigma,1,1,3);
alpha = calcAlpha(_h_ctheta_Sigma);
_h_alpha_Sigma->set(alpha.first, alpha.second);
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h_ctheta_pi0,_h_ctheta_gamma;
Histo1DGroupPtr _h_ctheta_Sigma;
CounterPtr _nSigma;
/// @}
};
RIVET_DECLARE_PLUGIN(NA48_2010_I868871);
}