Rivet analyses

// -*- 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);

}