Rivet analyses

title: OPAL_1998_I474012

Polarization of b-baryons at LEP1

Experiment: OPAL (LEP)

Inspire ID: 474012

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Lett. B444 (1998) 539-554

Beams: e- e+

Beam energies: (45.6, 45.6)GeV

Run details: - e+e- > hadrons

Measurement of the polarization of b-baryons, mainly $\Lambda_b$ at LEP1. The result is obtained by measuring the ratio of the neutrino (missing energy) to the charged lepton energy, rather than the more sophisticated fit used in the paper.

Source code:OPAL_1998_I474012.cc

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

include "Rivet/Analysis.hh"

include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {

/// @brief b-baryon polarization class OPAL_1998_I474012 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(OPAL_1998_I474012);


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

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

  // Initialise and register projections
  declare(UnstableParticles(), "UFS");

  // Book histograms
  book(_h_El, "El", 45, 0., 45.0);
  book(_h_Ev, "Ev", 45, 0., 45.0);
  book(_h_ratio, "ratio", 20, 0., 10.0);
}

void findDecayProducts(Particle p, Particles& lep, Particles& nu) {
  for (const Particle& child : p.children()) {
    if (PID::isHadron(child.pid())) continue;
    if (child.abspid() == 11 or child.abspid() == 13)
      lep.push_back(child);
    else if (child.abspid() == 12 or child.abspid() == 14)
      nu.push_back(child);
    else if (child.abspid() != 15)
      findDecayProducts(child, lep, nu);
  }
}

/// Perform the per-event analysis
void analyze(const Event& event) {
  const FinalState& ufs = apply<UnstableParticles>(event, "UFS");
  // loop over weakly decaying b-baryons
  for (const Particle& p : ufs.particles(Cuts::abspid == 5122)) {
    Particles lep, nu;
    findDecayProducts(p, lep, nu);
    if (lep.size() != 1 || nu.size() != 1) continue;
    _h_El->fill(lep[0].momentum().t());
    _h_Ev->fill(nu[0].momentum().t());
    _h_ratio->fill(nu[0].momentum().t() / lep[0].momentum().t());
  }
}


/// Normalise histograms etc., after the run
void finalize() {
  normalize(_h_El);
  normalize(_h_Ev);
  normalize(_h_ratio);
  if (_h_El->effNumEntries() != 0. and _h_Ev->effNumEntries() != 0.) {
    const double Ev = _h_Ev->xMean();
    const double El = _h_El->xMean();
    const double dEv = _h_Ev->xStdErr();
    const double dEl = _h_El->xStdErr();
    const double ratio = Ev / El;
    const double dr = (El * dEv - Ev * dEl) / sqr(El);
    const double rho = 0.091;
    const double P = 7. - 20. / (2. + ratio) + 10. * rho * ratio * (-4. + 3. * ratio) / sqr(2. + ratio);
    const double dP = (20. * (2. + ratio + rho * (-4. + 8. * ratio))) / pow(2. + ratio, 3) * dr;
    BinnedEstimatePtr<string> h_pol;
    book(h_pol, 1, 1, 1);
    h_pol->bin(1).set(P, dP);
  }
}

/// @}


/// @name Histograms
/// @{
Histo1DPtr _h_El, _h_Ev, _h_ratio;
/// @}

};

RIVET_DECLARE_PLUGIN(OPAL_1998_I474012);

} ```