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ARGUS_1992_I319105

Decay asymmetries in $\Lambda^+_c \to \Lambda^0\pi^+$
Experiment: ARGUS (DORIS)
Inspire ID: 319105
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Lett. B274 (1992) 239-245
Beams: * *
Beam energies: ANY
Run details:
  • Any process producing Lambda_c baryons

Measurement of the decay asymmetries in $\Lambda^+_c\to\Lambda^0\pi^+$ by the ARGUS experiment. The asymmetry parameter is extracted by fitting to normalised angular distribution. N.B. the product of the asymmetry parameters for the $\Lambda_c$ and daughter baryon is implemented as this is what is measured, rather than the extracted parameter for the $\Lambda_c$ 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: ARGUS_1992_I319105.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/UnstableParticles.hh"
  4
  5namespace Rivet {
  6
  7
  8  /// @brief Lambda_c -> Lambda pi asymmetry
  9  class ARGUS_1992_I319105 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(ARGUS_1992_I319105);
 14
 15
 16    /// @name Analysis methods
 17    /// @{
 18
 19    /// Book histograms and initialise projections before the run
 20    void init() {
 21      // Initialise and register projections
 22      declare(UnstableParticles(), "UFS" );
 23      // Book histograms
 24      book(_h_ctheta,3,1,1);
 25    }
 26
 27
 28    /// Perform the per-event analysis
 29    void analyze(const Event& event) {
 30      // loop over Lambda_c baryons
 31      for( const Particle& Lambdac : apply<UnstableParticles>(event, "UFS").particles(Cuts::abspid==4122)) {
 32	int sign = Lambdac.pid()/4122;
 33	if(Lambdac.children().size()!=2) continue;
 34	Particle baryon1,meson1;
 35	if(Lambdac.children()[0].pid()==sign*3122 &&
 36	   Lambdac.children()[1].pid()==sign*211) {
 37	  baryon1 = Lambdac.children()[0];
 38	  meson1  = Lambdac.children()[1];
 39	}
 40	else if(Lambdac.children()[1].pid()==sign*3122 &&
 41		Lambdac.children()[0].pid()==sign*211) {
 42	  baryon1 = Lambdac.children()[1];
 43	  meson1  = Lambdac.children()[0];
 44	}
 45	else
 46	  continue;
 47	Particle baryon2,meson2;
 48	if(baryon1.children()[0].pid()== sign*2212 &&
 49	   baryon1.children()[1].pid()==-sign*211) {
 50	  baryon2 = baryon1.children()[0];
 51	  meson2  = baryon1.children()[1];
 52	}
 53	else if(baryon1.children()[1].pid()== sign*2212 &&
 54		baryon1.children()[0].pid()==-sign*211) {
 55	  baryon2 = baryon1.children()[1];
 56	  meson2  = baryon1.children()[0];
 57	}
 58	else
 59	  continue;
 60	// first boost to the Lambdac rest frame
 61	LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
 62	FourMomentum pbaryon1 = boost1.transform(baryon1.momentum());
 63	FourMomentum pbaryon2 = boost1.transform(baryon2.momentum());
 64	// to lambda rest frame
 65	LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pbaryon1.betaVec());
 66	Vector3 axis = pbaryon1.p3().unit();
 67	FourMomentum pp = boost2.transform(pbaryon2);
 68	// calculate angle
 69	double cTheta = pp.p3().unit().dot(axis);
 70	_h_ctheta->fill(cTheta);
 71      }
 72    }
 73
 74    pair<double,double> calcAlpha(Histo1DPtr hist) {
 75      if(hist->numEntries()==0.) return make_pair(0.,0.);
 76      double sum1(0.),sum2(0.);
 77      for (const auto& bin : hist->bins() ) {
 78        double Oi = bin.sumW();
 79        if(Oi==0.) continue;
 80        double ai = 0.5*(bin.xMax()-bin.xMin());
 81        double bi = 0.5*ai*(bin.xMax()+bin.xMin());
 82        double Ei = bin.errW();
 83        sum1 += sqr(bi/Ei);
 84        sum2 += bi/sqr(Ei)*(Oi-ai);
 85      }
 86      return make_pair(sum2/sum1,sqrt(1./sum1));
 87    }
 88
 89
 90    /// Normalise histograms etc., after the run
 91    void finalize() {
 92      normalize(_h_ctheta);
 93      Estimate1DPtr _h_alpha;
 94      book(_h_alpha,2,1,1);
 95      pair<double,double> alpha = calcAlpha(_h_ctheta);
 96      _h_alpha->bin(1).set(alpha.first, alpha.second);
 97    }
 98
 99    /// @}
100
101
102    /// @name Histograms
103    /// @{
104    Histo1DPtr _h_ctheta;
105    /// @}
106
107
108  };
109
110
111  RIVET_DECLARE_PLUGIN(ARGUS_1992_I319105);
112
113}