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,1,1,1);
26    }
27
28    void findChildren(const Particle & p, Particles & pim, Particles & pip,
29		      Particles & pi0, unsigned int &ncount) {
30      for( const Particle &child : p.children()) {
31	if(child.pid()==PID::PIPLUS) {
32	  pip.push_back(child);
33	  ncount+=1;
34	}
35	else if(child.pid()==PID::PIMINUS) {
36	  pim.push_back(child);
37	  ncount+=1;
38	}
39	else if(child.pid()==PID::PI0) {
40	  pi0.push_back(child);
41	  ncount+=1;
42	}
43	else if(child.children().empty()) {
44	  ncount+=1;
45	}
46    	else
47    	  findChildren(child,pim,pip,pi0,ncount);
48      }
49    }
50
51    /// Perform the per-event analysis
52    void analyze(const Event& event) {
53      Particles B0 = apply<UnstableParticles>(event, "B0").particles();
54      for(const Particle & p : B0) {
55	// skip cases with mixing
56	if(p.children().size()==1 && p.children()[0].abspid()==p.abspid()) continue;
57	// particle antiparticle pair of a_1
58	if(p.children().size()!=2 || p.children()[0].pid()!=-p.children()[1].pid() ||
59	   p.children()[0].abspid()!=20213) continue;
60	Particle a1p = p.children()[0], a1m = p.children()[1];
61	if( (p.pid()>0&&a1p.pid()<0) || (p.pid()<0&&a1p.pid()>0) ) swap(a1p,a1m);
62	Particles pip,pim,pi0;
63	unsigned int ncount=0;
64	findChildren(a1p,pim,pip,pi0,ncount);
65	if(ncount!=3 || pip.size()!=2 || pim.size()!=1) continue;
66	LorentzTransform boost = LorentzTransform::mkFrameTransformFromBeta(p.momentum().betaVec());
67	FourMomentum pa1p = boost.transform(a1p.momentum());
68	FourMomentum pa1m = boost.transform(a1m.momentum());
69	LorentzTransform boost2 = LorentzTransform::mkFrameTransformFromBeta(pa1p.betaVec());
70	FourMomentum ppip = boost2.transform(boost.transform(pip[0].momentum()));
71	FourMomentum ppim = boost2.transform(boost.transform(pim[0].momentum()));
72	Vector3 n = ppip.p3().cross(ppim.p3()).unit();
73	double cTheta = n.dot(pa1m.p3().unit());
74	_p->fill(5.28,(2.-5.*sqr(cTheta)));
75      }
76    }
77
78
79    /// Normalise histograms etc., after the run
80    void finalize() {
81    }
82
83    /// @}
84
85
86    /// @name Histograms
87    /// @{
88    Profile1DPtr _p;
89    /// @}
90
91
92  };
93
94
95  RIVET_DECLARE_PLUGIN(BABAR_2009_I825406);
96
97}