Rivet analyses

Decay asymmetries in Λc+ → Λ0π+

Experiment: FOCUS ()

Inspire ID: 693639

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 Λc+ → Λ0π+ by the FOCUS experiment. The asymmetry parameter is extracted by fitting to normalised angular distribution. N.B. the product of the asymmetry parameters for the Λc and daughter baryon is implemented as this is what is measured, rather than the extracted parameter for the Λc which relies on other measurements of the parameter for the daughter baryon. This analysis is useful for testing spin correlations in hadron decays.

Source code:FOCUS_2006_I693639.cc

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

namespace Rivet {


  /// @brief Lambda_c -> Lambda pi asymmetry
  class FOCUS_2006_I693639 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(FOCUS_2006_I693639);


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

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS" );
      // Book histograms
      book(_h_ctheta1,2,1,1);
      book(_h_ctheta2,3,1,1);
    }


    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // loop over Lambda_c baryons
      for( const Particle& Lambdac : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==4122)) {
    int sign = Lambdac.pid()/4122;
    if(Lambdac.children().size()!=2) continue;
    Particle baryon1,meson1;
    if(Lambdac.children()[0].pid()==sign*3122 &&
       Lambdac.children()[1].pid()==sign*211) {
      baryon1 = Lambdac.children()[0];
      meson1  = Lambdac.children()[1];
    }
    else if(Lambdac.children()[1].pid()==sign*3122 &&
        Lambdac.children()[0].pid()==sign*211) {
      baryon1 = Lambdac.children()[1];
      meson1  = Lambdac.children()[0];
    }
    else
      continue;
    Particle baryon2,meson2;
    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;
    // first boost to the Lambdac rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.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(sign>0)
      _h_ctheta1->fill(cTheta);
    else
      _h_ctheta2->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_ctheta1);
      normalize(_h_ctheta2);
      Estimate0DPtr _h_alpha;
      book(_h_alpha,1,1,1);
      pair<double,double> alpha = calcAlpha(_h_ctheta1);
      _h_alpha->set(alpha.first, alpha.second);
    }

    /// @}


    /// @name Histograms
    /// @{
    Histo1DPtr _h_ctheta1, _h_ctheta2;
    /// @}


  };


  RIVET_DECLARE_PLUGIN(FOCUS_2006_I693639);


}