Rivet analyses

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 Λ_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++ -*-
#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);


}

Rivet analyses

Polarization of b-baryons at LEP1

Source code:OPAL_1998_I474012.cc

Source code:`OPAL_1998_I474012.cc`