Rivet analyses

Measurement of the asymmetry in Ξ− → Λ0π−

Experiment: CLEOII (CESR)

Inspire ID: 533575

Status: UNVALIDATED

Authors: - Peter Richardson

References: - hep-ex/0009037

Beams: * *

Beam energies: ANY

Run details: - Any process producing Xi- baryons

The CLEOII 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 $\frac12(1+\alpha\cos\theta)$. This analysis is useful for testing spin correlations in hadron decays.

Source code:CLEOII_2000_I533575.cc

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Xi-> Lambda pi asymmetry
  class CLEOII_2000_I533575 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CLEOII_2000_I533575);


    /// @name Analysis methods
    /// @{

    /// Book histograms and initialise projections before the run
    void init() {

      // Initialise and register projections
      declare(UnstableParticles(), "UFS" );

      // Book histograms
      book(_h_cthetaM, 2,1,1);
      book(_h_cthetaP, 2,1,2);
    }


    /// 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,pion1;
    if(Xi.children()[0].pid()==sign*3122 &&
       Xi.children()[1].pid()==-sign*211) {
      Lambda = Xi.children()[0];
      pion1   = Xi.children()[1];
    }
    else if(Xi.children()[1].pid()==sign*3122 &&
        Xi.children()[0].pid()==-sign*211) {
      Lambda = Xi.children()[1];
      pion1   = Xi.children()[0];
    }
    else
      continue;
    if(Lambda.children().size()!=2) continue;
    Particle proton,pion2;
    if(Lambda.children()[0].pid()==sign*2212 &&
       Lambda.children()[1].pid()==-sign*211) {
      proton = Lambda.children()[0];
      pion2   = Lambda.children()[1];
    }
    else if(Lambda.children()[1].pid()==sign*2212 &&
        Lambda.children()[0].pid()==-sign*211) {
      proton = Lambda.children()[1];
      pion2   = Lambda.children()[0];
    }
    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(CLEOII_2000_I533575);

}