TopTaggerBase.cc

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// $Id$
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// Copyright (c) 2005-2026, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
//
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// This file is part of FastJet.
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#include "fastjet/tools/TopTaggerBase.hh"
 
FASTJET_BEGIN_NAMESPACE
 
using namespace std;
 
// compute the W helicity angle
//
// The helicity angle is a standard observable in top decays, used to
// determine the Lorentz structure of the top- W coupling [13]. It is
// defined as the angle, measured in the rest frame of the
// reconstructed W, between the reconstructed top's flight direction
// and one of the W decay products. Normally, it is studied in
// semi-leptonic top decays, where the charge of the lepton uniquely
// identifies these decay products. In hadronic top decays there is an
// ambiguity which we resolve by choosing the lower pT subjet, as
// measured in the lab frame.
//
// The jet passed to this function is expected to already have
// the structure of a top, including a functional "W()" call;
// the W must be made of two pieces.
double TopTaggerBase::_cos_theta_W(const PseudoJet & res) const{
  // the two jets of interest: top and lower-pt prong of W
  const PseudoJet & W  = res.structure_of<TopTaggerBase>().W();
  vector<PseudoJet> W_pieces = W.pieces();
  assert(W_pieces.size() == 2);
  //assert(W_pieces[0].perp2() >= W_pieces[1].perp2());
  //PseudoJet W2  = W_pieces[1];
  // extract the softer of the two W pieces.
  PseudoJet W2 =  (W_pieces[0].perp2() < W_pieces[1].perp2())
                    ? W_pieces[0] 
                    : W_pieces[1];
  PseudoJet top = res;
  
  // transform these jets into jets in the rest frame of the W
  W2.unboost(W);
  top.unboost(W);
 
  return (W2.px()*top.px() + W2.py()*top.py() + W2.pz()*top.pz())/
    sqrt(W2.modp2() * top.modp2());
}
 
 
FASTJET_END_NAMESPACE