Rivet analyses

title: CLEOII_2005_I668268

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

Experiment: CLEOII (CESR)

Inspire ID: 668268

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. 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_2005_I668268.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief Lambda_c -> Lambda e+ nu_e asymmetry class CLEOII_2005_I668268 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(CLEOII_2005_I668268);


/// @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, "/TMP/hLambda", 20, -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, 1, 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_2005_I668268);

} ```