1 //FJSTARTHEADER
2 // $Id: TopTaggerBase.cc 4442 2020-05-05 07:50:11Z soyez $
3 //
4 // Copyright (c) 2005-2020, Matteo Cacciari, Gavin P. Salam and Gregory Soyez
5 //
6 //----------------------------------------------------------------------
7 // This file is part of FastJet.
8 //
9 // FastJet is free software; you can redistribute it and/or modify
10 // it under the terms of the GNU General Public License as published by
11 // the Free Software Foundation; either version 2 of the License, or
12 // (at your option) any later version.
13 //
14 // The algorithms that underlie FastJet have required considerable
15 // development. They are described in the original FastJet paper,
16 // hep-ph/0512210 and in the manual, arXiv:1111.6097. If you use
17 // FastJet as part of work towards a scientific publication, please
18 // quote the version you use and include a citation to the manual and
19 // optionally also to hep-ph/0512210.
20 //
21 // FastJet is distributed in the hope that it will be useful,
22 // but WITHOUT ANY WARRANTY; without even the implied warranty of
23 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
24 // GNU General Public License for more details.
25 //
26 // You should have received a copy of the GNU General Public License
27 // along with FastJet. If not, see <http://www.gnu.org/licenses/>.
28 //----------------------------------------------------------------------
29 //FJENDHEADER
30
31 #include <fastjet/tools/TopTaggerBase.hh>
32
33 FASTJET_BEGIN_NAMESPACE
34
35 using namespace std;
36
37 // compute the W helicity angle
38 //
39 // The helicity angle is a standard observable in top decays, used to
40 // determine the Lorentz structure of the top- W coupling [13]. It is
41 // defined as the angle, measured in the rest frame of the
42 // reconstructed W, between the reconstructed top's flight direction
43 // and one of the W decay products. Normally, it is studied in
44 // semi-leptonic top decays, where the charge of the lepton uniquely
45 // identifies these decay products. In hadronic top decays there is an
46 // ambiguity which we resolve by choosing the lower pT subjet, as
47 // measured in the lab frame.
48 //
49 // The jet passed to this function is expected to already have
50 // the structure of a top, including a functional "W()" call;
51 // the W must be made of two pieces.
_cos_theta_W(const PseudoJet & res) const52 double TopTaggerBase::_cos_theta_W(const PseudoJet & res) const{
53 // the two jets of interest: top and lower-pt prong of W
54 const PseudoJet & W = res.structure_of<TopTaggerBase>().W();
55 vector<PseudoJet> W_pieces = W.pieces();
56 assert(W_pieces.size() == 2);
57 //assert(W_pieces[0].perp2() >= W_pieces[1].perp2());
58 //PseudoJet W2 = W_pieces[1];
59 // extract the softer of the two W pieces.
60 PseudoJet W2 = (W_pieces[0].perp2() < W_pieces[1].perp2())
61 ? W_pieces[0]
62 : W_pieces[1];
63 PseudoJet top = res;
64
65 // transform these jets into jets in the rest frame of the W
66 W2.unboost(W);
67 top.unboost(W);
68
69 return (W2.px()*top.px() + W2.py()*top.py() + W2.pz()*top.pz())/
70 sqrt(W2.modp2() * top.modp2());
71 }
72
73
74 FASTJET_END_NAMESPACE
75