Rivet analyses

// -*- C++ -*-
#include "Rivet/Analysis.hh"
#include "Rivet/Projections/UnstableParticles.hh"

namespace Rivet {


  /// @brief D* polarization in B0 -> D* tau nu_tau
  class BELLE_2019_I1724068 : public Analysis {
  public:

    /// Constructor
    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2019_I1724068);


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

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

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

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

    void findChildren(const Particle & p, unsigned int & nprod,
              Particles & Dstar, Particles & tau, Particles & nu) {
      for(const Particle & child : p.children()) {
    if(child.pid()==-413) {
      ++nprod;
      Dstar.push_back(child);
    }
    else if(child.pid()==-15) {
      ++nprod;
      tau.push_back(child);
    }
    else if(child.pid()==16) {
      ++nprod;
      nu.push_back(child);
    }
    else if(child.pid()==22)
      continue;
    else if(child.children().empty() ||
        child.pid()==111 || child.pid()==221 || child.pid()==331) {
      ++nprod;
    }
    else {
      findChildren(child,nprod,Dstar,tau,nu);
    }
      }
    }

    /// Perform the per-event analysis
    void analyze(const Event& event) {
      // Loop over B0 mesons
      for(const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==PID::B0)) {
    // find the B decay
    unsigned int nprod = 0;
    Particles Dstar,tau,nu;
    findChildren(p,nprod,Dstar,tau,nu);
    if(nprod!=3 || Dstar.size()!=1 || tau.size() !=1 || nu.size()!=1)
      continue;
    // and the D* decay
    if(Dstar[0].children().size()!=2) continue;
    Particle D0;
    if(Dstar[0].children()[0].pid()==-421 &&
       Dstar[0].children()[1].pid()==-211) {
      D0 = Dstar[0].children()[0];
    }
    else if(Dstar[0].children()[1].pid()==-421 &&
        Dstar[0].children()[0].pid()==-211) {
      D0 = Dstar[0].children()[1];
    }
    else
      continue;
    // compute the helicity angle
    // boost to B0 rest frame
    LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
    FourMomentum pDstar = boost1.transform(Dstar[0].momentum());
    FourMomentum pD     = boost1.transform(D0      .momentum());
    LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pDstar.betaVec());
    pD = boost2.transform(pD);
    double cTheta = pD.p3().unit().dot(pDstar.p3().unit());
    if(cTheta<=0.) _h_cTheta->fill(cTheta);
      }
    }

    pair<double,double> calcF(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.xMin()*(sqr(bin.xMin())-3.)-bin.xMax()*(sqr(bin.xMax())-3.));
        double bi = 1.5*(bin.xMin()*(1.-sqr(bin.xMin()))-bin.xMax()*(1.-sqr(bin.xMax())));
        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() {
      normalize(_h_cTheta);
      Estimate1DPtr _h_F;
      book(_h_F,2,1,1);
      pair<double,double> F = calcF(_h_cTheta);
      _h_F->bin(1).set(F.first, F.second);

    }

    /// @}


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


  };


  RIVET_DECLARE_PLUGIN(BELLE_2019_I1724068);

}