Rivet analyses

title: 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

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Lambda_c -> Lambda pi asymmetry class ARGUS_1992_I319105 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(ARGUS_1992_I319105);


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

/// Book histograms and initialise projections before the run
void init() {
  // Initialise and register projections
  declare(UnstableParticles(), "UFS");
  // Book histograms
  book(_h_ctheta, 3, 1, 1);
}


/// 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;
    if (Lambdac.children().size() != 2) continue;
    Particle baryon1, meson1;
    if (Lambdac.children()[0].pid() == sign * 3122 && Lambdac.children()[1].pid() == sign * 211) {
      baryon1 = Lambdac.children()[0];
      meson1 = Lambdac.children()[1];
    }
    else if (Lambdac.children()[1].pid() == sign * 3122 && Lambdac.children()[0].pid() == sign * 211) {
      baryon1 = Lambdac.children()[1];
      meson1 = Lambdac.children()[0];
    }
    else
      continue;
    Particle baryon2, meson2;
    if (baryon1.children()[0].pid() == sign * 2212 && baryon1.children()[1].pid() == -sign * 211) {
      baryon2 = baryon1.children()[0];
      meson2 = baryon1.children()[1];
    }
    else if (baryon1.children()[1].pid() == sign * 2212 && baryon1.children()[0].pid() == -sign * 211) {
      baryon2 = baryon1.children()[1];
      meson2 = baryon1.children()[0];
    }
    else
      continue;
    // first boost to the Lambdac rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
    FourMomentum pbaryon1 = boost1.transform(baryon1.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_ctheta->fill(cTheta);
  }
}

pair<double, double> calcAlpha(Histo1DPtr hist) {
  if (hist->numEntries() == 0.) return make_pair(0., 0.);
  double sum1(0.), sum2(0.);
  for (const auto& bin : hist->bins()) {
    double Oi = bin.sumW();
    if (Oi == 0.) continue;
    double ai = 0.5 * (bin.xMax() - bin.xMin());
    double bi = 0.5 * ai * (bin.xMax() + bin.xMin());
    double Ei = bin.errW();
    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() {
  normalize(_h_ctheta);
  Estimate0DPtr _h_alpha;
  book(_h_alpha, 2, 1, 1);
  pair<double, double> alpha = calcAlpha(_h_ctheta);
  _h_alpha->set(alpha.first, alpha.second);
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(ARGUS_1992_I319105);

} ```