Rivet analyses
Measurement of the asymmetry in Ξ− → Λ0π−
Experiment: E756 ()
Inspire ID: 530367
Status: VALIDATED
Authors: - Peter Richardson
References: - Phys.Rev.Lett. 85 (2000) 4860-4863
Beams: * *
Beam energies: ANY
Run details: - Any process producing Xi- baryons
The Hyper CP experiment measured the asymmetry parameter in the decay Ξ− → Λ0π− and the charge conjugate mode, in practice this is a fit to a normalised distribution $\frac12(1+\alpha\cos\theta)$. The paper only gives the number for the α parameter and not the distribution, so the distribution is calculated. The α parameter is then extracted using a χ2 fit. This analysis is useful for testing spin correlations in hadron decays.
Source
code:E756_2000_I530367.cc
// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"
namespace Rivet {
/// @brief Xi-> Lambda pi asymmetry
class E756_2000_I530367 : public Analysis {
public:
/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(E756_2000_I530367);
/// @name Analysis methods
/// @{
/// Book histograms and initialise projections before the run
void init() {
// Initialise and register projections
declare(UnstableParticles(), "UFS" );
// Book histograms
book(_h_cthetaP, "cthetaP",20,-1,1);
book(_h_cthetaM, "cthetaM",20,-1,1);
}
/// Perform the per-event analysis
void analyze(const Event& event) {
// loop over Xi- baryons
for (const Particle& Xi : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==3312)) {
int sign = Xi.pid()/3312;
if(Xi.children().size()!=2) continue;
Particle Lambda;
if(Xi.children()[0].pid()==sign*3122 &&
Xi.children()[1].pid()==-sign*211) {
Lambda = Xi.children()[0];
}
else if(Xi.children()[1].pid()==sign*3122 &&
Xi.children()[0].pid()==-sign*211) {
Lambda = Xi.children()[1];
}
else
continue;
if(Lambda.children().size()!=2) continue;
Particle proton;
if(Lambda.children()[0].pid()==sign*2212 &&
Lambda.children()[1].pid()==-sign*211) {
proton = Lambda.children()[0];
}
else if(Lambda.children()[1].pid()==sign*2212 &&
Lambda.children()[0].pid()==-sign*211) {
proton = Lambda.children()[1];
}
else
continue;
// boost to xi rest frame first
LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xi.momentum().betaVec());
FourMomentum pLambda = boost1.transform(Lambda.momentum());
FourMomentum pproton = boost1.transform(proton.momentum());
// to lambda rest frame
LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pLambda.betaVec());
Vector3 axis = pLambda.p3().unit();
FourMomentum pp = boost2.transform(pproton);
// calculate angle
double cTheta = pp.p3().unit().dot(axis);
if(sign==1) {
_h_cthetaM->fill(cTheta);
}
else {
_h_cthetaP->fill(cTheta);
}
}
}
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));
}
/// Normalise histograms etc., after the run
void finalize() {
normalize(_h_cthetaP);
normalize(_h_cthetaM);
// calculate the values of alpha
// xibar+
Estimate0DPtr _h_alphaP;
book(_h_alphaP, 1,1,2);
pair<double,double> alpha = calcAlpha(_h_cthetaP);
_h_alphaP->set(alpha.first, alpha.second);
// xi-
Estimate0DPtr _h_alphaM;
book(_h_alphaM, 1,1,1);
alpha = calcAlpha(_h_cthetaM);
_h_alphaM->set(alpha.first, alpha.second);
}
/// @}
/// @name Histograms
/// @{
Histo1DPtr _h_cthetaP,_h_cthetaM;
/// @}
};
RIVET_DECLARE_PLUGIN(E756_2000_I530367);
}