FastJet  3.0.6
TopTaggerBase.cc
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2 // $Id: TopTaggerBase.cc 2689 2011-11-14 14:51:06Z soyez $
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28 
29 #include <fastjet/tools/TopTaggerBase.hh>
30 
31 FASTJET_BEGIN_NAMESPACE
32 
33 using namespace std;
34 
35 // compute the W helicity angle
36 //
37 // The helicity angle is a standard observable in top decays, used to
38 // determine the Lorentz structure of the top- W coupling [13]. It is
39 // defined as the angle, measured in the rest frame of the
40 // reconstructed W, between the reconstructed top's flight direction
41 // and one of the W decay products. Normally, it is studied in
42 // semi-leptonic top decays, where the charge of the lepton uniquely
43 // identifies these decay products. In hadronic top decays there is an
44 // ambiguity which we resolve by choosing the lower pT subjet, as
45 // measured in the lab frame.
46 //
47 // The jet passed to this function is expected to already have
48 // the structure of a top, including a functional "W()" call;
49 // the W must be made of two pieces.
50 double TopTaggerBase::_cos_theta_W(const PseudoJet & res) const{
51  // the two jets of interest: top and lower-pt prong of W
52  const PseudoJet & W = res.structure_of<TopTaggerBase>().W();
53  vector<PseudoJet> W_pieces = W.pieces();
54  assert(W_pieces.size() == 2);
55  //assert(W_pieces[0].perp2() >= W_pieces[1].perp2());
56  //PseudoJet W2 = W_pieces[1];
57  // extract the softer of the two W pieces.
58  PseudoJet W2 = (W_pieces[0].perp2() < W_pieces[1].perp2())
59  ? W_pieces[0]
60  : W_pieces[1];
61  PseudoJet top = res;
62 
63  // transform these jets into jets in the rest frame of the W
64  W2.unboost(W);
65  top.unboost(W);
66 
67  return (W2.px()*top.px() + W2.py()*top.py() + W2.pz()*top.pz())/
68  sqrt(W2.modp2() * top.modp2());
69 }
70 
71 
72 FASTJET_END_NAMESPACE
const TransformerType::StructureType & structure_of() const
this is a helper to access any structure created by a Transformer (that is, of type Transformer::Stru...
Definition: PseudoJet.hh:970
virtual std::vector< PseudoJet > pieces() const
retrieve the pieces that make up the jet.
Definition: PseudoJet.cc:666
A base class that provides a common interface for top taggers that are able to return a W (in additio...
PseudoJet & unboost(const PseudoJet &prest)
transform this jet (given in lab) into a jet in the rest frame of prest [NOT FULLY TESTED] ...
Definition: PseudoJet.cc:295
double modp2() const
return the squared 3-vector modulus = px^2+py^2+pz^2
Definition: PseudoJet.hh:161
Class to contain pseudojets, including minimal information of use to jet-clustering routines...
Definition: PseudoJet.hh:65