1// -*- C++ -*-
  2#include "Rivet/Analysis.hh"
  3#include "Rivet/Projections/UnstableParticles.hh"
  4
  5namespace Rivet {
  6
  7
  8  /// @brief B0 -> a1+ a1-
  9  class BABAR_2009_I825406 : public Analysis {
 10  public:
 11
 12    /// Constructor
 13    RIVET_DEFAULT_ANALYSIS_CTOR(BABAR_2009_I825406);
 14
 15
 16    /// @name Analysis methods
 17    /// @{
 18
 19    /// Book histograms and initialise projections before the run
 20    void init() {
 21      // Initialise and register projections
 22      UnstableParticles ufs = UnstableParticles(Cuts::pid==511);
 23      declare(ufs, "B0");
 24      // histograms
 25      book(_p[0],1,1,1);
 26      book(_p[1],"TMP/wgt");
 27    }
 28
 29    void findChildren(const Particle & p, Particles & pim, Particles & pip,
 30		      Particles & pi0, unsigned int &ncount) {
 31      for( const Particle &child : p.children()) {
 32	if(child.pid()==PID::PIPLUS) {
 33	  pip.push_back(child);
 34	  ncount+=1;
 35	}
 36	else if(child.pid()==PID::PIMINUS) {
 37	  pim.push_back(child);
 38	  ncount+=1;
 39	}
 40	else if(child.pid()==PID::PI0) {
 41	  pi0.push_back(child);
 42	  ncount+=1;
 43	}
 44	else if(child.children().empty()) {
 45	  ncount+=1;
 46	}
 47    	else
 48    	  findChildren(child,pim,pip,pi0,ncount);
 49      }
 50    }
 51
 52    /// Perform the per-event analysis
 53    void analyze(const Event& event) {
 54      Particles B0 = apply<UnstableParticles>(event, "B0").particles();
 55      for(const Particle & p : B0) {
 56	// skip cases with mixing
 57	if(p.children().size()==1 && p.children()[0].abspid()==p.abspid()) continue;
 58	// particle antiparticle pair of a_1
 59	if(p.children().size()!=2 || p.children()[0].pid()!=-p.children()[1].pid() ||
 60	   p.children()[0].abspid()!=20213) continue;
 61	Particle a1p = p.children()[0], a1m = p.children()[1];
 62	if( (p.pid()>0&&a1p.pid()<0) || (p.pid()<0&&a1p.pid()>0) ) swap(a1p,a1m);
 63	Particles pip,pim,pi0;
 64	unsigned int ncount=0;
 65	findChildren(a1p,pim,pip,pi0,ncount);
 66	if(ncount!=3 || pip.size()!=2 || pim.size()!=1) continue;
 67	LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
 68	FourMomentum pa1p = boost.transform(a1p.momentum());
 69	FourMomentum pa1m = boost.transform(a1m.momentum());
 70	LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pa1p.betaVec());
 71	FourMomentum ppip = boost2.transform(boost.transform(pip[0].momentum()));
 72	FourMomentum ppim = boost2.transform(boost.transform(pim[0].momentum()));
 73	Vector3 n = ppip.p3().cross(ppim.p3()).unit();
 74	double cTheta = n.dot(pa1m.p3().unit());
 75	_p[0]->fill((2.-5.*sqr(cTheta)));
 76        _p[1]->fill();
 77      }
 78    }
 79
 80
 81    /// Normalise histograms etc., after the run
 82    void finalize() {
 83      scale(_p[0], 1./ *_p[1]);
 84    }
 85
 86    /// @}
 87
 88
 89    /// @name Histograms
 90    /// @{
 91    CounterPtr _p[2];
 92    /// @}
 93
 94
 95  };
 96
 97
 98  RIVET_DECLARE_PLUGIN(BABAR_2009_I825406);
 99
100}