Rivet analyses

title: OPAL_2000_I474010

Correlations for $\Lambda^0$ and $\bar\Lambda^0$ production at LEP1

Experiment: OPAL (LEP)

Inspire ID: 474010

Status: VALIDATED

Authors: - Peter Richardson

References: - Eur.Phys.J. C13 (2000) 185-195

Beams: e- e+

Beam energies: ANY

Run details: - e+e- to hadrons

Measurement of the correlations between the production of $\Lambda^0$ and $\bar\Lambda^0$ baryons at LEP1, which is useful for testing models of baryon production.

Source code:OPAL_2000_I474010.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/FastJets.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Projections/Thrust.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace {

unsigned int factorial(unsigned int n) { if (n <= 1) return 0; unsigned int out = 1; for (unsigned int i = n; i > 0; --i) out *= i; return out; }

double rapidityAxis(Rivet::Vector4 momentum, Rivet::Vector3 axis) { // assume axis is unit vector double plong = momentum.vector3().dot(axis); return 0.5 * log((momentum.t() + plong) / (momentum.t() - plong)); }

double cosThetaStar(const Rivet::Particle& p1, const Rivet::Particle& p2, Rivet::Vector3 axis) { Rivet::FourMomentum psum = p1.momentum() + p2.momentum(); Rivet::LorentzTransform trans(Rivet::LorentzTransform::mkObjTransformFromBeta(-psum.betaVec())); Rivet::FourMomentum m1 = trans.transform(p1.momentum()); Rivet::FourVector a4; a4.setX(axis.x()); a4.setY(axis.y()); a4.setZ(axis.z()); a4.setT(1.); a4 = trans.transform(a4); return fabs(m1.vector3().dot(a4.vector3()) / m1.vector3().mod() / a4.vector3().mod()); }

}

namespace Rivet {

/// @brief lambda anti-lambda correlations class OPAL_2000_I474010 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(OPAL_2000_I474010);


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

/// Book histograms and initialise projections before the run
void init() {
  // projections
  const FinalState fs;
  declare(fs, "FS");
  FastJets durhamjets(fs, JetAlg::DURHAM, 0.7);
  durhamjets.useInvisibles(JetInvisibles::ALL);
  declare(durhamjets, "DurhamJets");
  const Thrust thrust(fs);
  declare(thrust, "Thrust");
  declare(UnstableParticles(), "UFS");
  // histograms
  book(_histLLBar, 1, 1, 1);
  book(_histLL, 1, 1, 2);
  book(_histLLBarCorr, 1, 1, 3);
  book(_histLLBar_2jet, 2, 1, 1);
  book(_histLL_2jet, 2, 1, 2);
  book(_histLLBarCorr_2jet, 2, 1, 3);
  book(_histLLBar_3jet, 3, 1, 1);
  book(_histLL_3jet, 3, 1, 2);
  book(_histLLBarCorr_3jet, 3, 1, 3);
  book(_histdcosTheta, 4, 1, 1);
  book(_histdy, 5, 1, 1);
  // weights
  book(_weight2Jet, "/TMP/W2Jet");
  book(_weight3Jet, "/TMP/W3Jet");
}


/// 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) {
    MSG_DEBUG("Failed leptonic event cut");
    vetoEvent;
  }
  MSG_DEBUG("Passed leptonic event cut");

  const Thrust& thrust = apply<Thrust>(event, "Thrust");
  Vector3 axis = thrust.thrustAxis();

  // Final state of unstable particles to get particle spectra
  const UnstableParticles& ufs = apply<UnstableParticles>(event, "UFS");

  Particles lambda, lambdaBar;
  for (const Particle& p : ufs.particles()) {
    const int id = p.pid();
    if (id == 3122)
      lambda.push_back(p);
    else if (id == -3122)
      lambdaBar.push_back(p);
  }

  // get the number of jets
  const FastJets& durjet = apply<FastJets>(event, "DurhamJets");

  unsigned int njet = durjet.clusterSeq()->n_exclusive_jets_ycut(0.005);
  if (njet == 2)
    _weight2Jet->fill();
  else if (njet == 3)
    _weight3Jet->fill();

  // need a pair of baryons
  if (lambda.size() + lambdaBar.size() < 2) vetoEvent;

  // multiplicities
  int nsame = (factorial(lambda.size()) + factorial(lambdaBar.size())) / 2;
  int ndiff = lambda.size() * lambdaBar.size();
  _histLLBar->fill(Ecm, ndiff);
  _histLL->fill(Ecm, nsame);
  _histLLBarCorr->fill(Ecm, (ndiff - nsame));

  // multiplicities for different numbers of jets
  if (njet == 2) {
    _histLLBar_2jet->fill(Ecm, ndiff);
    _histLL_2jet->fill(Ecm, nsame);
    _histLLBarCorr_2jet->fill(Ecm, (ndiff - nsame));
  }
  else if (njet == 3) {
    _histLLBar_3jet->fill(Ecm, ndiff);
    _histLL_3jet->fill(Ecm, nsame);
    _histLLBarCorr_3jet->fill(Ecm, (ndiff - nsame));
  }
  // uncorrelated lambda
  for (unsigned int ix = 0; ix < lambda.size(); ++ix) {
    double y1 = rapidityAxis(lambda[ix].momentum(), axis);
    for (unsigned int iy = ix + 1; iy < lambda.size(); ++iy) {
      double y2 = rapidityAxis(lambda[iy].momentum(), axis);
      if (njet == 2) _histdy->fill(fabs(y1 - y2), -1.0);
      double ctheta = cosThetaStar(lambda[ix], lambda[iy], axis);
      _histdcosTheta->fill(ctheta, -1.0);
    }
  }
  // uncorrelated lambdabar
  for (unsigned int ix = 0; ix < lambdaBar.size(); ++ix) {
    double y1 = rapidityAxis(lambdaBar[ix].momentum(), axis);
    for (unsigned int iy = ix + 1; iy < lambdaBar.size(); ++iy) {
      double y2 = rapidityAxis(lambdaBar[iy].momentum(), axis);
      if (njet == 2) _histdy->fill(fabs(y1 - y2), -1.0);
      double ctheta = cosThetaStar(lambdaBar[ix], lambdaBar[iy], axis);
      _histdcosTheta->fill(ctheta, -1.0);
    }
  }
  // all lambda lambdabar
  for (unsigned int ix = 0; ix < lambda.size(); ++ix) {
    double y1 = rapidityAxis(lambda[ix].momentum(), axis);
    for (unsigned int iy = 0; iy < lambdaBar.size(); ++iy) {
      double y2 = rapidityAxis(lambdaBar[iy].momentum(), axis);
      if (njet == 2) _histdy->fill(fabs(y1 - y2));
      double ctheta = cosThetaStar(lambda[ix], lambdaBar[iy], axis);
      _histdcosTheta->fill(ctheta);
    }
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  scale(_histLLBar, 1. / sumOfWeights());
  scale(_histLL, 1. / sumOfWeights());
  scale(_histLLBarCorr, 1. / sumOfWeights());
  scale(_histLLBar_2jet, 1. / *_weight2Jet);
  scale(_histLL_2jet, 1. / *_weight2Jet);
  scale(_histLLBarCorr_2jet, 1. / *_weight2Jet);
  scale(_histLLBar_3jet, 1. / *_weight3Jet);
  scale(_histLL_3jet, 1. / *_weight3Jet);
  scale(_histLLBarCorr_3jet, 1. / *_weight3Jet);
  normalize(_histdcosTheta, 1.);
  normalize(_histdy, 1.);
}

/// @}


/// @name Histograms
/// @{

CounterPtr _weight2Jet;
CounterPtr _weight3Jet;

BinnedHistoPtr<string> _histLLBar, _histLL, _histLLBarCorr;
BinnedHistoPtr<string> _histLLBar_2jet, _histLL_2jet, _histLLBarCorr_2jet;
BinnedHistoPtr<string> _histLLBar_3jet, _histLL_3jet, _histLLBarCorr_3jet;

Histo1DPtr _histdcosTheta;
Histo1DPtr _histdy;

const string Ecm = "91.2";
/// @}

};

RIVET_DECLARE_PLUGIN(OPAL_2000_I474010);

} ```