Rivet analyses

Decay asymmetries in Ξc0 → Ξ−π+

Experiment: CLEO (CESR)

Inspire ID: 537236

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B634 (2006) 165-172

Beams: * *

Beam energies: ANY

Run details: - Any process producing Lambda_c baryons

Measurement of the decay asymmetries in Ξc0 → Ξ−π+ by the CLEO 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:CLEO_2000_I537236.cc

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

namespace Rivet {


  /// @brief Xi_c0 -> Xi-pi+ asymmetry
  class CLEO_2000_I537236 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(CLEO_2000_I537236);


    /// @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, "ctheta"  , 20,-1,1);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Xi_c0 baryons
      for( const Particle& Xic : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==4132)) {
    int sign = Xic.pid()/4132;
    if(Xic.children().size()!=2) continue;
    Particle baryon1,meson1;
    if(Xic.children()[0].pid()==sign*3312 &&
       Xic.children()[1].pid()==sign*211) {
      baryon1 = Xic.children()[0];
      meson1  = Xic.children()[1];
    }
    else if(Xic.children()[1].pid()==sign*3312 &&
        Xic.children()[0].pid()==sign*211) {
      baryon1 = Xic.children()[1];
      meson1  = Xic.children()[0];
    }
    else
      continue;
    Particle baryon2,meson2;
    if(baryon1.children()[0].pid()== sign*3122 &&
       baryon1.children()[1].pid()==-sign*211) {
      baryon2 = baryon1.children()[0];
      meson2  = baryon1.children()[1];
    }
    else if(baryon1.children()[1].pid()== sign*3122 &&
        baryon1.children()[0].pid()==-sign*211) {
      baryon2 = baryon1.children()[1];
      meson2  = baryon1.children()[0];
    }
    else
      continue;
    // first boost to the Xic rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Xic.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);
    _h_ctheta->fill(cTheta,1.);
      }
    }

    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_ctheta);
      Estimate0DPtr _h_alpha;
      book(_h_alpha,1,1,1);
      pair<double,double> alpha = calcAlpha(_h_ctheta);
      _h_alpha->set(alpha.first, alpha.second);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_ctheta;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(CLEO_2000_I537236);


}