1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/UnstableParticles.hh"
  4
  5namespace Rivet {
  6
  7
  8  /// @brief D* polarization in B0 -> D* tau nu_tau
  9  class BELLE_2019_I1724068 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(BELLE_2019_I1724068);
 14
 15
 16    /// @name Analysis methods
 17    /// @{
 18
 19    /// Book histograms and initialise projections before the run
 20    void init() {
 21
 22      // Initialise and register projections
 23      declare(UnstableParticles(), "UFS");
 24
 25      // Book histograms
 26      book(_h_cTheta,1,1,1);
 27    }
 28
 29    void findChildren(const Particle & p, unsigned int & nprod,
 30		      Particles & Dstar, Particles & tau, Particles & nu) {
 31      for(const Particle & child : p.children()) {
 32	if(child.pid()==-413) {
 33	  ++nprod;
 34	  Dstar.push_back(child);
 35	}
 36	else if(child.pid()==-15) {
 37	  ++nprod;
 38	  tau.push_back(child);
 39	}
 40	else if(child.pid()==16) {
 41	  ++nprod;
 42	  nu.push_back(child);
 43	}
 44	else if(child.pid()==22)
 45	  continue;
 46	else if(child.children().empty() ||
 47		child.pid()==111 || child.pid()==221 || child.pid()==331) {
 48	  ++nprod;
 49	}
 50	else {
 51	  findChildren(child,nprod,Dstar,tau,nu);
 52	}
 53      }
 54    }
 55
 56    /// Perform the per-event analysis
 57    void analyze(const Event& event) {
 58      // Loop over B0 mesons
 59      for(const Particle& p : apply<UnstableParticles>(event, "UFS").particles(Cuts::pid==PID::B0)) {
 60	// find the B decay
 61	unsigned int nprod = 0;
 62	Particles Dstar,tau,nu;
 63	findChildren(p,nprod,Dstar,tau,nu);
 64	if(nprod!=3 || Dstar.size()!=1 || tau.size() !=1 || nu.size()!=1)
 65	  continue;
 66	// and the D* decay
 67	if(Dstar[0].children().size()!=2) continue;
 68	Particle D0;
 69	if(Dstar[0].children()[0].pid()==-421 &&
 70	   Dstar[0].children()[1].pid()==-211) {
 71	  D0 = Dstar[0].children()[0];
 72	}
 73	else if(Dstar[0].children()[1].pid()==-421 &&
 74		Dstar[0].children()[0].pid()==-211) {
 75	  D0 = Dstar[0].children()[1];
 76	}
 77	else
 78	  continue;
 79	// compute the helicity angle
 80	// boost to B0 rest frame
 81	LorentzTransform boost1 = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
 82	FourMomentum pDstar = boost1.transform(Dstar[0].momentum());
 83	FourMomentum pD     = boost1.transform(D0      .momentum());
 84	LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pDstar.betaVec());
 85	pD = boost2.transform(pD);
 86	double cTheta = pD.p3().unit().dot(pDstar.p3().unit());
 87	if(cTheta<=0.) _h_cTheta->fill(cTheta);
 88      }
 89    }
 90
 91    pair<double,double> calcF(Histo1DPtr hist) {
 92      if(hist->numEntries()==0.) return make_pair(0.,0.);
 93      double sum1(0.),sum2(0.);
 94      for (const auto& bin : hist->bins() ) {
 95      	double Oi = bin.sumW();
 96      	if(Oi==0.) continue;
 97      	double ai = 0.5*(bin.xMin()*(sqr(bin.xMin())-3.)-bin.xMax()*(sqr(bin.xMax())-3.));
 98        double bi = 1.5*(bin.xMin()*(1.-sqr(bin.xMin()))-bin.xMax()*(1.-sqr(bin.xMax())));
 99      	double Ei = bin.errW();
100       	sum1 += sqr(bi/Ei);
101      	sum2 += bi/sqr(Ei)*(Oi-ai);
102      }
103      return make_pair(sum2/sum1,sqrt(1./sum1));
104    }
105
106    /// Normalise histograms etc., after the run
107    void finalize() {
108      normalize(_h_cTheta);
109      Estimate1DPtr _h_F;
110      book(_h_F,2,1,1);
111      pair<double,double> F = calcF(_h_cTheta);
112      _h_F->bin(1).set(F.first, F.second);
113
114    }
115
116    /// @}
117
118
119    /// @name Histograms
120    /// @{
121    Histo1DPtr _h_cTheta;
122    /// @}
123
124
125  };
126
127
128  RIVET_DECLARE_PLUGIN(BELLE_2019_I1724068);
129
130}