Rivet analyses

title: 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

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/ChargedFinalState.hh"

include "Rivet/Projections/Sphericity.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Lambda and Lambda bar dists class DELPHI_1993_I360638 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(DELPHI_1993_I360638);


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

/// Book histograms and initialise projections before the run
void init() {

  // Initialise and register projections
  const ChargedFinalState cfs;
  declare(cfs, "FS");
  declare(UnstableParticles(), "UFS");
  declare(Sphericity(cfs), "Sphericity");

  // Book histograms
  book(_h_x, 1, 1, 1);
  book(_h_rap, 3, 1, 1);
  book(_h_cos, 4, 1, 1);
  book(_m_single, 2, 1, 1);
  book(_m_like, 5, 1, 1);
  book(_m_opposite, 6, 1, 1);
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // First, veto on leptonic events by requiring at least 4 charged FS particles
  const FinalState& fs = apply<FinalState>(event, "FS");
  const size_t numParticles = fs.particles().size();
  // Even if we only generate hadronic events, we still need a cut on numCharged >= 2.
  if (numParticles < 2) vetoEvent;
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");
  // lambda
  Particles lambda = ufs.particles(Cuts::pid == PID::LAMBDA);
  Particles lambdabar = ufs.particles(Cuts::pid == -PID::LAMBDA);
  // multiplicities
  _m_single->fill(Ecm, (lambda.size() + lambdabar.size()));
  if (lambda.empty() && lambdabar.empty()) vetoEvent;
  for (const Particle& p : lambda) {
    double xP = 2. * p.p3().mod() / sqrtS();
    _h_x->fill(xP);
  }
  for (const Particle& p : lambdabar) {
    double xP = 2. * p.p3().mod() / sqrtS();
    _h_x->fill(xP);
  }
  if (lambda.size() >= 2) {
    unsigned int npair = lambda.size() / 2;
    _m_like->fill(Ecm, double(npair));
  }
  if (lambdabar.size() >= 2) {
    unsigned int npair = lambdabar.size() / 2;
    _m_like->fill(Ecm, double(npair));
  }
  if (lambda.size() == 0 || lambdabar.size() == 0) return;
  _m_opposite->fill(Ecm, double(max(lambda.size(), lambdabar.size())));
  const Sphericity& sphericity = apply<Sphericity>(event, "Sphericity");
  for (const Particle& p : lambda) {
    const Vector3 momP = p.p3();
    const double enP = p.E();
    const double modP = dot(sphericity.sphericityAxis(), momP);
    const double rapP = 0.5 * std::log((enP + modP) / (enP - modP));
    for (const Particle& pb : lambdabar) {
      const Vector3 momB = pb.p3();
      const double enB = pb.E();
      const double modB = dot(sphericity.sphericityAxis(), momB);
      const double rapB = 0.5 * std::log((enB + modB) / (enB - modB));
      _h_rap->fill(abs(rapP - rapB));
      _h_cos->fill(momP.unit().dot(momB.unit()));
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_h_x, 1. / sumOfWeights());
  scale(_h_rap, 1. / sumOfWeights());
  scale(_h_cos, 1. / sumOfWeights());
  scale(_m_single, 1. / sumOfWeights());
  scale(_m_like, 1. / sumOfWeights());
  scale(_m_opposite, 1. / sumOfWeights());
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_x, _h_rap, _h_cos;
BinnedHistoPtr<string> _m_single, _m_like, _m_opposite;
const string Ecm = "91.2";
/// @}

};

RIVET_DECLARE_PLUGIN(DELPHI_1993_I360638);

} ```