Rivet analyses

title: CLEOII_1994_I371611

Decay asymmetry in $\Lambda^+_c\to\Lambda^0 e^+ \nu_e$

Experiment: CLEOII (CESR)

Inspire ID: 371611

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett.B 323 (1994) 219-226

Beams: * *

Beam energies: ANY

Run details: - Any process producing Lambda_c+, original e+e-

Measurement of the decay asymmetry in $\Lambda^+c\to\Lambda^0 e^+ \nu_e$ by CLEOII. While this result is superseeded by later CLEO measurements it is one of the few to present the anglular distribution as well as the fitted asymmetry parameter. The product of assymetries $\alpha{\Lambda_c^+}\alpha_\Lambda$ is extracted as that is what is measured not $\alpha_{\Lambda_c^+}$ and there has been a significant change in $\alpha_\Lambda$ since this paper was published

Source code:CLEOII_1994_I371611.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Lambda_c -> Lambda e+ nu_e asymmetry class CLEOII_1994_I371611 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CLEOII_1994_I371611);


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

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

  // Book histograms
  book(_h_Lambda, 1, 1, 1);
}

void findChildren(Particle parent,
                  int sign,
                  unsigned int& npart,
                  Particles& lambda,
                  Particles& e,
                  Particles& nu) {
  for (const Particle& child : parent.children()) {
    if (child.pid() == sign * PID::LAMBDA) {
      lambda.push_back(child);
      ++npart;
    }
    else if (child.pid() == -sign * PID::EMINUS) {
      e.push_back(child);
      ++npart;
    }
    else if (child.pid() == sign * PID::NU_E) {
      nu.push_back(child);
      ++npart;
    }
    else if (!child.children().empty()) {
      findChildren(child, sign, npart, lambda, e, nu);
    }
    else {
      ++npart;
    }
  }
}

/// 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;
    Particles lambda, e, nu;
    unsigned int npart(0);
    findChildren(Lambdac, sign, npart, lambda, e, nu);
    if (npart != 3 || lambda.size() != 1 || e.size() != 1 || nu.size() != 1) continue;
    Particle baryon2;
    if (lambda[0].children()[0].pid() == sign * 2212 && lambda[0].children()[1].pid() == -sign * 211) {
      baryon2 = lambda[0].children()[0];
    }
    else if (lambda[0].children()[1].pid() == sign * 2212
             && lambda[0].children()[0].pid() == -sign * 211) {
      baryon2 = lambda[0].children()[1];
    }
    else
      continue;
    // first boost to the Lambdac rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(Lambdac.momentum().betaVec());
    FourMomentum pbaryon1 = boost1.transform(lambda[0].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_Lambda->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() {
  //  asymmetry
  normalize(_h_Lambda);
  Estimate0DPtr _h_alpha;
  book(_h_alpha, 2, 1, 1);
  pair<double, double> alpha = calcAlpha(_h_Lambda);
  _h_alpha->set(alpha.first, alpha.second);
}

/// @}


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

};

RIVET_DECLARE_PLUGIN(CLEOII_1994_I371611);

} ```