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DELPHI_1993_I360638

Correlations between $\Lambda^0$ and $\bar{\Lambda}^0$ production in hadronic $Z^0$ decays
Experiment: DELPHI (LEP)
Inspire ID: 360638
Status: VALIDATED
Authors:
  • Peter Richardson
References:
  • Phys.Lett. B318 (1993) 249-262, 1993
Beams: e+ e-
Beam energies: (45.6, 45.6) GeV
Run details:
  • $\sqrt{s} = 91.2$ GeV, $e^+ e^- -> Z^0$ production with hadronic decays only

The spectrum for the production of $\Lambda^0$ and $\bar{\Lambda}^0$ in hadronic $Z^0$ decays. Importantly the rapidity difference and cosine of the angle between $\Lambda^0$ and $\bar{\Lambda}^0$ baryons is measured. This is sensitive to different models of baryon production.

Source code: DELPHI_1993_I360638.cc
  1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/Sphericity.hh"
  4#include "Rivet/Projections/ChargedFinalState.hh"
  5#include "Rivet/Projections/UnstableParticles.hh"
  6
  7namespace Rivet {
  8
  9
 10  /// @brief Lambda and Lambda bar dists
 11  class DELPHI_1993_I360638 : public Analysis {
 12  public:
 13
 14    /// Constructor
 15    RIVET_DEFAULT_ANALYSIS_CTOR(DELPHI_1993_I360638);
 16
 17
 18    /// @name Analysis methods
 19    //@{
 20
 21    /// Book histograms and initialise projections before the run
 22    void init() {
 23
 24      // Initialise and register projections
 25      const ChargedFinalState cfs;
 26      declare(cfs, "FS");
 27      declare(UnstableParticles(), "UFS");
 28      declare(Sphericity(cfs), "Sphericity");
 29
 30      // Book histograms
 31      book(_h_x       , 1, 1, 1);
 32      book(_h_rap     , 3, 1, 1);
 33      book(_h_cos     , 4, 1, 1);
 34      book(_m_single  , 2, 1, 1);
 35      book(_m_like    , 5, 1, 1);
 36      book(_m_opposite, 6, 1, 1);
 37
 38    }
 39
 40
 41    /// Perform the per-event analysis
 42    void analyze(const Event& event) {
 43      // First, veto on leptonic events by requiring at least 4 charged FS particles
 44      const FinalState& fs = apply<FinalState>(event, "FS");
 45      const size_t numParticles = fs.particles().size();
 46      // Even if we only generate hadronic events, we still need a cut on numCharged >= 2.
 47      if (numParticles < 2) vetoEvent;
 48      const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
 49      // lambda
 50      Particles lambda    = ufs.particles(Cuts::pid== PID::LAMBDA);
 51      Particles lambdabar = ufs.particles(Cuts::pid==-PID::LAMBDA);
 52      // multiplicities
 53      _m_single->fill(91.2,(lambda.size()+lambdabar.size()));
 54      if(lambda.empty()&&lambdabar.empty()) vetoEvent;
 55      for(const Particle& p : lambda) {
 56	double xP = 2.*p.p3().mod()/sqrtS();
 57	_h_x->fill(xP);
 58      }
 59      for(const Particle& p : lambdabar) {
 60	double xP = 2.*p.p3().mod()/sqrtS();
 61	_h_x->fill(xP);
 62      }
 63      if(lambda.size()>=2) {
 64	unsigned int npair=lambda.size()/2;
 65	_m_like->fill(91.2,double(npair));
 66      }
 67      if(lambdabar.size()>=2) {
 68	unsigned int npair=lambdabar.size()/2;
 69	_m_like->fill(91.2,double(npair));
 70      }
 71      if(lambda.size()==0 || lambdabar.size()==0)
 72	return;
 73      _m_opposite->fill(91.2,double(max(lambda.size(),lambdabar.size())));
 74      const Sphericity& sphericity = apply<Sphericity>(event, "Sphericity");
 75      for(const Particle & p : lambda) {
 76        const Vector3 momP = p.p3();
 77        const double  enP  = p.E();
 78        const double  modP = dot(sphericity.sphericityAxis(), momP);
 79        const double rapP = 0.5 * std::log((enP + modP) / (enP - modP));
 80	for(const Particle & pb : lambdabar) {
 81	  const Vector3 momB = pb.p3();
 82	  const double  enB  = pb.E();
 83	  const double  modB = dot(sphericity.sphericityAxis(), momB);
 84	  const double rapB = 0.5 * std::log((enB + modB) / (enB - modB));
 85	  _h_rap->fill(abs(rapP-rapB));
 86	  _h_cos->fill(momP.unit().dot(momB.unit()));
 87	}
 88      }
 89    }
 90
 91
 92    /// Normalise histograms etc., after the run
 93    void finalize() {
 94      scale( _h_x       , 1./sumOfWeights());
 95      scale( _h_rap     , 1./sumOfWeights());
 96      scale( _h_cos     , 1./sumOfWeights());
 97      scale( _m_single  , 1./sumOfWeights());
 98      scale( _m_like    , 1./sumOfWeights());
 99      scale( _m_opposite, 1./sumOfWeights());
100    }
101
102    //@}
103
104
105    /// @name Histograms
106    //@{
107    Histo1DPtr _h_x, _h_rap ,_h_cos;
108    Histo1DPtr _m_single, _m_like, _m_opposite;
109    //@}
110
111
112  };
113
114
115  // The hook for the plugin system
116  RIVET_DECLARE_PLUGIN(DELPHI_1993_I360638);
117
118
119}