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CLEOII_2005_I668268

Decay asymmetry in $\Lambda^+_c\to\Lambda^0 e^+ \nu_e$
Experiment: CLEOII (CESR)
Inspire ID: 668268
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Lett.B 323 (1994) 219-226
Beams: * *
Beam energies: ANY
Run details:
  • Any process producing Lambda_c+, original e+e-

Measurement of the decay asymmetry in $\Lambda^+_c\to\Lambda^0 e^+ \nu_e$ by CLEOII. The product of assymetries $\alpha_{\Lambda_c^+}\alpha_\Lambda$ is extracted as that is what is measured not $\alpha_{\Lambda_c^+}$ and there has been a significant change in $\alpha_\Lambda$ since this paper was published

Source code: CLEOII_2005_I668268.cc
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// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief Lambda_c -> Lambda e+ nu_e asymmetry
  class CLEOII_2005_I668268 : public Analysis {
  public:

    /// Constructor
    DEFAULT_RIVET_ANALYSIS_CTOR(CLEOII_2005_I668268);


    /// @name Analysis methods
    //@{

    /// Book histograms and initialise projections before the run
    void init() {
      // Initialise and register projections
      declare(UnstableParticles(), "UFS" );
      
      // Book histograms
      book(_h_Lambda, "/TMP/hLambda", 20, -1.,1.);
    }

    void findChildren(Particle parent, int sign, unsigned int & npart,
		      Particles & lambda, Particles & e, Particles & nu) {
      for(const Particle & child : parent.children()) {
	if(child.pid()==sign*PID::LAMBDA) {
	  lambda.push_back(child);
	  ++npart;
	}
	else if(child.pid()==-sign*PID::EMINUS) {
	  e.push_back(child);
	  ++npart;
	}
	else if(child.pid()==sign*PID::NU_E) {
	  nu.push_back(child);
	  ++npart;
	}
	else if(!child.children().empty()) {
	  findChildren(child,sign,npart,lambda,e,nu);
	}
	else {
	  ++npart;
	}
      }
    }

    /// 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;
        Particles lambda,e,nu;
        unsigned int npart(0);
        findChildren(Lambdac,sign,npart,lambda,e,nu);
	if(npart!=3 || lambda.size()!=1 || e.size()!=1 || nu.size()!=1) continue;
	Particle baryon2;
	if(lambda[0].children()[0].pid()== sign*2212 && 
	   lambda[0].children()[1].pid()== -sign*211) {
	  baryon2 = lambda[0].children()[0];
	}
	else if(lambda[0].children()[1].pid()== sign*2212 && 
		lambda[0].children()[0].pid()== -sign*211) {
	  baryon2 = lambda[0].children()[1];
	}
	else
	  continue;
	// first boost to the Lambdac rest frame
	LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
	FourMomentum pbaryon1 = boost1.transform(lambda[0].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_Lambda->fill(cTheta);
      }
    }

    pair<double,double> calcAlpha(Histo1DPtr hist) {
      if(hist->numEntries()==0.) return make_pair(0.,0.);
      double sum1(0.),sum2(0.);
      for (auto bin : hist->bins() ) {
	double Oi = bin.area();
	if(Oi==0.) continue;
	double ai = 0.5*(bin.xMax()-bin.xMin());
	double bi = 0.5*ai*(bin.xMax()+bin.xMin());
	double Ei = bin.areaErr();
	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() {
      //  asymmetry
      normalize(_h_Lambda);
      Scatter2DPtr _h_alpha;
      book(_h_alpha,1,1,1);
      pair<double,double> alpha = calcAlpha(_h_Lambda);
      _h_alpha->addPoint(0.5, alpha.first, make_pair(0.5,0.5), make_pair(alpha.second,alpha.second) );
    }
    //@}


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


  };


  DECLARE_RIVET_PLUGIN(CLEOII_2005_I668268);

}